Supporting article V: The role of Predation in ecosystems
Published: December 28, 2009, 3:50 pm
Lead Author: Mark McGinley
Contributing Author: J. Emmett Duffy
This article has been reviewed by the following Topic Editor: Judith S. Weis
Predation is an interaction between species in which one species uses another species as food. Predation is a process of major importance in influencing the distribution, abundance, and diversity of species in ecological communities. Generally, successful predation leads to an increase in the population size of the predator and a decrease in population size of the prey. These effects on the prey population may then ripple out through the ecological community, indirectly changing the abundances of other species. One example of such indirect effects of predation involves the trophic cascade. As the name implies, a trophic cascade occurs when the effects of predation “cascade” down the food chain to affect plants or other species that are not direcrtly eaten by the predator. Typically, a trophic cascade involves a predator feeding on herbivores and reducing their abundance, which then releases plants from grazing pressure and increases the biomass of vegetation. In addition to such ecological effects of predation, which occur on time scales of one or a few generations of the organisms involved, predation has also played, and continues to play, a major role over evolutionary time in molding the phenotypes of many species.
Types of predation
By the most general definition, predation is a class of ecological interactions in which one species benefits (the predator) while the second species is harmed (the prey). Cannibalism is simply predation on another individual of the same species. Another example of this general class of interactions is parasitism; as in predation, one species benefits (the parasite) while the second is harmed (the host). The distinction between these two types of interactions is that, typically, a predator kills its prey more or less immediately (e.g., a shark eating a tuna or a venus fly trap consuming a fly) whereas a parasite feeds for an extended period on a living host (e.g., a tapeworm living in the body of a deer or a mistletoe “feeding” on a mesquite tree). Finally, herbivory occurs when an animal uses a plant as food. In most cases, a single act of herbivory does not kill a plant).
Predation and population dynamics
In many cases predation has a strong influence on the population sizes of predator and prey. In general, increasing the population size of prey will result in a corresponding increase in the population size of the predator because the predator has more food. Similarly, prey populations are expected to decline as the population size of a predator increases because of increased predation pressure. Because the population response of one species to a change in the other requires time for the population to grow, predator-prey interactions sometimes result in population cycles, in which both predator and prey populations each undergo regular increases and decreases, but the population cycles are out of phase with one another.
Predation and species richness
Predation can either increase or decrease the number of species that coexist in a community, depending on the favorability of the environment and on the competitive status of the preferred prey species. For example, a keystone predator is one that feeds on a competitively dominant prey species. By reducing the dominant prey’s abundance, the keystone predator releases competitively inferior prey from suppression by that dominant species. As a result, keystone predation allows more prey species to coexist within the community than would be possible in the absence of predation, and thus increases species richness within the community (predator-mediated coexistence). Conversely, when a predator feeds preferentially on competitively inferior prey species, predation can further reduce the number of species in the community. In environments that are favorable for prey, competition among prey species will be stronger, such that keystone predation can be important in reducing competitive exclusion among prey and thus increasing species richness. In unfavorable environments, on the other hand, most prey species are stressed or living at low population densities such that predation is likely to have negative effects on all prey species, thus lowering species richness.
Predation as a force of natural selection
Given the strong impacts of predators on the fitness and population dynamics of their prey, it is not surprising that predation has been an important factor molding the evolution of traits of both predators and prey. The process of natural selection favors individual predators with traits that make them more effective in obtaining prey, thus driving the evolution of predator traits that allow them to more effectively capture or feed on prey, while prey have evolved traits that enhance their ability to escape or deter predation.
Predators have evolved a variety of techniques to catch, subdue, or exploit their prey (e.g., fast running speed, sharp teeth and talons and camouflage) while prey have evolved a variety of predator defense mechanisms to allow them to escape their predators or reduce their desirability as prey (e.g., fast speed, camouflage, wary behavior, physical defenses, and chemical defenses). Even plants can use chemical defenses to deter herbivores. While some predators actively pursue their prey, others use a “sit-and-wait” ambush strategy. Angler fishes actually lure their prey with an appendage on their heads that resembles a worm or small fish that they wave around to attract their prey. Generally, selective pressures are stronger on the prey than on the predator. This maxim has been called the “life-dinner principle”: if a fox unsuccessfully attacks a rabbit then he loses only a meal, whereas if a rabbit attempting to escape from a fox is unsuccessful, it loses its life.
Because predators impose a selective force on their prey, while prey reciprocally impose selective pressures on their predators, coevolution between predators and prey should be common. For example, over time, predation by foxes should select for faster rabbits which, in turn, selects for faster foxes, which in turn selects for faster rabbits, and so on. Such “arms races” between predators and prey (and between parasites and their hosts) have strongly influenced the traits of many plants and animals. For example, the intense predation and grazing characteristic of coral reefs has caused evolution of noxious chemical defenses (toxins) and physical defenses (spines and hard shells) in many of the seaweeds and sessile invertebrates that dominate reef ecosystems.