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What prevents predator overpopulation?

What prevents predator overpopulation?


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I've often heard that a population, human or otherwise, will continue to grow as long as there is food available (assuming nothing else is killing them off). It makes sense: if you have food you can live, and if nothing is hunting you you'll survive to reproduce.

I recently designed a piece of software to simulate an ecosystem, with groups of creatures of different species eating and hunting and reproducing alongside each other. It was very simplified (each animal had simple attack/defense/speed/stealth values, etc), but something became rapidly apparent: in every simulation the predators overwhelmed the prey, reproducing until their numbers could not be sustained by the herbivores, and leading to an inevitable die-off of both groups. I could delay the die-off by adjusting different values and initial population counts, but it would always happen eventually. The predators would eat and breed and eat and breed until the entire system collapsed.

At first I thought it was just the product of my over-simplified system, but it got me thinking: what prevents predators from overpopulating in real life?

It seems like the natural tendency would be for (for example) the sharks to continue breeding and eating until all the fish are gone, or the wolves to eat all the deer, etc. Obviously some predators have predators of their own, but that's just putting off the question: if the hyenas don't overpopulate because the lions eat them, then what's keeping the lions from overpopulating? I can't come up with anything that would prevent the apex predators from growing too numerous, then fighting each other over a dwindling prey population, then dying off entirely when there was no more food to find.

Do predator populations self-regulate to prevent putting undo stress on their prey populations? Or is there some other mechanism to keep the predator hierarchy from becoming top-heavy?


No, I don't think auto-regulation explain much in the population sizes of predators. Group selection may explain such auto-regulation but I don't think it is of any considerable importance for this discussion.

The short answer is, as @shigeta said

[predators] tend to starve to death as they are too many!

To have a better understanding of what @shigeta said, you'll be interested in understanding various model of prey-predator or of consumer-resource interactions. For example the famous Lotka-Volterra equations describe the population dynamics of two co-existing species where one is the prey and the other is a predator. Let's first define some variables…

  • $x$ : Number of preys
  • $y$ : number of predators
  • $t$ : time
  • $alpha$, $eta$, $xi$ and $gamma$ are parameters describing how one species influence the population size of the other one.

The Lotka-Voltera equations are:

$$frac{dx}{dt} = x(alpha - eta y)$$ $$frac{dy}{dt} = -y(gamma - xi x)$$

You can show that for some parameters the matrix for these equations have a complex eigenvalue meaning that the long term behavior of this system is cyclic (periodic behavior). If you simulate such systems you'll see that the population sizes of the two species fluctuate like this:

where the blue line represents the predators and the red line represents the preys.

Representing the same data in phase space, meaning with the population size of the two species on axes $x$ and $y$ you get:

where the arrows shows the direction toward which the system moves. If the population size of the predators ($y$) reaches 0 (extinction), then $frac{dx}{dt} = x(alpha - eta y)space$ becomes $frac{dx}{dt} = xalpha space$ (which general solution is $x_t = e^{alpha t}x_0$) and therefore the populations of preys will grow exponentially. If the population size of preys ($x$) reaches 0 (extinction), then $frac{dy}{dt} = -y(gamma - xi x)space$ becomes $frac{dy}{dt} = -ygamma space$, and therefore the population of predators will decrease exponentially.

Following this model, your question is actually: Why are the parameters $alpha$, $eta$, $xi$ and $gamma$ not "set" in a way that predators cause the extinction of preys (and therefore their own extinction)? One might equivalently ask the opposite question? Why don't preys evolve in order to escape predators so that the population of predators crushes?

As showed, you don't need a complex model to allow the co-existence of predators and preys. You could describe your model a bit more accurately in another post and ask why in your model the preys always get extinct. But there are tons of possibilities to render your model more realistic such as adding spatial heterogeneities (places to hide for example as suggested by @AudriusMeškauskas). One can also consider other trophic levels, stochastic effects, varying selection pressure through time (and other types of balancing selection), age, sex or health-specific mortality rate due to predation (e.g. predators may target preferentially young ones or diseased ones), several competing species, etc…


I would also like to talk about other things that might be of interest in your model (two of them need you to allow evolutionary processes in your model):

1) lineage selection: predators that eat too much end up disappearing because they caused their preys to get extinct. This hypothesis has nothing to do with some kind of auto-regulation for the good of species. Of course you'd need several species of predators and preys in your model. This kind of hypothesis are usually considered as very unlikely to have any explanatory power.

2) Life-dinner principle. While the wolf runs for its dinner, the rabbit runs for its life. Therefore, there is higher selection pressure on the rabbits which yield the rabbits to run in average slightly faster than wolves. This evolutionary process protects the rabbits from extinction.

3) You may consider…

  • more than one species of preys or predators
  • environmental heterogeneity
  • partial overlapping of distribution ranges between predators and preys
  • When one species is absent, the model behave just like an exponential model. You might want to make a model of logistic growth for each species by including $K_x$ and $K_y$ the carrying capacity for each species.
  • Adding a predator (or parasite) to the predator species of interest

    … and you might get very different results.


Remi.b's answer is an excellent one, and this should be taken as a supplement to it:

It's possible your simulation is correct

The Lotka-Volterra equations are what is known as a deterministic model, and it describes the behavior of predator-prey systems (in a somewhat simplified fashion) in large populations. Small populations are subject to what is known as stochastic extinction - as the predator and prey curves approach their minimums, they may predict populations less than 1, which in reality are either 0 or 1, and when they're 0… well, someone's gone extinct.

Odds are your simulation is on a small population, and if its a simulation, rather than calculus, you should be seeing those stochastic effects (to be sure - if your simulation keeps track of integer animals, rather than continuous animals, and random chance is involved, this is going to be something you have to worry about).

In a similar model I've been working with, that's a pretty simple adaptation of a L-V model that should, deterministically, result in a stable system like in Remi.b's picture, the predators go extinct 20% of the time, and the prey 80% of the time.


One of the possible adjustments of these mathematical models is to introduce a "place to hide", making some (small) percent of the prey population not accessible (or much more difficult to access) for predators. After the number of predators decreases from starvation, prey individuals are relatively safer outside the "place to hide" and can grow over this limit before the number of predators increases again.


You need to add Bell curves to your simulation. The most important curve to simulate is the nutritional quality of the prey though there are plenty more thing to curve like speed and virility for prey and predators both. Nature uses lots of Bell curves so they must be good for something, such as softening the harsh effects of pure exponential growth. I suspect that the more Bell curves you implement the more stable your populations will become.

If the food value of your prey is all equal then there's no reason for your predators to not eat every last one. That's what I do with a plate full of foodstuffs, all equally delicious and with enough surplus that all the bad food can be thrown in the trash. Problems arise when you are forced to eat the trash because there's nothing else to eat.

Let's eat our prey from all 3 sides of the curve. If we eat from the weakest and easiest to catch on the left this makes the prey population stronger and more resistant to the predator. If we eat from the most common on the top the prey rebound more rapidly. If we eat from the most desirable on the right the rapid quality (but not the virility) reduction of the prey population has serious negative health consequences for the predators. Notice how each direction we eat has the necessary corrective action against the predator as a response. Due to the wild randomness of genetics the weak population can always rebound in quality when the predatory pressure eases. Looks like Nature didn't screw that one up either.

An easy example is seen in the human vs plant food supply. Population should rise with no detriment as we produce more and more food, and it would if the quality could stay the same. The population does rise but because the nutritional quality keeps dropping, the detriment is rapidly increasing on numerous health and population charts.

When plants are forced to make do with what little they have, you are forced to make do with what little they provide.


This boils down to one main reason: competition.

Animals, in general, don't like sharing resources with direct competitors, but this violence over food, territory, and in the case of intraspecific relations, breeding rights seems to be more extreme the higher up you go on the food chain.

An excellent example of this regards cougars. A dominant tom cat in Montana, for instance, rules over a domain that can easily exceed a hundred square miles. This territory is several times larger than what he'd require just to feed himself, so why is it so large? It's quite simple: breeding rights. Within the borders of his, hundred or so square miles of territory are lands also used by two or three females and the cubs he's fathered with them. Other tom cougars enter at their own risk, as he'd be more than happy to run them out or kill them. The same fate befalls any of his sons who reach maturity, so they either disperse to lands unknown or die trying. Dispersing males naturally die frequently in most species as a result.

Females also defend their territories, but since it's not advantageous for them to have multiple potential breeding partners, their lands aren't near as large (maybe forty square miles or smaller), just big enough to keep them and their offspring safe and well fed with all the deer, elk, coyotes, and/or bighorns they need, in addition to smaller prey. Female offspring who disperse have an easier time finding territories than their brothers and are less likely to disperse widely because of this fact.

To answer your question, predators self regulate their populations, often violently. In a stable ecosystem, there's enough of them to keep their prey in check, but due to factors of real estate, breeding rights, and interactions with other predatory species, they simply don't overpopulate.


What you are missing is that not all prey are equally easy to catch. The old, sick animals living in exposed places are much easier to catch than animals that are young, healthy and living in well-protected places. As the predator catches the easy meat, it becomes progressively harder and harder for the predator to get a meal.


If I missed seeing this in one of the other answers, I apologize, but I don't believe anyone has mentioned a very relevant fact about some predators that directly affects their populations at any given time. The fact is that wolves and probably other predators living in a pack-style grouping, allow only the alpha male and female to mate. This obviously severely limits the number of offspring born each year as well as leaving the pack vulnerable to catastrophe when something happens to either or both alphas. This mode of self-regulation is independent of food availability so works in years of abundance as well as famine.


Trophic cascade

Trophic cascades are powerful indirect interactions that can control entire ecosystems, occurring when a trophic level in a food web is suppressed. For example, a top-down cascade will occur if predators are effective enough in predation to reduce the abundance, or alter the behavior, of their prey, thereby releasing the next lower trophic level from predation (or herbivory if the intermediate trophic level is a herbivore).

The trophic cascade is an ecological concept which has stimulated new research in many areas of ecology. For example, it can be important for understanding the knock-on effects of removing top predators from food webs, as humans have done in many places through hunting and fishing.

A top-down cascade is a trophic cascade where the top consumer/predator controls the primary consumer population. In turn, the primary producer population thrives. The removal of the top predator can alter the food web dynamics. In this case, the primary consumers would overpopulate and exploit the primary producers. Eventually there would not be enough primary producers to sustain the consumer population. Top-down food web stability depends on competition and predation in the higher trophic levels. Invasive species can also alter this cascade by removing or becoming a top predator. This interaction may not always be negative. Studies have shown that certain invasive species have begun to shift cascades and as a consequence, ecosystem degradation has been repaired. [1] [2]

For example, if the abundance of large piscivorous fish is increased in a lake, the abundance of their prey, smaller fish that eat zooplankton, should decrease. The resulting increase in zooplankton should, in turn, cause the biomass of its prey, phytoplankton, to decrease.

In a bottom-up cascade, the population of primary producers will always control the increase/decrease of the energy in the higher trophic levels. Primary producers are plants, phytoplankton and zooplankton that require photosynthesis. Although light is important, primary producer populations are altered by the amount of nutrients in the system. This food web relies on the availability and limitation of resources. All populations will experience growth if there is initially a large amount of nutrients. [3] [4]

In a subsidy cascade, species populations at one trophic level can be supplemented by external food. For example, native animals can forage on resources that don't originate in their same habitat, such as native predators eating livestock. This may increase their local abundances thereby affecting other species in the ecosystem and causing an ecological cascade. For example, Luskin et al. (2017) found that native animals living in protected primary rainforest in Malaysia found food subsidies in neighboring oil palm plantations. [5] This subsidy allowed native animal populations to increase, which then triggered powerful secondary ‘cascading’ effects on forest tree community. Specifically, crop-raiding wild boar (Sus scrofa) built thousands of nests from the forest understory vegetation and this caused a 62% decline in forest tree sapling density over a 24-year study period. Such cross-boundary subsidy cascades may be widespread in both terrestrial and marine ecosystems and present significant conservation challenges.

These trophic interactions shape patterns of biodiversity globally. Humans and climate change have affected these cascades drastically. One example can be seen with sea otters (Enhydra lutris) on the Pacific coast of the United States of America. Over time, human interactions caused a removal of sea otters. One of their main prey, the pacific purple sea urchin (Strongylocentrotus purpuratus) eventually began to overpopulate. The overpopulation caused increased predation of giant kelp (Macrocystis pyrifera). As a result, there was extreme deterioration of the kelp forests along the California coast. This is why it is important for countries to regulate marine and terrestrial ecosystems. [6] [7]

Predator-induced interactions could heavily influence the flux of atmospheric carbon if managed on a global scale. For example, a study was conducted to determine the cost of potential stored carbon in living kelp biomass in sea otter (Enhydra lutris) enhanced ecosystems. The study valued the potential storage between $205 million and $408 million dollars (US) on the European Carbon Exchange (2012). [8]


Contents

The first experiments with predation and spatial heterogeneity were conducted by G. F. Gause in the 1930s, based on the Lotka–Volterra equation, which was formulated in the mid-1920s, but no further application had been conducted. [3] The Lotka-Volterra equation suggested that the relationship between predators and their prey would result in population oscillations over time based on the initial densities of predator and prey. Gause's early experiments to prove the predicted oscillations of this theory failed because the predator–prey interactions were not influenced by immigration. However, once immigration was introduced, the population cycles accurately depicted the oscillations predicted by the Lotka-Volterra equation, with the peaks in prey abundance shifted slightly to the left of the peaks of the predator densities. Huffaker's experiments expanded on those of Gause by examining how both the factors of migration and spatial heterogeneity lead to predator–prey oscillations.

In order to study predation and population oscillations, Huffaker used mite species, one being the predator and the other being the prey. [4] He set up a controlled experiment using oranges, which the prey fed on, as the spatially structured habitat in which the predator and prey would interact. [5] At first, Huffaker experienced difficulties similar to those of Gause in creating a stable predator–prey interaction. By using oranges only, the prey species quickly became extinct followed consequently with predator extinction. However, he discovered that by modifying the spatial structure of the habitat, he could manipulate the population dynamics and allow the overall survival rate for both species to increase. He did this by altering the distance between the prey and oranges (their food), establishing barriers to predator movement, and creating corridors for the prey to disperse. [3] These changes resulted in increased habitat patches and in turn provided more areas for the prey to seek temporary protection. When the prey would become extinct locally at one habitat patch, they were able to reestablish by migrating to new patches before being attacked by predators. This habitat spatial structure of patches allowed for coexistence between the predator and prey species and promoted a stable population oscillation model. [6] Although the term metapopulation had not yet been coined, the environmental factors of spatial heterogeneity and habitat patchiness would later describe the conditions of a metapopulation relating to how groups of spatially separated populations of species interact with one another. Huffaker's experiment is significant because it showed how metapopulations can directly affect the predator–prey interactions and in turn influence population dynamics. [7]

Levins' original model applied to a metapopulation distributed over many patches of suitable habitat with significantly less interaction between patches than within a patch. Population dynamics within a patch were simplified to the point where only presence and absence were considered. Each patch in his model is either populated or not.

Let N be the fraction of patches occupied at a given time. During a time dt, each occupied patch can become unoccupied with an extinction probability edt. Additionally, 1 − N of the patches are unoccupied. Assuming a constant rate c of propagule generation from each of the N occupied patches, during a time dt, each unoccupied patch can become occupied with a colonization probability cNdt . Accordingly, the time rate of change of occupied patches, dN/dt, is

This equation is mathematically equivalent to the logistic model, with a carrying capacity K given by

At equilibrium, therefore, some fraction of the species's habitat will always be unoccupied.

Huffaker's [4] studies of spatial structure and species interactions are an example of early experimentation in metapopulation dynamics. Since the experiments of Huffaker [4] and Levins, [1] models have been created which integrate stochastic factors. These models have shown that the combination of environmental variability (stochasticity) and relatively small migration rates cause indefinite or unpredictable persistence. However, Huffaker's experiment almost guaranteed infinite persistence because of the controlled immigration variable.

One major drawback of the Levins model is that it is deterministic, whereas the fundamental metapopulation processes are stochastic. Metapopulations are particularly useful when discussing species in disturbed habitats, and the viability of their populations, i.e., how likely they are to become extinct in a given time interval. The Levins model cannot address this issue. A simple way to extend the Levins' model to incorporate space and stochastic considerations is by using the contact process. Simple modifications to this model can also incorporate for patch dynamics. At a given percolation threshold, habitat fragmentation effects take place in these configurations predicting more drastic extinction thresholds. [8]

For conservation biology purposes, metapopulation models must include (a) the finite nature of metapopulations (how many patches are suitable for habitat), and (b) the probabilistic nature of extinction and colonisation. Also, note that in order to apply these models, the extinctions and colonisations of the patches must be asynchronous.

By combining nanotechnology with landscape ecology, a habitat landscape can be nanofabricated on-chip by building a collection of nanofabricated bacterial habitats, and connecting them by corridors in different topological arrangements and with nano-scale channels providing them with the local ecosystem service of habitat renewal. These landscapes of MHPs can be used as physical implementations of an adaptive landscape: [9] by generating a spatial mosaic of patches of opportunity distributed in space and time. The patchy nature of these fluidic landscapes allows for the study of adapting bacterial cells in a metapopulation system operating on-chip within a synthetic ecosystem. The metapopulation biology and evolutionary ecology of these bacterial systems, in these synthetic ecosystems, can be addressed using experimental biophysics.

Metapopulation models have been used to explain life-history evolution, such as the ecological stability of amphibian metamorphosis in small vernal ponds. Alternative ecological strategies have evolved. For example, some salamanders forgo metamorphosis and sexually mature as aquatic neotenes. The seasonal duration of wetlands and the migratory range of the species determines which ponds are connected and if they form a metapopulation. The duration of the life history stages of amphibians relative to the duration of the vernal pool before it dries up regulates the ecological development of metapopulations connecting aquatic patches to terrestrial patches. [10]


Effects of Overpopulation

  1. Environmental degradation
  2. Depletion of natural resources
  3. Conflicts
  4. Mortality
  5. Loss in biodiversity
  6. Increase in pollution
  7. Ecological collapse
  8. Water and food conflicts
  9. Overextensive farming
  10. Excessive use of fossil fuels
  11. Loss of arable land and desertification
  12. Antibiotic-resistant bacteria
  13. Increasing probability for epidemics
  14. Malnutrition and starvation
  15. Increasing costs of living and housing
  16. Rise in unemployment

Environmental degradation

Overpopulation has a vast adverse effect on the environmental system. Since we have to produce large amounts of food to provide for an increasing number of people, we have to use more land and resources to accomplish this. However, the use of more land comes with deforestation, which is especially severe in the Amazonian Rainforest.

This deforestation leads to a decline in the variety of species. Moreover, local tribes lose their homes and their whole culture. In addition, in order to produce more goods for our daily lives, a bigger amount of fossil fuels is needed. Burning fossil fuels lead to the emission of harmful chemicals and to acid rain.

Additionally, it contributes to the global warming issue. In addition, overpopulation is also likely to lead to an increase in noise pollution due to higher population density. Overpopulation can also contribute to the urban sprawl issue.

Depletion of natural resources

Another related problem of overpopulation is the depletion of natural resources. Our planet can only provide a limited amount of food and water.

When our population grows further, we will soon come to a point where we will no longer be able to supply the food and water necessary to feed all of them.

Even right now, people are starving every day. This problem will get even worse in the future due to overpopulation.

Conflicts

Another consequence of overpopulation is an increasing number of conflicts. Since people have to fight for their chunk of resources in order to survive, they are more willing to participate in conflicts to be able to provide for their families.

As population grows further, the probability of serious conflicts will do as well since a limited amount of resources has to be distributed to a growing number of people.

Mortality

The mortality rate, especially in poor countries, is likely to increase due to overpopulation. This relates to the limited capacity of resources of our planet. With an increasing number of people, everyone has to fight harder to get enough food. Those who are not able to do that will be left behind, which results in an increased mortality rate.

Loss in biodiversity

Population growth can also lead to a loss of biodiversity. As we need more space for farming or living, we destruct the natural environment of many species. This problem is especially severe in the Amazonian Rainforest. Due to farming and the related issues, many species go extinct each year.

Increase in pollution

Pollution is likely to increase due to overpopulation. This is quite logical since with a larger number of people on our planet, we have to produce more food and other things for daily life. This production of material goods leads to an increase in all sorts of pollution.

The air is polluted through the burning of fossil fuels. Our rivers, lakes and also the sea are polluted through illegal dumping and waste disposal.

Our groundwater is polluted through the excessive use of fertilizers and pesticides. Agricultural pollution results since there is an excessive need for food production and huge amounts of fertilizers and pesticides have to be used in order to meet the demand for food.

Moreover, overpopulation may also lead to an increase in light pollution due to a higher population density.

Ecological collapse

Overpopulation can also cause a kind of ecological collapse. The risk for such an event is especially severe if we look at our oceans. Since humans like some kinds of fish much more than others, this can lead to a severe ecological problem. Many of our preferred fishes consume large amounts of plankton.

If the number of these fishes is diminished, the plankton levels in our oceans will increase. This leads to a reduction in oxygen levels, which in turn can lead fishes to leave the affected areas. In the worst case, many sea animals will die from this development.

Water and food conflicts

Due to an increasing number of people who have basic needs, the likelihood of conflicts relating to water and food will increase dramatically. This may lead to states where people literally fight for water and other resources for their daily lives.

Overextensive farming

In order to meet the demand for food, farmers are likely to use an increasing amount of fertilizers and pesticides. This leads to soil pollution and also affects our groundwater in an adverse way since chemicals through the soil. Moreover, some animals will be contaminated by eating crops containing high levels of pesticides.

Excessive use of fossil fuels

Overpopulation is likely to lead to increased use of fossil fuels. Fossil fuels are used in many different occasions in our lives, for example for driving our cars or also for the production of other daily life products.

However, the use of fossil fuels is harmful to our environment since the gases produced in industrial processes contribute to global warming and also to air pollution.

Loss of arable land and desertification

If an area is used too extensively for farming or other purposes, it is likely to lose its fertility and thus this will lead to desertification. With an increasing number of people, the probability of desertification will increase since the available land will be used more extensively in order to provide bigger crop yields.

Antibiotic-resistant bacteria

With an increasing population, there is an increasing demand for meat. Intensive factory farming is used to be able to meet this demand. However, this increases the likelihood of the development of antibiotic-resistant bacteria which can have immense adverse effects on humans as well as on human-environmental interactions.

Increasing probability for epidemics

Another related topic is the increase in the likelihood of epidemics. A combination of overcrowded living conditions and malnutrition in conjunction with low hygienic standards can lead to severe epidemics or pandemics.

This problem will be especially severe for poor people in developing countries since they also lack the money to afford medical treatment if necessary.

Malnutrition and starvation

Due to distribution problems of wealth, not all people will be able to meet their daily energy supply. As a result, overpopulation will lead to death for many people, especially for the poorest among us.

Increasing costs of living and housing

With a limited amount of space to build houses and other infrastructure, a growing population is likely to lead to an increased cost of living. We already see this nowadays as in many cities, it is almost not possible for people with average salary jobs to live there. This problem will become even worse with overpopulation.

An increasing number of people will strive to live in the best places on earth, which will likely lead to an explosion of housing costs. This is also true for goods of our daily lives. Since all sorts of commodities are limited, increasing demand due to overpopulation will lead commodity prices to go up.

Rise in unemployment

The unemployment rate is likely to increase as a consequence of overpopulation. There are simply not that many jobs to employ all people.

Moreover, due to artificial intelligence, it is likely that the unemployment rate continues to increase since many tasks will be performed by machines instead of humans in the future.


Contents

At the most basic level, predators kill and eat other organisms. However, the concept of predation is broad, defined differently in different contexts, and includes a wide variety of feeding methods and some relationships that result in the prey's death are not generally called predation. A parasitoid, such as an ichneumon wasp, lays its eggs in or on its host the eggs hatch into larvae, which eat the host, and it inevitably dies. Zoologists generally call this a form of parasitism, though conventionally parasites are thought not to kill their hosts. A predator can be defined to differ from a parasitoid in that it has many prey, captured over its lifetime, where a parasitoid's larva has just one, or at least has its food supply provisioned for it on just one occasion. [1] [2]

There are other difficult and borderline cases. Micropredators are small animals that, like predators, feed entirely on other organisms they include fleas and mosquitoes that consume blood from living animals, and aphids that consume sap from living plants. However, since they typically do not kill their hosts, they are now often thought of as parasites. [3] [4] Animals that graze on phytoplankton or mats of microbes are predators, as they consume and kill their food organisms but herbivores that browse leaves are not, as their food plants usually survive the assault. [5] When animals eat seeds (seed predation or granivory) or eggs (egg predation), they are consuming entire living organisms, which by definition makes them predators. [6] [7] [8]

Scavengers, organisms that only eat organisms found already dead, are not predators, but many predators such as the jackal and the hyena scavenge when the opportunity arises. [9] [10] [5] Among invertebrates, social wasps (yellowjackets) are both hunters and scavengers of other insects. [11]

While examples of predators among mammals and birds are well known, [12] predators can be found in a broad range of taxa including arthropods. They are common among insects, including mantids, dragonflies, lacewings and scorpionflies. In some species such as the alderfly, only the larvae are predatory (the adults do not eat). Spiders are predatory, as well as other terrestrial invertebrates such as scorpions centipedes some mites, snails and slugs nematodes and planarian worms. [13] In marine environments, most cnidarians (e.g., jellyfish, hydroids), ctenophora (comb jellies), echinoderms (e.g., sea stars, sea urchins, sand dollars, and sea cucumbers) and flatworms are predatory. [14] Among crustaceans, lobsters, crabs, shrimps and barnacles are predators, [15] and in turn crustaceans are preyed on by nearly all cephalopods (including octopuses, squid and cuttlefish). [16]

Seed predation is restricted to mammals, birds, and insects but is found in almost all terrestrial ecosystems. [8] [6] Egg predation includes both specialist egg predators such as some colubrid snakes and generalists such as foxes and badgers that opportunistically take eggs when they find them. [17] [18] [19]

Some plants, like the pitcher plant, the Venus fly trap and the sundew, are carnivorous and consume insects. [12] Methods of predation by plants varies greatly but often involves a food trap, mechanical stimulation, and electrical impulses to eventually catch and consume its prey. [20] Some carnivorous fungi catch nematodes using either active traps in the form of constricting rings, or passive traps with adhesive structures. [21]

Many species of protozoa (eukaryotes) and bacteria (prokaryotes) prey on other microorganisms the feeding mode is evidently ancient, and evolved many times in both groups. [22] [12] [23] Among freshwater and marine zooplankton, whether single-celled or multi-cellular, predatory grazing on phytoplankton and smaller zooplankton is common, and found in many species of nanoflagellates, dinoflagellates, ciliates, rotifers, a diverse range of meroplankton animal larvae, and two groups of crustaceans, namely copepods and cladocerans. [24]

To feed, a predator must search for, pursue and kill its prey. These actions form a foraging cycle. [26] [27] The predator must decide where to look for prey based on its geographical distribution and once it has located prey, it must assess whether to pursue it or to wait for a better choice. If it chooses pursuit, its physical capabilities determine the mode of pursuit (e.g., ambush or chase). [28] [29] Having captured the prey, it may also need to expend energy handling it (e.g., killing it, removing any shell or spines, and ingesting it). [25] [26]

Search Edit

Predators have a choice of search modes ranging from sit-and-wait to active or widely foraging. [30] [25] [31] [32] The sit-and-wait method is most suitable if the prey are dense and mobile, and the predator has low energy requirements. [30] Wide foraging expends more energy, and is used when prey is sedentary or sparsely distributed. [28] [30] There is a continuum of search modes with intervals between periods of movement ranging from seconds to months. Sharks, sunfish, Insectivorous birds and shrews are almost always moving while web-building spiders, aquatic invertebrates, praying mantises and kestrels rarely move. In between, plovers and other shorebirds, freshwater fish including crappies, and the larvae of coccinellid beetles (ladybirds), alternate between actively searching and scanning the environment. [30]

Prey distributions are often clumped, and predators respond by looking for patches where prey is dense and then searching within patches. [25] Where food is found in patches, such as rare shoals of fish in a nearly empty ocean, the search stage requires the predator to travel for a substantial time, and to expend a significant amount of energy, to locate each food patch. [33] For example, the black-browed albatross regularly makes foraging flights to a range of around 700 kilometres (430 miles), up to a maximum foraging range of 3,000 kilometres (1,860 miles) for breeding birds gathering food for their young. [a] [34] With static prey, some predators can learn suitable patch locations and return to them at intervals to feed. [33] The optimal foraging strategy for search has been modelled using the marginal value theorem. [35]

Search patterns often appear random. One such is the Lévy walk, that tends to involve clusters of short steps with occasional long steps. It is a good fit to the behaviour of a wide variety of organisms including bacteria, honeybees, sharks and human hunter-gatherers. [36] [37]

Assessment Edit

Having found prey, a predator must decide whether to pursue it or keep searching. The decision depends on the costs and benefits involved. A bird foraging for insects spends a lot of time searching but capturing and eating them is quick and easy, so the efficient strategy for the bird is to eat every palatable insect it finds. By contrast, a predator such as a lion or falcon finds its prey easily but capturing it requires a lot of effort. In that case, the predator is more selective. [28]

One of the factors to consider is size. Prey that is too small may not be worth the trouble for the amount of energy it provides. Too large, and it may be too difficult to capture. For example, a mantid captures prey with its forelegs and they are optimized for grabbing prey of a certain size. Mantids are reluctant to attack prey that is far from that size. There is a positive correlation between the size of a predator and its prey. [28]

A predator may also assess a patch and decide whether to spend time searching for prey in it. [25] This may involve some knowledge of the preferences of the prey for example, ladybirds can choose a patch of vegetation suitable for their aphid prey. [38]

Capture Edit

To capture prey, predators have a spectrum of pursuit modes that range from overt chase (pursuit predation) to a sudden strike on nearby prey (ambush predation). [25] [39] [12] Another strategy in between ambush and pursuit is ballistic interception, where a predator observes and predicts a prey's motion and then launches its attack accordingly. [40]

Ambush Edit

Ambush or sit-and-wait predators are carnivorous animals that capture prey by stealth or surprise. In animals, ambush predation is characterized by the predator's scanning the environment from a concealed position until a prey is spotted, and then rapidly executing a fixed surprise attack. [41] [40] Vertebrate ambush predators include frogs, fish such as the angel shark, the northern pike and the eastern frogfish. [40] [42] [43] [44] Among the many invertebrate ambush predators are trapdoor spiders and Australian Crab spiders on land and mantis shrimps in the sea. [41] [45] [46] Ambush predators often construct a burrow in which to hide, improving concealment at the cost of reducing their field of vision. Some ambush predators also use lures to attract prey within striking range. [40] The capturing movement has to be rapid to trap the prey, given that the attack is not modifiable once launched. [40]

Ballistic interception Edit

Ballistic interception is the strategy where a predator observes the movement of a prey, predicts its motion, works out an interception path, and then attacks the prey on that path. This differs from ambush predation in that the predator adjusts its attack according to how the prey is moving. [40] Ballistic interception involves a brief period for planning, giving the prey an opportunity to escape. Some frogs wait until snakes have begun their strike before jumping, reducing the time available to the snake to recalibrate its attack, and maximising the angular adjustment that the snake would need to make to intercept the frog in real time. [40] Ballistic predators include insects such as dragonflies, and vertebrates such as archerfish (attacking with a jet of water), chameleons (attacking with their tongues), and some colubrid snakes. [40]

Pursuit Edit

In pursuit predation, predators chase fleeing prey. If the prey flees in a straight line, capture depends only on the predator's being faster than the prey. [40] If the prey manoeuvres by turning as it flees, the predator must react in real time to calculate and follow a new intercept path, such as by parallel navigation, as it closes on the prey. [40] Many pursuit predators use camouflage to approach the prey as close as possible unobserved (stalking) before starting the pursuit. [40] Pursuit predators include terrestrial mammals such as humans, African wild dogs, spotted hyenas and wolves marine predators such as dolphins, orcas and many predatory fishes, such as tuna [47] [48] predatory birds (raptors) such as falcons and insects such as dragonflies. [49]

An extreme form of pursuit is endurance or persistence hunting, in which the predator tires out the prey by following it over a long distance, sometimes for hours at a time. The method is used by human hunter-gatherers and by canids such as African wild dogs and domestic hounds. The African wild dog is an extreme persistence predator, tiring out individual prey by following them for many miles at relatively low speed. [50]

A specialised form of pursuit predation is the lunge feeding of baleen whales. These very large marine predators feed on plankton, especially krill, diving and actively swimming into concentrations of plankton, and then taking a huge gulp of water and filtering it through their feathery baleen plates. [51] [52]

Pursuit predators may be social, like the lion and wolf that hunt in groups, or solitary. [2]

Handling Edit

Once the predator has captured the prey, it has to handle it: very carefully if the prey is dangerous to eat, such as if it possesses sharp or poisonous spines, as in many prey fish. Some catfish such as the Ictaluridae have spines on the back (dorsal) and belly (pectoral) which lock in the erect position as the catfish thrashes about when captured, these could pierce the predator's mouth, possibly fatally. Some fish-eating birds like the osprey avoid the danger of spines by tearing up their prey before eating it. [53]

Solitary versus social predation Edit

In social predation, a group of predators cooperates to kill prey. This makes it possible to kill creatures larger than those they could overpower singly for example, hyenas, and wolves collaborate to catch and kill herbivores as large as buffalo, and lions even hunt elephants. [54] [55] [56] It can also make prey more readily available through strategies like flushing of prey and herding it into a smaller area. For example, when mixed flocks of birds forage, the birds in front flush out insects that are caught by the birds behind. Spinner dolphins form a circle around a school of fish and move inwards, concentrating the fish by a factor of 200. [57] By hunting socially chimpanzees can catch colobus monkeys that would readily escape an individual hunter, while cooperating Harris hawks can trap rabbits. [54] [58]

Predators of different species sometimes cooperate to catch prey. In coral reefs, when fish such as the grouper and coral trout spot prey that is inaccessible to them, they signal to giant moray eels, Napoleon wrasses or octopuses. These predators are able to access small crevices and flush out the prey. [59] [60] Killer whales have been known to help whalers hunt baleen whales. [61]

Social hunting allows predators to tackle a wider range of prey, but at the risk of competition for the captured food. Solitary predators have more chance of eating what they catch, at the price of increased expenditure of energy to catch it, and increased risk that the prey will escape. [62] [63] Ambush predators are often solitary to reduce the risk of becoming prey themselves. [64] Of 245 terrestrial carnivores, 177 are solitary and 35 of the 37 wild cats are solitary, [65] including the cougar and cheetah. [62] [2] However, the solitary cougar does allow other cougars to share in a kill, [66] and the coyote can be either solitary or social. [67] Other solitary predators include the northern pike, [68] wolf spiders and all the thousands of species of solitary wasps among arthropods, [69] [70] and many microorganisms and zooplankton. [22] [71]

Physical adaptations Edit

Under the pressure of natural selection, predators have evolved a variety of physical adaptations for detecting, catching, killing, and digesting prey. These include speed, agility, stealth, sharp senses, claws, teeth, filters, and suitable digestive systems. [72]

For detecting prey, predators have well-developed vision, smell, or hearing. [12] Predators as diverse as owls and jumping spiders have forward-facing eyes, providing accurate binocular vision over a relatively narrow field of view, whereas prey animals often have less acute all-round vision. Animals such as foxes can smell their prey even when it is concealed under 2 feet (60 cm) of snow or earth. Many predators have acute hearing, and some such as echolocating bats hunt exclusively by active or passive use of sound. [73]

Predators including big cats, birds of prey, and ants share powerful jaws, sharp teeth, or claws which they use to seize and kill their prey. Some predators such as snakes and fish-eating birds like herons and cormorants swallow their prey whole some snakes can unhinge their jaws to allow them to swallow large prey, while fish-eating birds have long spear-like beaks that they use to stab and grip fast-moving and slippery prey. [73] Fish and other predators have developed the ability to crush or open the armoured shells of molluscs. [74]

Many predators are powerfully built and can catch and kill animals larger than themselves this applies as much to small predators such as ants and shrews as to big and visibly muscular carnivores like the cougar and lion. [73] [2] [75]

Skull of brown bear has large pointed canines for killing prey, and self-sharpening carnassial teeth at rear for cutting flesh with a scissor-like action

Crab spider, an ambush predator with forward-facing eyes, catching another predator, a field digger wasp

Red-tailed hawk uses sharp hooked claws and beak to kill and tear up its prey

Specialist: a great blue heron with a speared fish

Indian python unhinges its jaw to swallow large prey like this chital

Diet and behaviour Edit

Predators are often highly specialized in their diet and hunting behaviour for example, the Eurasian lynx only hunts small ungulates. [76] Others such as leopards are more opportunistic generalists, preying on at least 100 species. [77] [78] The specialists may be highly adapted to capturing their preferred prey, whereas generalists may be better able to switch to other prey when a preferred target is scarce. When prey have a clumped (uneven) distribution, the optimal strategy for the predator is predicted to be more specialized as the prey are more conspicuous and can be found more quickly [79] this appears to be correct for predators of immobile prey, but is doubtful with mobile prey. [80]

In size-selective predation, predators select prey of a certain size. [81] Large prey may prove troublesome for a predator, while small prey might prove hard to find and in any case provide less of a reward. This has led to a correlation between the size of predators and their prey. Size may also act as a refuge for large prey. For example, adult elephants are relatively safe from predation by lions, but juveniles are vulnerable. [82]

Camouflage and mimicry Edit

Members of the cat family such as the snow leopard (treeless highlands), tiger (grassy plains, reed swamps), ocelot (forest), fishing cat (waterside thickets), and lion (open plains) are camouflaged with coloration and disruptive patterns suiting their habitats. [83]

In aggressive mimicry, certain predators, including insects and fishes, make use of coloration and behaviour to attract prey. Female Photuris fireflies, for example, copy the light signals of other species, thereby attracting male fireflies, which they capture and eat. [84] Flower mantises are ambush predators camouflaged as flowers, such as orchids, they attract prey and seize it when it is close enough. [85] Frogfishes are extremely well camouflaged, and actively lure their prey to approach using an esca, a bait on the end of a rod-like appendage on the head, which they wave gently to mimic a small animal, gulping the prey in an extremely rapid movement when it is within range. [86]

Venom Edit

Many smaller predators such as the box jellyfish use venom to subdue their prey, [87] and venom can also aid in digestion (as is the case for rattlesnakes and some spiders). [88] [89] The marbled sea snake that has adapted to egg predation has atrophied venom glands, and the gene for its three finger toxin contains a mutation (the deletion of two nucleotides) that inactives it. These changes are explained by the fact that its prey does not need to be subdued. [90]

Electric fields Edit

Several groups of predatory fish have the ability to detect, track, and sometimes, as in the electric ray, to incapacitate their prey by generating electric fields using electric organs. [91] [92] [93] The electric organ is derived from modified nerve or muscle tissue. [94]

Physiology Edit

Physiological adaptations to predation include the ability of predatory bacteria to digest the complex peptidoglycan polymer from the cell walls of the bacteria that they prey upon. [23] Carnivorous vertebrates of all five major classes (fishes, amphibians, reptiles, birds, and mammals) have lower relative rates of sugar to amino acid transport than either herbivores or omnivores, presumably because they acquire plenty of amino acids from the animal proteins in their diet. [95]

To counter predation, prey have a great variety of defences. They can try to avoid detection. They can detect predators and warn others of their presence. If detected, they can try to avoid being the target of an attack, for example, by signalling that a chase would be unprofitable or by forming groups. If they become a target, they can try to fend off the attack with defences such as armour, quills, unpalatability or mobbing and they can escape an attack in progress by startling the predator, shedding body parts such as tails, or simply fleeing. [96] [97] [12] [98]

Avoiding detection Edit

Prey can avoid detection by predators with morphological traits and coloration that make them hard to detect. They can also adopt behaviour that avoids predators by, for example, avoiding the times and places where predators forage. [99]

Misdirection Edit

Prey animals make use of a variety of mechanisms including camouflage and mimicry to misdirect the visual sensory mechanisms of predators, enabling the prey to remain unrecognized for long enough to give it an opportunity to escape. Camouflage delays recognition through coloration, shape, and pattern. [73] [100] Among the many mechanisms of camouflage are countershading [83] and disruptive coloration. [101] The resemblance can be to the biotic or non-living environment, such as a mantis resembling dead leaves, or to other organisms. In mimicry, an organism has a similar appearance to another species, as in drone flies (Eristalis), which resembles a bee, yet has no sting. [102]

Behavioural mechanisms Edit

Animals avoid predators with behavioural mechanisms such as changing their habitats (particularly when raising young), reducing their activity, foraging less and forgoing reproduction when they sense that predators are about. [103]

Eggs and nestlings are particularly vulnerable to predation, so birds take measures to protect their nests. [99] Where birds locate their nests can have a large effect on the frequency of predation. It is lowest for those such as woodpeckers that excavate their own nests and progressively higher for those on the ground, in canopies and in shrubs. [104] To compensate, shrub nesters must have more broods and shorter nesting times. Birds also choose appropriate habitat (e.g., thick foliage or islands) and avoid forest edges and small habitats. Similarly, some mammals raise their young in dens. [103]

By forming groups, prey can often reduce the frequency of encounters with predators because the visibility of a group does not rise in proportion to its size. However, there are exceptions: for example, human fishermen can only detect large shoals of fish with sonar. [105]

Detecting predators Edit

Recognition Edit

Prey species use sight, sound and odor to detect predators, and they can be quite discriminating. For example, Belding's ground squirrel can distinguish several aerial and ground predators from each other and from harmless species. Prey also distinguish between the calls of predators and non-predators. Some species can even distinguish between dangerous and harmless predators of the same species. In the northeastern Pacific Ocean, transient killer whales prey on seals, but the local killer whales only eat fish. Seals rapidly exit the water if they hear calls between transients. Prey are also more vigilant if they smell predators. [106]

The abilities of prey to detect predators do have limits. Belding's ground squirrel cannot distinguish between harriers flying at different heights, although only the low-flying birds are a threat. [106] Wading birds sometimes take flight when there does not appear to be any predator present. Although such false alarms waste energy and lose feeding time, it can be fatal to make the opposite mistake of taking a predator for a harmless animal. [107]

Vigilance Edit

Prey must remain vigilant, scanning their surroundings for predators. This makes it more difficult to feed and sleep. Groups can provide more eyes, making detection of a predator more likely and reducing the level of vigilance needed by individuals. [108] Many species, such as Eurasian jays, give alarm calls warning of the presence of a predator these give other prey of the same or different species an opportunity to escape, and signal to the predator that it has been detected. [109] [110]

Avoiding an attack Edit

Signalling unprofitability Edit

If predator and prey have spotted each other, the prey can signal to the predator to decrease the likelihood of an attack. These honest signals may benefit both the prey and predator, because they save the effort of a fruitless chase. [111] Signals that appear to deter attacks include stotting, for example by Thomson's gazelle [112] [111] push-up displays by lizards [111] and good singing by skylarks after a pursuit begins. [111] Simply indicating that the predator has been spotted, as a hare does by standing on its hind legs and facing the predator, may sometimes be sufficient. [111]

Many prey animals are aposematically coloured or patterned as a warning to predators that they are distasteful or able to defend themselves. [73] [113] [114] Such distastefulness or toxicity is brought about by chemical defences, found in a wide range of prey, especially insects, but the skunk is a dramatic mammalian example. [115]

Forming groups Edit

By forming groups, prey can reduce attacks by predators. There are several mechanisms that produce this effect. One is dilution, where, in the simplest scenario, if a given predator attacks a group of prey, the chances of a given individual being the target is reduced in proportion to the size of the group. However, it is difficult to separate this effect from other group-related benefits such as increased vigilance and reduced encounter rate. [116] [117] Other advantages include confusing predators such as with motion dazzle, making it more difficult to single out a target. [118] [119]

Fending off an attack Edit

Chemical defences include toxins, such as bitter compounds in leaves absorbed by leaf-eating insects, are used to dissuade potential predators. [120] Mechanical defences include sharp spines, hard shells and tough leathery skin or exoskeletons, all making prey harder to kill. [121]

Some species mob predators cooperatively, reducing the likelihood of attack. [122]

Escaping an attack Edit

When a predator is approaching an individual and attack seems imminent, the prey still has several options. One is to flee, whether by running, jumping, climbing, burrowing or swimming. [123] The prey can gain some time by startling the predator. Many butterflies and moths have eyespots, wing markings that resemble eyes. [124] When a predator disturbs the insect, it reveals its hind wings in a deimatic or bluffing display, startling the predator and giving the insect time to escape. [125] [126] Some other strategies include playing dead and uttering a distress call. [123]

Predators and prey are natural enemies, and many of their adaptations seem designed to counter each other. For example, bats have sophisticated echolocation systems to detect insects and other prey, and insects have developed a variety of defences including the ability to hear the echolocation calls. [127] [128] Many pursuit predators that run on land, such as wolves, have evolved long limbs in response to the increased speed of their prey. [129] Their adaptations have been characterized as an evolutionary arms race, an example of the coevolution of two species. [130] In a gene centered view of evolution, the genes of predator and prey can be thought of as competing for the prey's body. [130] However, the "life-dinner" principle of Dawkins and Krebs predicts that this arms race is asymmetric: if a predator fails to catch its prey, it loses its dinner, while if it succeeds, the prey loses its life. [130]

The metaphor of an arms race implies ever-escalating advances in attack and defence. However, these adaptations come with a cost for instance, longer legs have an increased risk of breaking, [131] while the specialized tongue of the chameleon, with its ability to act like a projectile, is useless for lapping water, so the chameleon must drink dew off vegetation. [132]

The "life-dinner" principle has been criticized on multiple grounds. The extent of the asymmetry in natural selection depends in part on the heritability of the adaptive traits. [132] Also, if a predator loses enough dinners, it too will lose its life. [131] [132] On the other hand, the fitness cost of a given lost dinner is unpredictable, as the predator may quickly find better prey. In addition, most predators are generalists, which reduces the impact of a given prey adaption on a predator. Since specialization is caused by predator-prey coevolution, the rarity of specialists may imply that predator-prey arms races are rare. [132]

It is difficult to determine whether given adaptations are truly the result of coevolution, where a prey adaptation gives rise to a predator adaptation that is countered by further adaptation in the prey. An alternative explanation is escalation, where predators are adapting to competitors, their own predators or dangerous prey. [133] Apparent adaptations to predation may also have arisen for other reasons and then been co-opted for attack or defence. In some of the insects preyed on by bats, hearing evolved before bats appeared and was used to hear signals used for territorial defence and mating. [134] Their hearing evolved in response to bat predation, but the only clear example of reciprocal adaptation in bats is stealth echolocation. [135]

A more symmetric arms race may occur when the prey are dangerous, having spines, quills, toxins or venom that can harm the predator. The predator can respond with avoidance, which in turn drives the evolution of mimicry. Avoidance is not necessarily an evolutionary response as it is generally learned from bad experiences with prey. However, when the prey is capable of killing the predator (as can a coral snake with its venom), there is no opportunity for learning and avoidance must be inherited. Predators can also respond to dangerous prey with counter-adaptations. In western North America, the common garter snake has developed a resistance to the toxin in the skin of the rough-skinned newt. [132]

Predators affect their ecosystems not only directly by eating their own prey, but by indirect means such as reducing predation by other species, or altering the foraging behaviour of a herbivore, as with the biodiversity effect of wolves on riverside vegetation or sea otters on kelp forests. This may explain population dynamics effects such as the cycles observed in lynx and snowshoe hares. [136] [137] [138]

Trophic level Edit

One way of classifying predators is by trophic level. Carnivores that feed on herbivores are secondary consumers their predators are tertiary consumers, and so forth. [139] At the top of this food chain are apex predators such as lions. [140] Many predators however eat from multiple levels of the food chain a carnivore may eat both secondary and tertiary consumers. [141] This means that many predators must contend with intraguild predation, where other predators kill and eat them. For example, coyotes compete with and sometimes kill gray foxes and bobcats. [142]

Biodiversity maintained by apex predation Edit

Predators may increase the biodiversity of communities by preventing a single species from becoming dominant. Such predators are known as keystone species and may have a profound influence on the balance of organisms in a particular ecosystem. [143] Introduction or removal of this predator, or changes in its population density, can have drastic cascading effects on the equilibrium of many other populations in the ecosystem. For example, grazers of a grassland may prevent a single dominant species from taking over. [144]

The elimination of wolves from Yellowstone National Park had profound impacts on the trophic pyramid. In that area, wolves are both keystone species and apex predators. Without predation, herbivores began to over-graze many woody browse species, affecting the area's plant populations. In addition, wolves often kept animals from grazing near streams, protecting the beavers' food sources. The removal of wolves had a direct effect on the beaver population, as their habitat became territory for grazing. Increased browsing on willows and conifers along Blacktail Creek due to a lack of predation caused channel incision because the reduced beaver population was no longer able to slow the water down and keep the soil in place. The predators were thus demonstrated to be of vital importance in the ecosystem. [145]

Population dynamics Edit

In the absence of predators, the population of a species can grow exponentially until it approaches the carrying capacity of the environment. [146] Predators limit the growth of prey both by consuming them and by changing their behavior. [147] Increases or decreases in the prey population can also lead to increases or decreases in the number of predators, for example, through an increase in the number of young they bear.

Cyclical fluctuations have been seen in populations of predator and prey, often with offsets between the predator and prey cycles. A well-known example is that of the snowshoe hare and lynx. Over a broad span of boreal forests in Alaska and Canada, the hare populations fluctuate in near synchrony with a 10-year period, and the lynx populations fluctuate in response. This was first seen in historical records of animals caught by fur hunters for the Hudson Bay Company over more than a century. [148] [138] [149] [150]

A simple model of a system with one species each of predator and prey, the Lotka–Volterra equations, predicts population cycles. [151] However, attempts to reproduce the predictions of this model in the laboratory have often failed for example, when the protozoan Didinium nasutum is added to a culture containing its prey, Paramecium caudatum, the latter is often driven to extinction. [152]

The Lotka-Volterra equations rely on several simplifying assumptions, and they are structurally unstable, meaning that any change in the equations can stabilize or destabilize the dynamics. [153] [154] For example, one assumption is that predators have a linear functional response to prey: the rate of kills increases in proportion to the rate of encounters. If this rate is limited by time spent handling each catch, then prey populations can reach densities above which predators cannot control them. [152] Another assumption is that all prey individuals are identical. In reality, predators tend to select young, weak, and ill individuals, leaving prey populations able to regrow. [155]

Many factors can stabilize predator and prey populations. [156] One example is the presence of multiple predators, particularly generalists that are attracted to a given prey species if it is abundant and look elsewhere if it is not. [157] As a result, population cycles tend to be found in northern temperate and subarctic ecosystems because the food webs are simpler. [158] The snowshoe hare-lynx system is subarctic, but even this involves other predators, including coyotes, goshawks and great horned owls, and the cycle is reinforced by variations in the food available to the hares. [159]

A range of mathematical models have been developed by relaxing the assumptions made in the Lotka-Volterra model these variously allow animals to have geographic distributions, or to migrate to have differences between individuals, such as sexes and an age structure, so that only some individuals reproduce to live in a varying environment, such as with changing seasons [160] [161] and analysing the interactions of more than just two species at once. Such models predict widely differing and often chaotic predator-prey population dynamics. [160] [162] The presence of refuge areas, where prey are safe from predators, may enable prey to maintain larger populations but may also destabilize the dynamics. [163] [164] [165] [166]

Predation dates from before the rise of commonly recognized carnivores by hundreds of millions (perhaps billions) of years. Predation has evolved repeatedly in different groups of organisms. [5] [167] The rise of eukaryotic cells at around 2.7 Gya, the rise of multicellular organisms at about 2 Gya, and the rise of mobile predators (around 600 Mya - 2 Gya, probably around 1 Gya) have all been attributed to early predatory behavior, and many very early remains show evidence of boreholes or other markings attributed to small predator species. [5] It likely triggered major evolutionary transitions including the arrival of cells, eukaryotes, sexual reproduction, multicellularity, increased size, mobility (including insect flight [168] ) and armoured shells and exoskeletons. [5]

The earliest predators were microbial organisms, which engulfed or grazed on others. Because the fossil record is poor, these first predators could date back anywhere between 1 and over 2.7 Gya (billion years ago). [5] Predation visibly became important shortly before the Cambrian period—around 550 million years ago —as evidenced by the almost simultaneous development of calcification in animals and algae, [169] and predation-avoiding burrowing. However, predators had been grazing on micro-organisms since at least 1,000 million years ago , [5] [170] [171] with evidence of selective (rather than random) predation from a similar time. [172]

The fossil record demonstrates a long history of interactions between predators and their prey from the Cambrian period onwards, showing for example that some predators drilled through the shells of bivalve and gastropod molluscs, while others ate these organisms by breaking their shells. [173] Among the Cambrian predators were invertebrates like the anomalocaridids with appendages suitable for grabbing prey, large compound eyes and jaws made of a hard material like that in the exoskeleton of an insect. [174] Some of the first fish to have jaws were the armoured and mainly predatory placoderms of the Silurian to Devonian periods, one of which, the 6 m (20 ft) Dunkleosteus, is considered the world's first vertebrate "superpredator", preying upon other predators. [175] [176] Insects developed the ability to fly in the Early Carboniferous or Late Devonian, enabling them among other things to escape from predators. [168] Among the largest predators that have ever lived were the theropod dinosaurs such as Tyrannosaurus from the Cretaceous period. They preyed upon herbivorous dinosaurs such as hadrosaurs, ceratopsians and ankylosaurs. [177]

The Cambrian substrate revolution saw life on the sea floor change from minimal burrowing (left) to a diverse burrowing fauna (right), probably to avoid new Cambrian predators.

Mouth of the anomalocaridid Laggania cambria, a Cambrian invertebrate, probably an apex predator

Dunkleosteus, a Devonian placoderm, perhaps the world's first vertebrate superpredator, reconstruction

Meganeura monyi, a predatory Carboniferous insect related to dragonflies, could fly to escape terrestrial predators. Its large size, with a wingspan of 65 cm (30 in), may reflect the lack of vertebrate aerial predators at that time.

Practical uses Edit

Humans, as omnivores, are to some extent predatory, [178] using weapons and tools to fish, [179] hunt and trap animals. [180] They also use other predatory species such as dogs, cormorants, [181] and falcons to catch prey for food or for sport. [182] Two mid-sized predators, dogs and cats, are the animals most often kept as pets in western societies. [183] [184] Human hunters, including the San of southern Africa, use persistence hunting, a form of pursuit predation where the pursuer may be slower than prey such as a kudu antelope over short distances, but follows it in the midday heat until it is exhausted, a pursuit that can take up to five hours. [185] [186]

In biological pest control, predators (and parasitoids) from a pest's natural range are introduced to control populations, at the risk of causing unforeseen problems. Natural predators, provided they do no harm to non-pest species, are an environmentally friendly and sustainable way of reducing damage to crops and an alternative to the use of chemical agents such as pesticides. [187]

Symbolic uses Edit

In film, the idea of the predator as a dangerous if humanoid enemy is used in the 1987 science fiction horror action film Predator and its three sequels. [188] [189] A terrifying predator, a gigantic man-eating great white shark, is central, too, to Steven Spielberg's 1974 thriller Jaws. [190]

Among poetry on the theme of predation, a predator's consciousness might be explored, such as in Ted Hughes's Pike. [191] The phrase "Nature, red in tooth and claw" from Alfred, Lord Tennyson's 1849 poem "In Memoriam A.H.H." has been interpreted as referring to the struggle between predators and prey. [192]

In mythology and folk fable, predators such as the fox and wolf have mixed reputations. [193] The fox was a symbol of fertility in ancient Greece, but a weather demon in northern Europe, and a creature of the devil in early Christianity the fox is presented as sly, greedy, and cunning in fables from Aesop onwards. [193] The big bad wolf is known to children in tales such as Little Red Riding Hood, but is a demonic figure in the Icelandic Edda sagas, where the wolf Fenrir appears in the apocalyptic ending of the world. [193] In the Middle Ages, belief spread in werewolves, men transformed into wolves. [193] In ancient Rome, and in ancient Egypt, the wolf was worshipped, the she-wolf appearing in the founding myth of Rome, suckling Romulus and Remus. [193] More recently, in Rudyard Kipling's 1894 The Jungle Book, Mowgli is raised by the wolf pack. [193] Attitudes to large predators in North America, such as wolf, grizzly bear and cougar, have shifted from hostility or ambivalence, accompanied by active persecution, towards positive and protective in the second half of the 20th century. [194]

  1. ^ A range of 3000 kilometres means a flight distance of at least 6000 kilometres out and back.
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Contents

Staying out of sight Edit

Animals may avoid becoming prey by living out of sight of predators, whether in caves, burrows, or by being nocturnal. [2] [3] [4] [5] Nocturnality is an animal behavior characterized by activity during the night and sleeping during the day. This is a behavioral form of detection avoidance called crypsis used by animals to either avoid predation or to enhance prey hunting. Predation risk has long been recognized as critical in shaping behavioral decisions. For example, this predation risk is of prime importance in determining the time of evening emergence in echolocating bats. Although early access during brighter times permits easier foraging, it also leads to a higher predation risk from bat hawks and bat falcons. This results in an optimum evening emergence time that is a compromise between the conflicting demands. [4]

Another nocturnal adaptation can be seen in kangaroo rats. They forage in relatively open habitats, and reduce their activity outside their nest burrows in response to moonlight. During a full moon, they shift their activity towards areas of relatively dense cover to compensate for the extra brightness. [5]

Camouflage Edit

Camouflage uses any combination of materials, coloration, or illumination for concealment to make the organism hard to detect by sight. It is common in both terrestrial and marine animals. Camouflage can be achieved in many different ways, such as through resemblance to surroundings, disruptive coloration, shadow elimination by countershading or counter-illumination, self-decoration, cryptic behavior, or changeable skin patterns and colour. [6] [7] Animals such as the flat-tail horned lizard of North America have evolved to eliminate their shadow and blend in with the ground. The bodies of these lizards are flattened, and their sides thin towards the edge. This body form, along with the white scales fringed along their sides, allows the lizards to effectively hide their shadows. In addition, these lizards hide any remaining shadows by pressing their bodies to the ground. [2]

Masquerade Edit

Animals can hide in plain sight by masquerading as inedible objects. For example, the potoo, a South American bird, habitually perches on a tree, convincingly resembling a broken stump of a branch, [8] while a butterfly, Kallima, looks just like a dead leaf. [9]

Apostatic selection Edit

Another way to remain unattacked in plain sight is to look different from other members of the same species. Predators such as tits selectively hunt for abundant types of insect, ignoring less common types that were present, forming search images of the desired prey. This creates a mechanism for negative frequency-dependent selection, apostatic selection. [10]

Many species make use of behavioral strategies to deter predators. [11]

Startling the predator Edit

Many weakly-defended animals, including moths, butterflies, mantises, phasmids, and cephalopods such as octopuses, make use of patterns of threatening or startling behaviour, such as suddenly displaying conspicuous eyespots, so as to scare off or momentarily distract a predator, thus giving the prey animal an opportunity to escape. In the absence of toxins or other defences, this is essentially bluffing, in contrast to aposematism which involves honest signals. [12] [13] [14]

Pursuit-deterrent signals Edit

Pursuit-deterrent signals are behavioral signals used by prey that convince predators not to pursue them. For example, gazelles stot, jumping high with stiff legs and an arched back. This is thought to signal to predators that they have a high level of fitness and can outrun the predator. As a result, predators may choose to pursue a different prey that is less likely to outrun them. [15] White-tailed deer and other prey mammals flag with conspicuous (often black and white) tail markings when alarmed, informing the predator that it has been detected. [16] Warning calls given by birds such as the Eurasian jay are similarly honest signals, benefiting both predator and prey: the predator is informed that it has been detected and might as well save time and energy by giving up the chase, while the prey is protected from attack. [17] [18]

Playing dead Edit

Another pursuit-deterrent signal is thanatosis or playing dead. Thanatosis is a form of bluff in which an animal mimics its own dead body, feigning death to avoid being attacked by predators seeking live prey. Thanatosis can also be used by the predator in order to lure prey into approaching. [19] An example of this is seen in white-tailed deer fawns, which experience a drop in heart rate in response to approaching predators. This response, referred to as "alarm bradycardia", causes the fawn's heart rate to drop from 155 to 38 beats per minute within one beat of the heart. This drop in heart rate can last up to two minutes, causing the fawn to experience a depressed breathing rate and decrease in movement, called tonic immobility. Tonic immobility is a reflex response that causes the fawn to enter a low body position that simulates the position of a dead corpse. Upon discovery of the fawn, the predator loses interest in the "dead" prey. Other symptoms of alarm bradycardia, such as salivation, urination, and defecation, can also cause the predator to lose interest. [20]

Distraction Edit

Marine molluscs such as sea hares, cuttlefish, squid and octopuses give themselves a last chance to escape by distracting their attackers. To do this, they eject a mixture of chemicals, which may mimic food or otherwise confuse predators. [21] [22] In response to a predator, animals in these groups release ink, creating a cloud, and opaline, affecting the predator's feeding senses, causing it to attack the cloud. [21] [23]

Distraction displays attract the attention of predators away from an object, typically the nest or young, that is being protected. [24] Distraction displays are performed by some species of birds, which may feign a broken wing while hopping about on the ground, and by some species of fish. [25]

Mimicry and aposematism Edit

Mimicry occurs when an organism (the mimic) simulates signal properties of another organism (the model) to confuse a third organism. This results in the mimic gaining protection, food, and mating advantages. [26] There are two classical types of defensive mimicry: Batesian and Müllerian. Both involve aposematic coloration, or warning signals, to avoid being attacked by a predator. [27] [28]

In Batesian mimicry, a palatable, harmless prey species mimics the appearance of another species that is noxious to predators, thus reducing the mimic's risk of attack. [27] This form of mimicry is seen in many insects. The idea behind Batesian mimicry is that predators that have tried to eat the unpalatable species learn to associate its colors and markings with an unpleasant taste. This results in the predator learning to avoid species displaying similar colours and markings, including Batesian mimics, which are in effect parasitic on the chemical or other defences of the unprofitable models. [29] [30] Some species of octopus can mimic a selection of other animals by changing their skin color, skin pattern and body motion. When a damselfish attacks an octopus, the octopus mimics a banded sea-snake. [31] The model chosen varies with the octopus's predator and habitat. [32] Most of these octopuses use Batesian mimicry, selecting an organism repulsive to predators as a model. [33] [34]

In Müllerian mimicry, two or more aposematic forms share the same warning signals, [27] [35] as in viceroy and monarch butterflies. Birds avoid eating both species because their wing patterns honestly signal their unpleasant taste. [28]

Defensive structures Edit

Many animals are protected against predators with armour in the form of hard shells (such as most molluscs), leathery or scaly skin (as in reptiles), or tough chitinous exoskeletons (as in arthropods). [25]

A spine is a sharp, needle-like structure used to inflict pain on predators. An example of this seen in nature is in the Sohal surgeonfish. These fish have a sharp scalpel-like spine on the front of each of their tail fins, able to inflict deep wounds. The area around the spines is often brightly colored to advertise the defensive capability [36] predators often avoid the Sohal surgeonfish. [37] Defensive spines may be detachable, barbed or poisonous. Porcupine spines are long, stiff, break at the tip, and are barbed to stick into a would-be predator. In contrast, the hedgehog's short spines, which are modified hairs, [38] readily bend, and are barbed into the body, so they are not easily lost they may be jabbed at an attacker. [37]

Many species of slug caterpillar, Limacodidae, have numerous protuberances and stinging spines along their dorsal surfaces. Species that possess these stinging spines suffer less predation than larvae that lack them, and a predator, the paper wasp, chooses larvae without spines when given a choice. [39]

Group living can decrease the risk of predation to the individual in a variety of ways, [40] as described below.

Dilution effect Edit

A dilution effect is seen when animals living in a group "dilute" their risk of attack, each individual being just one of many in the group. George C. Williams and W.D. Hamilton proposed that group living evolved because it provides benefits to the individual rather than to the group as a whole, which becomes more conspicuous as it becomes larger. One common example is the shoaling of fish. Experiments provide direct evidence for the decrease in individual attack rate seen with group living, for example in Camargue horses in Southern France. The horse-fly often attacks these horses, sucking blood and carrying diseases. When the flies are most numerous, the horses gather in large groups, and individuals are indeed attacked less frequently. [41] Water striders are insects that live on the surface of fresh water, and are attacked from beneath by predatory fish. Experiments varying the group size of the water striders showed that the attack rate per individual water strider decreases as group size increases. [42]

Selfish herd Edit

The selfish herd theory was proposed by W.D. Hamilton to explain why animals seek central positions in a group. [43] The theory's central idea is to reduce the individual's domain of danger. A domain of danger is the area within the group in which the individual is more likely to be attacked by a predator. The center of the group has the lowest domain of danger, so animals are predicted to strive constantly to gain this position. Testing Hamilton's selfish herd effect, Alta De Vos and Justin O'Rainn (2010) studied brown fur seal predation from great white sharks. Using decoy seals, the researchers varied the distance between the decoys to produce different domains of danger. The seals with a greater domain of danger had an increased risk of shark attack. [44]

Predator satiation Edit

A radical strategy for avoiding predators which may otherwise kill a large majority of the emerging stage of a population is to emerge very rarely, at irregular intervals. Predators with a life-cycle of one or a few years are unable to reproduce rapidly enough in response to such an emergence. Predators may feast on the emerging population, but are unable to consume more than a fraction of the brief surfeit of prey. Periodical cicadas, which emerge at intervals of 13 or 17 years, are often used as an example of this predator satiation, though other explanations of their unusual life-cycle have been proposed. [45]

Alarm calls Edit

Animals that live in groups often give alarm calls that give warning of an attack. For example, vervet monkeys give different calls depending on the nature of the attack: for an eagle, a disyllabic cough for a leopard or other cat, a loud bark for a python or other snake, a "chutter". The monkeys hearing these calls respond defensively, but differently in each case: to the eagle call, they look up and run into cover to the leopard call, they run up into the trees to the snake call, they stand on two legs and look around for snakes, and on seeing the snake, they sometimes mob it. Similar calls are found in other species of monkey, while birds also give different calls that elicit different responses. [46]

Improved vigilance Edit

In the improved vigilance effect, groups are able to detect predators sooner than solitary individuals. [47] For many predators, success depends on surprise. If the prey is alerted early in an attack, they have an improved chance of escape. For example, wood pigeon flocks are preyed upon by goshawks. Goshawks are less successful when attacking larger flocks of wood pigeons than they are when attacking smaller flocks. This is because the larger the flock size, the more likely it is that one bird will notice the hawk sooner and fly away. Once one pigeon flies off in alarm, the rest of the pigeons follow. [48] Wild ostriches in Tsavo National Park in Kenya feed either alone or in groups of up to four birds. They are subject to predation by lions. As the ostrich group size increases, the frequency at which each individual raises its head to look for predators decreases. Because ostriches are able to run at speeds that exceed those of lions for great distances, lions try to attack an ostrich when its head is down. By grouping, the ostriches present the lions with greater difficulty in determining how long the ostriches' heads stay down. Thus, although individual vigilance decreases, the overall vigilance of the group increases. [49]

Predator confusion Edit

Individuals living in large groups may be safer from attack because the predator may be confused by the large group size. As the group moves, the predator has greater difficulty targeting an individual prey animal. The zebra has been suggested by the zoologist Martin Stevens and his colleagues as an example of this. When stationary, a single zebra stands out because of its large size. To reduce the risk of attack, zebras often travel in herds. The striped patterns of all the zebras in the herd may confuse the predator, making it harder for the predator to focus in on an individual zebra. Furthermore, when moving rapidly, the zebra stripes create a confusing, flickering motion dazzle effect in the eye of the predator. [50]

Defensive structures such as spines may be used both to ward off attack as already mentioned, and if need be to fight back against a predator. [37] Methods of fighting back include chemical defences, [51] mobbing, [52] defensive regurgitation, [53] and suicidal altruism. [54]

Chemical defences Edit

Many prey animals, and to defend against seed predation also seeds of plants, [55] make use of poisonous chemicals for self-defence. [51] [56] These may be concentrated in surface structures such as spines or glands, giving an attacker a taste of the chemicals before it actually bites or swallows the prey animal: many toxins are bitter-tasting. [51] A last-ditch defence is for the animal's flesh itself to be toxic, as in the puffer fish, danaid butterflies and burnet moths. Many insects acquire toxins from their food plants Danaus caterpillars accumulate toxic cardenolides from milkweeds (Asclepiadaceae). [56]

Some prey animals are able to eject noxious materials to deter predators actively. The bombardier beetle has specialized glands on the tip of its abdomen that allows it to direct a toxic spray towards predators. The spray is generated explosively through oxidation of hydroquinones and is sprayed at a temperature of 100 °C. [57] Armoured crickets similarly release blood at their joints when threatened (autohaemorrhaging). [58] Several species of grasshopper including Poecilocerus pictus, [59] Parasanaa donovani, [59] Aularches miliaris, [59] and Tegra novaehollandiae secrete noxious liquids when threatened, sometimes ejecting these forcefully. [59] Spitting cobras accurately squirt venom from their fangs at the eyes of potential predators, [60] striking their target eight times out of ten, and causing severe pain. [61] Termite soldiers in the Nasutitermitinae have a fontanellar gun, a gland on the front of their head which can secrete and shoot an accurate jet of resinous terpenes "many centimeters". The material is sticky and toxic to other insects. One of the terpenes in the secretion, pinene, functions as an alarm pheromone. [62] Seeds deter predation with combinations of toxic non-protein amino acids, cyanogenic glycosides, protease and amylase inhibitors, and phytohemaglutinins. [55]

A few vertebrate species such as the Texas horned lizard are able to shoot squirts of blood from their eyes, by rapidly increasing the blood pressure within the eye sockets, if threatened. Because an individual may lose up to 53% of blood in a single squirt, [63] this is only used against persistent predators like foxes, wolves and coyotes (Canidae), as a last defence. [64] Canids often drop horned lizards after being squirted, and attempt to wipe or shake the blood out of their mouths, suggesting that the fluid has a foul taste [65] they choose other lizards if given the choice, [66] suggesting a learned aversion towards horned lizards as prey. [66]

The slime glands along the body of the hagfish secrete enormous amounts of mucus when it is provoked or stressed. The gelatinous slime has dramatic effects on the flow and viscosity of water, rapidly clogging the gills of any fish that attempt to capture hagfish predators typically release the hagfish within seconds (pictured above). Common predators of hagfish include seabirds, pinnipeds and cetaceans, but few fish, suggesting that predatory fish avoid hagfish as prey. [67]

Communal defence Edit

In communal defence, prey groups actively defend themselves by grouping together, and sometimes by attacking or mobbing a predator, rather than allowing themselves to be passive victims of predation. Mobbing is the harassing of a predator by many prey animals. Mobbing is usually done to protect the young in social colonies. For example, red colobus monkeys exhibit mobbing when threatened by chimpanzees, a common predator. The male red colobus monkeys group together and place themselves between predators and the group's females and juveniles. The males jump together and actively bite the chimpanzees. [52] Fieldfares are birds which may nest either solitarily or in colonies. Within colonies, fieldfares mob and defecate on approaching predators, shown experimentally to reduce predation levels. [68]

Defensive regurgitation Edit

Some birds and insects use defensive regurgitation to ward off predators. The northern fulmar vomits a bright orange, oily substance called stomach oil when threatened. [53] The stomach oil is made from their aquatic diets. It causes the predator's feathers to mat, leading to the loss of flying ability and the loss of water repellency. [53] This is especially dangerous for aquatic birds because their water repellent feathers protect them from hypothermia when diving for food. [53]

European roller chicks vomit a bright orange, foul smelling liquid when they sense danger. This repels prospective predators and may alert their parents to danger: they respond by delaying their return. [69]

Numerous insects utilize defensive regurgitation. The eastern tent caterpillar regurgitates a droplet of digestive fluid to repel attacking ants. [70] Similarly, larvae of the noctuid moth regurgitate when disturbed by ants. The vomit of noctuid moths has repellent and irritant properties that help to deter predator attacks. [71]

Suicidal altruism Edit

An unusual type of predator deterrence is observed in the Malaysian exploding ant. Social hymenoptera rely on altruism to protect the entire colony, so the self-destructive acts benefit all individuals in the colony. [54] When a worker ant's leg is grasped, it suicidally expels the contents of its hypertrophied submandibular glands, [54] expelling corrosive irritant compounds and adhesives onto the predator. These prevent predation and serve as a signal to other enemy ants to stop predation of the rest of the colony. [72]

Flight Edit

The normal reaction of a prey animal to an attacking predator is to flee by any available means, whether flying, gliding, [73] falling, swimming, running, jumping, burrowing [74] or rolling, [75] according to the animal's capabilities. [76] Escape paths are often erratic, making it difficult for the predator to predict which way the prey will go next: for example, birds such as snipe, ptarmigan and black-headed gulls evade fast raptors such as peregrine falcons with zigzagging or jinking flight. [76] In the tropical rain forests of Southeast Asia in particular, many vertebrates escape predators by falling and gliding. [73] Among the insects, many moths turn sharply, fall, or perform a powered dive in response to the sonar clicks of bats. [76] Among fish, the stickleback follows a zigzagging path, often doubling back erratically, when chased by a fish-eating merganser duck. [76]

Autotomy Edit

Some animals are capable of autotomy (self-amputation), shedding one of their own appendages in a last-ditch attempt to elude a predator's grasp or to distract the predator and thereby allow escape. The lost body part may be regenerated later. Certain sea slugs discard stinging papillae arthropods such as crabs can sacrifice a claw, which can be regrown over several successive moults among vertebrates, many geckos and other lizards shed their tails when attacked: the tail goes on writhing for a while, distracting the predator, and giving the lizard time to escape a smaller tail slowly regrows. [77]

Aristotle recorded observations (around 350 BC) of the antipredator behaviour of cephalopods in his History of Animals, including the use of ink as a distraction, camouflage, and signalling. [78]

In 1940, Hugh Cott wrote a compendious study of camouflage, mimicry, and aposematism, Adaptive Coloration in Animals. [6]

By the 21st century, adaptation to life in cities had markedly reduced the antipredator responses of animals such as rats and pigeons similar changes are observed in captive and domesticated animals. [79]


Contents

The competitive exclusion principle is classically attributed to Georgyii Gause, [3] although he actually never formulated it. [1] The principle is already present in Darwin's theory of natural selection. [2] [4]

Throughout its history, the status of the principle has oscillated between a priori ('two species coexisting must have different niches') and experimental truth ('we find that species coexisting do have different niches'). [2]

Based on field observations, Joseph Grinnell formulated the principle of competitive exclusion in 1904: "Two species of approximately the same food habits are not likely to remain long evenly balanced in numbers in the same region. One will crowd out the other". [5] Georgy Gause formulated the law of competitive exclusion based on laboratory competition experiments using two species of Paramecium, P. aurelia and P. caudatum. The conditions were to add fresh water every day and input a constant flow of food. Although P. caudatum initially dominated, P. aurelia recovered and subsequently drove P. caudatum extinct via exploitative resource competition. However, Gause was able to let the P. caudatum survive by differing the environmental parameters (food, water). Thus, Gause's law is valid only if the ecological factors are constant.

Gause also studied competition between two species of yeast, finding that Saccharomyces cerevisiae consistently outcompeted Schizosaccharomyces kefir [ clarification needed ] by producing a higher concentration of ethyl alcohol. [6]

Competitive exclusion is predicted by mathematical and theoretical models such as the Lotka–Volterra models of competition. However, for poorly understood reasons, competitive exclusion is rarely observed in natural ecosystems, and many biological communities appear to violate Gause's law. The best-known example is the so-called "paradox of the plankton". [7] All plankton species live on a very limited number of resources, primarily solar energy and minerals dissolved in the water. According to the competitive exclusion principle, only a small number of plankton species should be able to coexist on these resources. Nevertheless, large numbers of plankton species coexist within small regions of open sea.

Some communities that appear to uphold the competitive exclusion principle are MacArthur's warblers [8] and Darwin's finches, [9] though the latter still overlap ecologically very strongly, being only affected negatively by competition under extreme conditions. [10]

A partial solution to the paradox lies in raising the dimensionality of the system. Spatial heterogeneity, trophic interactions, multiple resource competition, competition-colonization trade-offs, and lag may prevent exclusion (ignoring stochastic extinction over longer time-frames). However, such systems tend to be analytically intractable. In addition, many can, in theory, support an unlimited number of species. A new paradox is created: Most well-known models that allow for stable coexistence allow for unlimited number of species to coexist, yet, in nature, any community contains just a handful of species.

Recent studies addressing some of the assumptions made for the models predicting competitive exclusion have shown these assumptions need to be reconsidered. For example, a slight modification of the assumption of how growth and body size are related leads to a different conclusion, namely that, for a given ecosystem, a certain range of species may coexist while others become outcompeted. [11] [12]

One of the primary ways niche-sharing species can coexist is the competition-colonization trade-off. In other words, species that are better competitors will be specialists, whereas species that are better colonizers are more likely to be generalists. Host-parasite models are effective ways of examining this relationship, using host transfer events. There seem to be two places where the ability to colonize differs in ecologically closely related species. In feather lice, Bush and Clayton [13] provided some verification of this by showing two closely related genera of lice are nearly equal in their ability to colonize new host pigeons once transferred. Harbison [14] continued this line of thought by investigating whether the two genera differed in their ability to transfer. This research focused primarily on determining how colonization occurs and why wing lice are better colonizers than body lice. Vertical transfer is the most common occurrence, between parent and offspring, and is much-studied and well understood. Horizontal transfer is difficult to measure, but in lice seems to occur via phoresis or the "hitchhiking" of one species on another. Harbison found that body lice are less adept at phoresis and excel competitively, whereas wing lice excel in colonization.

An ecological community is the assembly of species which is maintained by ecological (Hutchinson, 1959 [15] Leibold, 1988 [16] ) and evolutionary process (Weiher and Keddy, 1995 [17] Chase et al., 2003). These two processes play an important role in shaping the existing community and will continue in the future (Tofts et al., 2000 Ackerly, 2003 Reich et al., 2003). In a local community, the potential members are filtered first by environmental factors such as temperature or availability of required resources and then secondly by its ability to co-exist with other resident species.

In an approach of understanding how two species fit together in a community or how the whole community fits together, The Origin of Species (Darwin, 1859) proposed that under homogeneous environmental condition struggle for existence is greater between closely related species than distantly related species. He also hypothesized that the functional traits may be conserved across phylogenies. Such strong phylogenetic similarities among closely related species are known as phylogenetic effects (Derrickson et al., 1988. [18] )

With field study and mathematical models, ecologist have pieced together a connection between functional traits similarity between species and its effect on species co-existence. According to competitive-relatedness hypothesis (Cahil et al., 2008 [19] ) or phylogenetic limiting similarity hypothesis (Violle et al., 2011 [20] ) interspecific competition [21] is high among the species which have similar functional traits, and which compete for similar resources and habitats. Hence, it causes reduction in the number of closely related species and even distribution of it, known as phylogenetic overdispersion (Webb et al., 2002 [22] ). The reverse of phylogenetic overdispersion is phylogenetic clustering in which case species with conserved functional traits are expected to co-occur due to environmental filtering (Weiher et al.,1 995 Webb, 2000). In the study performed by Webb et al., 2000, they showed that a small-plots of Borneo forest contained closely related trees together. This suggests that closely related species share features that are favored by the specific environmental factors that differ among plots causing phylogenetic clustering.

For both phylogenetic patterns (phylogenetic overdispersion and phylogenetic clustering), the baseline assumption is that phylogenetically related species are also ecologically similar (H. Burns et al, 2011 [23] ). There are no significant number of experiments answering to what degree the closely related species are also similar in niche. Due to that, both phylogenetic patterns are not easy to interpret. It’s been shown that phylogenetic overdispersion may also result from convergence of distantly related species (Cavender-Bares et al. 2004 [24] Kraft et al. 2007 [25] ). In their study, they have shown that traits are convergent rather than conserved. While, in another study, it’s been shown that phylogenetic clustering may also be due to historical or bio-geographical factors which prevents species from leaving their ancestral ranges. So, more phylogenetic experiments are required for understanding the strength of species interaction in community assembly.

Evidence showing that the competitive exclusion principle operates in human groups has been reviewed and integrated into regality theory to explain warlike and peaceful societies. [26] For example, hunter-gatherer groups surrounded by other hunter-gatherer groups in the same ecological niche will fight, at least occasionally, while hunter-gatherer groups surrounded by groups with a different means of subsistence can coexist peacefully. [26]


Predator-induced behaviour shifts and natural selection in field-experimental lizard populations

The role of behaviour in evolutionary change has long been debated. On the one hand, behavioural changes may expose individuals to new selective pressures by altering the way that organisms interact with the environment, thus driving evolutionary divergence. Alternatively, behaviour can act to retard evolutionary change: by altering behavioural patterns in the face of new environmental conditions, organisms can minimize exposure to new selective pressures. This constraining influence of behaviour has been put forward as an explanation for evolutionary stasis within lineages and niche conservatism within clades. Nonetheless, the hypothesis that behavioural change prevents natural selection from operating in new environments has never been experimentally tested. We conducted a controlled and replicated experimental study of selection in entirely natural populations we demonstrate that lizards alter their habitat use in the presence of an introduced predator, but that these behavioural shifts do not prevent patterns of natural selection from changing in experimental populations.


Killing off wild predators is a stupid idea

The headline looked like a joke: "To truly end animal suffering, the most ethical choice is to kill wild predators (especially Cecil the lion)." Instead, it was an apparently serious opinion piece published earlier today on the news site Quartz. Walter Palmer — the infamous dentist who shot and killed Cecil the Lion — actually did the world a favor, the article says, since Cecil would have killed many more animals before he died. The authors argue that humans should hunt and kill predators in order to save prey animals from dying horrible deaths.

Several readers of the Quartz piece assumed it was satire. But Quartz's Editor-In-Chief, Kevin Delaney, told us in a Twitter direct message that it's "not satire it’s an ideas piece on the issue." (The authors, Amanda MacAskill and William MacAskill, are — respectively — a philosophy Ph.D. student at New York University and an associate professor of philosophy at Lincoln College, part of Oxford University.) It's too bad they're serious the arguments put forth are ridiculous. The piece is riddled with generalizations, faulty analogies, and propositions that have no evidence to support them. It is an embarrassment to ethicists, who typically have higher standards for public arguments.

"When I read the article in Quartz, I had to laugh."

"When I read the article in Quartz, I had to laugh, because I thought the article was written as a straw man to illustrate the many potential ethical points of view you can take regarding killing of top predators," Karl Cottenie, an associate professor of ecology at the University of Guelph, told The Verge. "The arguments made in the article seems relatively far-fetched."

The authors describe predators as murderers, comparing them to serial killers "intent on murdering several people over the next year." This is the anthropomorphic fallacy at work — projecting human emotions and attributes onto animals. It also serves as what philosophers call an "intuition pump," a problem framed in a specific fashion in order to elicit an author’s desired intuitive conclusion. Actually, predators aren't cold-blooded killers bent on destroying other animals' lives. Predators’ kills aren’t just necessary for their own survival the kills also maintain a balanced ecosystem. It’s not a coincidence that keystone species — so named because they support an environment like a keystone supports a building — are predators. They eat quickly-reproducing prey that could otherwise overwhelm an area’s resources this service lets animals that have slower reproductive cycles also have access to food.

So reducing the size of predator populations, even by a little, could have dire consequences. Prey animals might live longer, but their lives would be dominated by competing for food with other animals, and many — if not all — would eventually starve. This is to say nothing of the increased incidence of disease that comes with tightly-packed populations. "Let's say you kill the predators to prevent the prey from suffering." said Dan Blumstein, an ecologist at the University of California, Los Angeles. "That means there will be more prey, but they're going to suffer anyway. They'll become overabundant."

Red wolf stalking a deer. (Flickr CC by 2.0)

Case studies have shown that prey populations drastically increase when left unchecked by predation. For example, scientists found that in North American forests, deer populations averaged six times higher in places without wolves, compared to areas with the predator. A paper published in Science in 2011 found that the loss of lions and leopards in sub-Saharan Africa resulted in a giant increase in the region's baboon populations.

The increase in baboons, predictably, had consequences. The now more-numerous animals had to find new sources of food, so they started foraging closer to human-populated settlements. As a result, more people were infected with intestinal parasites that originated with the baboons. Arguably, intestinal parasites increase suffering in their hosts.

The authors use "predator" freely but don’t define it

This is something that the MacAskills actually seem to address. They write that "the cases that we are considering don’t involve a large-scale intervention." Rather, they want individual hunts, since those "are unlikely to have knock-on effects on the ecosystem of the region." On what evidence do they base this statement? None.

What evidence does show is that small changes, like a decline in a certain kind of predator, can have very large effects. A decline in insect-eating bird species worldwide, for instance, led to an increase in the pests they normally consume. That has, in turn, led to plant damage — damage that affects humans who rely on plants to live. These are the types of cascading effects that the authors seem to ignore entirely, probably because scientists barely understand these food web connections in the first place.

"The historical wildlife management record makes it quite clear that it is very tricky to anticipate the full range of complex ecological ramifications put into play whenever humans choose to cull top predators," says John Fryxell, a biologist also at the University of Guelph.

Male jumping spider, which is generally carnivorous. (Lukas Jonaitis/Wikimedia Commons)

That's not the only problem. There's a more basic one: the authors use "predator" freely but don’t define it. "Predators are everywhere," Blumstein said. Some predators, like cats or minks, are obligate carnivores — they cannot survive on plant-based diets. Other predators — humans among them — have more flexibility. Some predators are microscopic — do they count for the purposes of this argument? What about predators, like spiders or frogs, that eat pests like mosquitoes — which are themselves the source of suffering in their bites, and also in the diseases those bites transmit. What about predators like the killer whale, which prey largely on other predators like seals and sea lions?

The MacAskills suggest taking predators out of their natural environment and giving them "good lives that don't involve hunting prey." However, they offer no explanations for how to pull off such an incredibly labor-intensive and land-consuming endeavor, nor do they explain how obligate carnivores will survive on plant-based diets asking a tiger to eat plants, food its body cannot process, for the rest of its natural life seems like a sure-fire cause of animal suffering. You know, for the tiger that is now starving to death.

Life is suffering the only sure-fire way to avoid suffering is to die

It’s not enough to state that "interventions" — what interventions? — "could be justified following a rigorous risk analysis." (One thinks of Ben Franklin: "So convenient a thing it is to be a reasonable creature, since it enables one to find or make a reason for everything one has a mind to do.") What the MacAskills are asking is that we undertake tremendous ecosystem upheaval and damage, at staggering expense and at considerable risk to the lives of people involved — to "prevent suffering."

Life is suffering nature is red in tooth and claw, as Lord Tennyson rightly noted. Pain is an exquisite warning system, meant to keep an animal alive the only sure-fire way to avoid suffering is to die. Perhaps, then, the true way to alleviate suffering is to increase, rather than decrease, the number of predators. There will be more deaths — and more animals that are no longer suffering, for that very reason.


The Crucial Role of Predators: A New Perspective on Ecology

Scientists have recently begun to understand the vital role played by top predators in ecosystems and the profound impacts that occur when those predators are wiped out. Now, researchers are citing new evidence that shows the importance of lions, wolves, sharks, and other creatures at the top of the food chain.

Found in the North Palace at Ninevah, stone panels depicting the Royal Lion Hunt of the last Assyrian king, Ashurbanipal, are as violent as any video game: A female lion flies upside down, arrows protruding from her back and belly. Beneath her, a male rears back, arrows piercing his nasal passages while another male drags his hindquarters behind him. From the king’s chariot, attendants drive spears through the chest of another.

The panels are two-and-a-half thousand years old, and the story they tell is nearly over. In Africa, the lion’s numbers have declined sharply in the past decade, to as low as 23,000. The tiger is near extinction. Earlier this year, a mountain lion walked 1,800 miles from the Black Hills of South Dakota to the East Coast — one of the world’s longest recorded journey by a land mammal — only to be killed by a sport utility vehicle near Milford, Connecticut, 50 miles from New York City.

Just as the world’s lions, tigers, and bears are disappearing worldwide, a scientific consensus is emerging that they are critical to ecosystem function, exerting control over smaller predators, prey, and the plant world. Studies of predation — a so-called “top-down” force in nature — have always run a weak second to ecology’s traditional focus, which holds that the foundation of life springs from bottom-up processes enabled by plants capturing energy from the sun. While no one disputes the importance of photosynthesis and nutrient cycling, experts on predation have become increasingly convinced that ecosystems are ruled from the top.

Beginning with aquatic experiments, they have amassed considerable evidence of damage done to food chains by predator removal and have extended such studies to land: Predation may be as consequential, if not more so, than bottom-up forces. With a comprehensive new book (Trophic Cascades) and a major Science review published this summer, these specialists present the case that our persecution of predators menaces the marine and terrestrial ecosystems that produce food, hold human and zoonotic diseases in abeyance, and stabilize climate.

Using such terms as “deep anxiety” and “grave concern” to signal their alarm, the authors contend that the loss of large animals, and apex predators in particular, constitutes humanity’s “most pervasive influence” on the environment. It amounts, they argue, to a “global decapitation” of the systems that support life on Earth.

These are hardly new ideas: Both publications catalogue decades of work examining the power of predators. Charles Elton, an Oxford ecologist, first conceptualized food webs in the 1920s, speculating that wolf removal would unleash hordes of deer, a notion that weighed on Aldo Leopold’s mind as he compared the consequences of wolf-extirpation in German forests to still-thriving, intact systems in Mexico’s Sierra Madre Mountains.

These insights gave rise to the 1960s “green world” hypothesis, which held that plants prevail because predators hold herbivores in check. Profound food chain effects — caused by adding or removing top species — are now known as “trophic cascades.” In a classic 1966 experiment, biologist Robert Paine removed the purple seastar, Pisaster ochraceus — a voracious mussel-feeder — from an area of coastline in Washington state. Their predator gone, mussels sprouted like corn in Kansas, crowding out algae, chitons, and limpets, replacing biodiversity with monoculture.

Corroborating evidence multiplied. Less than a decade after Pisaster, marine ecologists James Estes and John Palmisano reached the astonishing and widely reported conclusion that hunting of sea otters had caused the collapse of kelp forests around the Aleutian Islands. While the cat was away, the prey (sea urchins) stripped the larder bare. When otters returned, they regulated urchins, allowing “luxuriant” regrowth of biodiverse kelp communities. Around islands farther out to sea, where the mammals had not reestablished themselves, “urchin barrens” remained.

The Science review this summer and other recent research have highlighted the cost of cascades in other marine systems. Extirpation of great sharks along the eastern seaboard caused an irruption of rays and the collapse of a century-old scallop fishery, a glimpse of the future as shark populations crash worldwide. Overfishing of cod, a top predator of lobster and sea urchins, upended the coastal North Atlantic, producing hyper-abundant lobster and a market glut in the Gulf of Maine, as well as an urchin boom-and-bust cycle off Nova Scotia, where urchins have been periodically wiped out by disease.

Yet, as data from aquatic systems proliferated, skeptics suggested that top-down forces might be “all wet” — limited to marine or freshwater systems, with a dearth of evidence for cascades in terrestrial systems.

Where was that evidence? Designing experiments to reveal cascades on land, across large-scales and over long time periods, seemed nearly impossible. So many ecosystems had already been irreparably altered that predator-related effects — including damage done to food chains, so-called “trophic downgrading” — could not be measured with certainty. Long-term trials teasing out wide-ranging interactions among predators and other species promised to be unwieldy and expensive.

Nonetheless, startling revelations continued to crop up. In a Venezuelan valley flooded by construction of a dam in the 1980s, Duke University ecologist John Terborgh and his students documented the strange perturbations that afflicted the “islands” of Lago Guri. Top predators — jaguar, mountain lion, harpy eagle — fled rising waters. Multiplying out of control, howler monkeys went mad as their numbers soared and the plants they ate increased toxins in self-defense. Some islands were cloaked in thorns as leaf-cutter ants — undeterred by armadillos or other predators — starved the soil of nutrients by carrying every leaf down to their lairs.

In 1995, the terrestrial camp landed an extraordinary boon as Yellowstone National Park gave William Ripple, director of Oregon State University’s Trophic Cascades Program, the chance to study top-down forcing in action. Ripple watched in amazement as the wolf’s return to Yellowstone — an ecosystem where elk had had the browse of the place for 75 years — gave willow and other trees the chance to take hold along stream banks, cooling water temperatures for trout and encouraging the return of beaver, whose ponds host long-absent amphibians and songbirds. Yellowstone proved that damage to a terrestrial food web could be reversed and an ecosystem restored with the return of a single species. It is a sobering lesson for the eastern U.S., where the explosion of white-tailed deer has eradicated hemlock, a keystone species in once-biodiverse hardwood forests.

Yet despite such developments, researchers of trophic cascades have despaired of securing the money and means to examine predator removal in large-scale, long-term trials on land. Some have dealt with constrictions by adopting a more manageable, meadow-sized scale. In a three-year experiment, ecologist Oswald Schmitz of the Yale School of Forestry & Environmental Studies found that even the tiniest of predators (spiders) exercise a more significant top-down influence on plants than bottom-up factors. The type of predation — active versus ambush hunting — also appears to be consequential, affecting the composition of plant communities and nitrogen levels. Spiders that hunt actively reduce grasshopper density, allowing grass and goldenrod to dominate other plants and increasing available nitrogen. Ambush hunting has an opposite effect, forcing grasshoppers, which would rather feed on grass, to shelter in goldenrod, yielding a more diverse plant community and less nitrogen. Taken together, Schmitz says, “it’s the richness of the functional role of predators that becomes important to conserve.”

Estes and Terborgh, editors of Trophic Cascades, question whether spiders and grasshoppers will “convince anyone that orcas, great white sharks, wolves, tigers, and jaguars are important.” But Schmitz, who grew up north of Toronto where wolf-hunting was a way of life, thinks the process is underway: “Piece by piece, it’s taken 20 years to accumulate the evidence, and the culmination is in that Science paper — that the world is driven by predators as well as nutrients. We have to pay attention to their health and well-being if we want a healthy ecosystem. Simply eliminating them because we want more prey or because we don’t think they’re important is very misguided.”

Indeed, the Science review presses the trophic case into new territory, extending predation’s impact to human health. A reduction in lion and leopard populations in Ghana has led to an explosion of olive baboons. The release of such “mesopredators” — mid-sized carnivores such as cats or raccoons that run rampant without control — has wreaked havoc around local villages, where baboons attack livestock, damage crops, and spread intestinal parasites to the human population.

In the Science paper, the authors call for “a paradigm shift in ecology.” Scientists and land-managers, they argue, must adopt top-down forcing as a given “if there is to be any real hope of understanding and managing the workings of nature.”

In Trophic Cascades, Terborgh and Estes go farther, criticizing national science agencies for failing to fund research on predator removal in terrestrial systems, accusing them of clinging to old views and “retarding progress” while ecosystems are undermined. “The idea that plants are affected by the things that eat them,” Estes says dryly, “has not been widely appreciated.”

But Alan James Tessier, program director of the National Science Foundation’s Environmental Biology Division, disagrees, asserting that the agency has funded much research into top-down processes. “It’s ridiculous to talk only about top-down or bottom-up control,” said Tessier. “Both are happening all the time.”

In science, new ideas are rightly met with skepticism, if not denials and dismissals. But as the consequences of predator loss become increasingly measurable and predictable, they implicitly call for a reassessment of our ancient foes. Estes is as reluctant as any scientist to weigh in on the wolf wars, but his frustration is clear. “That’s not the way we should be behaving as a species,” he says.

Caroline Fraser traveled on six continents to write Rewilding the World: Dispatches from the Conservation Revolution. Her first book, God's Perfect Child: Living and Dying in the Christian Science Church, was selected as a New York Times Book Review Notable Book and a Los Angeles Times Book Review Best Book. She has written widely about animal rights, natural history, and the environment, and her work has appeared in The New Yorker, The New York Review of Books, and Outside magazine, among others. More about Caroline Fraser →


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