Predation

Predation is an interaction between two organisms in which one of them, the predator, derives nutrition by killing and eating the other, the prey. Obvious examples include lions feeding on zebras and hawks eating rodents, but predation is not limited to interactions among animals. Birds that feed on seeds are legitimate predators since they are killing individual organisms (embryonic plants) to derive energy. There are a number of species of carnivorous plants, such as sundews and pitcher plants, that capture and consume small animals to obtain nitrogen in habitats lacking sufficient quantities of that nutrient. Most animals that feed on plants (herbivores) do not kill the entire plant and, therefore, are not really predators. Exceptions to this generalization are some insects that reach infestation levels, such as gypsy moths or locusts, and can kill the plants upon which they feed. Most herbivore-plant associations, however, are more properly described as parasite-host interactions in which the host plant may suffer damage but does not die.

There are special cases in which parasitism and predation may be combined. One of these is the interaction between parasitoid wasps and their hosts, usually flies. Adult female parasitoids attack and inject eggs into fly pupae (the resting stage, during which fly larvae metamorphose into adults), and the larvae of the wasp consume the fly. The adult parasitoid is, therefore, a parasite, while the larval wasp acts as a predator.

Predator-Prey Interactions

Predator-prey interactions can be divided into two considerations: the effects of prey on predators and the effects of predators on their prey. Predators respond to changes in prey density (the number of prey organisms in the habitat) in two principal ways. The first is called numerical response, which means that predators change their numbers in response to changes in prey density. This may be accomplished by increasing or decreasing reproduction or by immigrating to or emigrating from a habitat. If prey density increases, predators may immigrate from other habitats to take advantage of this increased resource, or those predators already present may produce more offspring. When prey density decreases, the opposite will occur. Some predators, which are known as fugitive species, are specialized at finding habitats with abundant prey, migrating to them, and reproducing rapidly once they are established.

Cape May warblers are good at finding high densities of spruce budworms (a serious pest of conifers) and then converting the energy from their prey into offspring. This strategy allows the birds to persist only because the budworms are never completely wiped out. They are better at dispersing to new habitats than are the birds.

The second response of predators to changing prey density is called functional response. The rate at which predators capture and consume prey depends upon the rate at which they encounter prey, which is a function of prey density. If the predator has a choice of several prey species, it may learn to prefer one of them. If that prey is sufficiently abundant, this situation results in a phenomenon known as switching, the concentration by the predator on the preferred prey. It may entail a change in searching behavior on the part of the predator, such that former prey items will no longer be encountered as frequently.

Animals have evolved several defense mechanisms that reduce their probability of being eaten by predators. Spines on horned lizards, threatening displays by harmless snakes, acoustic decoys moths use to trick bats, camouflage of many cryptic animals, toxic or distasteful chemicals in insects and amphibians, and simply rapid movement—all are adaptations that may have evolved in response to natural selection by predators. A predator that can learn to prefer one prey item over another is smart enough to learn to avoid less desirable prey. That capability is the basis for a phenomenon—known as aposematism—among potential prey species that are toxic or distasteful to their predators.

Aposematic organisms advertise their toxicity by bright coloration, making it easy for predators to learn to avoid them, which in turn saves the prey population from frequent taste-testing. Many species of insects are aposematic. Monarch butterflies are bright orange with black stripes, an easy signal to recognize. They owe their toxicity to the milkweed plant, which they eat as caterpillars. The plant contains cardiac glycosides, which are very toxic. The monarch caterpillar is immune to the poison and stores it in its body so that the adult has a high concentration of it in its wings. If a bird grabs the butterfly in flight, it is likely to get a piece of wing first, and this will teach it not to try orange butterflies in the future.

Some potential non-toxic prey species have evolved to resemble toxic species. These are called Batesian mimics. Viceroy butterflies are not toxic but mimic monarchs very closely, so birds cannot tell them apart. One limitation of Batesian mimicry is that mimics can never get very numerous, or their predators will not get a strong enough message to leave them alone. Another kind of mimicry involves mimics that are as toxic as their models. The advantage of this type, Müllerian mimicry is that the predator must learn only one coloration signal, which reduces risk for both prey populations. In this relationship, the mimic population does not have to remain at low levels relative to the model population. A third type of mimicry is more insidious—aggressive mimicry, in which a predator resembles a prey or the resource of that prey to lure it close enough to capture. There are tropical praying mantis that closely resemble orchid flowers, thus attracting the bees upon which they prey. Some species of fireflies eat other species of fireflies by using the flashing light signal of their prey to lure them within range. This method is effective, but some scientists posit that this may lead to predator-prey confusion. If a predator remains healthy after eating an animal that appears poisonous, they may continue to engage in this behavior, eventually leading them to a truly poisonous species. Additionally, this behavior may lead to instinct confusion in species that learn from other group members.

The Choice of Prey

What determines predator preference for prey? Since prey are a source of energy for the predator, it might be expected that predators would simply attack the largest prey they could handle. To an extent, this choice holds true for many predators, but there is a cost to be considered. The cost involves the energy a predator must expend to search for, capture, handle, and consume prey. To be profitable, a prey item must yield much more energy than it costs. Natural selection should favor a reduction in energetic cost relative to energetic gain, the basis for optimal foraging theory. According to this theory, many predators have evolved hunting strategies to optimize the time and energy spent in searching for and capturing prey. Some predators, such as web-building spiders and boa constrictors, ambush their prey. The low energetic cost of sit-and-wait is an advantage in environments that provide plentiful prey. If encounters with prey become less predictably reliable. However, an ambush predator may experience starvation. Spiders can lower their metabolic energy requirements when prey is unavailable, whereas more mobile predators, such as boa constrictors, can simply shift to active searching. Probably because of the likelihood of facing starvation for extended periods of time, ambush predation is more common among animals that do not expend metabolic energy to regulate their body temperatures (ectotherms) than among those that do (endotherms). Some predators, such as wolves and lions, hunt in groups. This allows them to tackle larger (more profitable) prey than if they hunted alone. Solitary hunters generally have to hunt smaller prey.

Natural communities consist of food webs constructed of links (feeding relationships) among trophic levels. Each prey species is linked to one or more predators. Most predators in nature are generalists with respect to their prey. Spiders, snakes, hawks, lions, and wolves all feed on a variety of prey. Some of these prey are herbivores, but some are themselves predators. Praying mantis eat grasshoppers (herbivores), but they also eat spiders (carnivores) and each other. Thus, generalist predators have a bitrophic niche, in that they occupy two trophic levels at the same time.

Predatory Relationships and Population Fluctuation

It is an open question whether predators and prey commonly regulate each other’s numbers in nature. There are many examples of cyclic changes in abundance over time, in which an increase in prey density is followed by an increase in the numbers of predators, and then the availability of prey decreases, also followed by a decrease in predators. Are predators causing their prey to fluctuate, or are prey responding to some other environmental factor, such as their own food supply? In the second case, prey may be regulated by food and, in turn, may be regulating predators, but not the reverse.

Predators can sometimes determine the number of prey species that can coexist in a habitat. If a predator feeds on a prey species that could outcompete (competitively exclude) other prey species in a habitat, it may free more resources for those other species. This relationship is known as the keystone effect. Empirical studies have indicated that the number of prey species in some communities is directly related to the intensity of predation (numerical and functional responses of predators) such that at low intensity, few species coexist because of competitive exclusion; at intermediate intensity, the diversity of the prey community is greatest; and at high intensity, diversity decreases because overgrazing begins to eliminate species. This intermediate predation hypothesis depends upon competition among prey species, which is not always the case.

Population fluctuations and the nature of predator-prey relationships are linked, but predation plays a more extensive and understudied role in maintaining the health and stability of ecosystems. Predators control the population of other animals, ensuring that mating among prey animals remains competitive and that birth rates are appropriate so as not to negatively impact other species. However, the carcass that remains after their kill creates an ecological hotspot that other animals in the ecosystem benefit from and depend on. For example, after lions eat most of the meat from a kill and move on, scavenger birds, hyenas, worms, flies, and microscopic organisms break down the rest of the body as they feed. This process also fertilizes the land, allowing plants to grow to feed plant-eating animals. Predation controls the population, but it also ensures a habitable, stable, and healthy ecosystem for future generations.

Studying Predation

The central question in the study of predation is: To what extent do predators and their prey regulate one another? Most studies suggest that predators are usually food limited, but the extent to which they regulate their prey is uncertain. It is one thing to observe predators in nature and another to assess their importance to the dynamics of natural communities. Like other aspects of ecology, studies of predation can be descriptive, experimental, or mathematical.

At the descriptive level, characteristics of both predator and prey populations are assessed: rates of birth and mortality, age structure, environmental requirements, and behavioral traits. Qualitative and quantitative information of this type is necessary before predictions can be made about the interactions between predator and prey populations. A general lack of such information in natural ecosystems is largely responsible for failures at biological control of pests and management of exploited populations.

Experimental studies of predation involve the manipulation of predator or prey populations. A powerful method of testing the importance of predation is to exclude a predator from portions of its accustomed habitat, leaving other portions intact as experimental controls. In one such experiment, excluding starfish from marine intertidal communities of sessile invertebrates resulted in domination by mussels and exclusion of barnacles and other attached species; in the absence of the predator, one prey species was capable of competitively excluding others. This keystone effect depends on two factors—that the prey assemblage structure is determined by competition and that the predator preferentially feeds on the species that is the best competitor in the assemblage. Clearly, not all food webs are likely to be structured in this way.

Another method of experimental manipulation is to enhance the number of predators in a community. For complex natural communities, both additions and exclusions of predators have revealed direct (depression of prey) and indirect (enhancement) effects. Since generalist predators are bitrophic in nature, they may interact with other carnivores in such a way as to enhance the survival of herbivores that normally would fall victim. In one experiment, adding praying mantids to an insect community resulted in a decrease in spiders and a consequent increase in aphids, normally eaten by these spiders. Such results are not uncommon and contribute to the uncertainty of prediction. Mathematical models have been constructed to depict predator-prey interactions in terms of how each population affects the growth of the other. The simplest of these models, known as the Lotka-Volterra model for the mathematicians who developed it, describes a situation in which prey and predator populations are assumed to be mutually regulating. This model, which was developed for a single prey population and single predator species, has been modified by many workers to provide more realism, but it is far from predicting many competitive situations in complex natural communities.

All parts of an ecosystem work together to create balance. For example, lions in an area of Kenya became less effective in hunting their primary food source, zebra, after a species of big-headed ants invaded the ecosystem. These ants kill the ecosystem's native acacia ants, which protect whistling-thorn trees by deterring elephants, giraffes, and other herbivores from eating, knocking down, or otherwise killing these trees. Lions use whistling-thorn trees for cover when they stalk their prey, and without them, they are less effective and exert more energy during the hunt.

As with the rest of modern ecology, these approaches must be blended to build a robust picture of how important predators are in natural ecosystems. This knowledge would allow for a more successful prediction of the outcomes of human intervention and more intelligent management of exploited populations. Predation is a key interaction in natural ecosystems. Understanding the nature of this interaction is central to any understanding of nature itself.

Principal Terms

Competition: The interaction among two or more organisms of the same or different species that results when they share a limited resource

Food Web: The sum of the feeding relationships (links) between trophic levels in ecosystems

Functional Response: The rate at which an individual predator consumes prey, dependent upon the abundance of that prey in a habitat

Mimicry: The resemblance of one species (the model) to one or more other species (mimics), such that a predator cannot distinguish among them

Numerical Response: The abundance of predators dependent upon the abundance of prey in a habitat

Population Regulation: Stabilization of population size by factors such as predation and competition, the relative impact of which depends on abundance of the population in a habitat

Trophic Level: A level at which energy is acquired in a food web—the herbivore level obtains energy from plants; the carnivore level from herbivores and other carnivores

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