Prey refers to any living entities that are hunted and consumed by predators. Usually the term is used in reference to animals that are stalked, killed, and consumed by other animals, as when a deer is killed by a mountain lion. However, plants may also be considered to be the prey of herbivorous animals, and hosts may be considered the prey of their parasites.
Often, predators are important sources of mortality for populations of their prey. As such, predators may act as significant agents of natural selection, with some prey individuals being favored because they are less vulnerable to predation, while less-fit individuals of the same species suffer a disproportionate risk of mortality from this source. If differences among individuals in the vulnerability to predation have a genetic basis, then evolution will occur at the population level, and the prey will become more difficult to capture. This evolutionary change in the vulnerability of prey in turn exerts a selective pressure on the predators, so that the more capable individual hunters are favored and the population of predators becomes more effective at catching prey. This is an example of coevolution of populations of predators and prey.
There are limits, however, to how evasive prey can become, and to how effective predators can become. Eventually, extreme expression in the prey of anatomical, physiological, or behavioural characteristics that help to reduce the risks of predation may become maladaptive in other respects. For example, adaptive changes in the coloration of prey may make them more cryptic, so they blend in better with the background environment and are therefore less visible to predators. However, in many species bright coloration is an important cue in terms of species recognition and mate selection, as is the case of birds in which the males are garishly colored and marked. In such cases, a balance must be struck among adaptations that make prey more difficult to catch, and those that are important in terms of coping with other environmental or biological factors that exert selective pressures.
Predator-prey associations of plants and herbivores also develop coevolutionary dynamics. To deter their predators, plants may evolve bad tastes, toxic chemicals, or physical defenses such as thorns and spines. At the same time, the herbivores evolve ways to overcome these defenses.
Predator satiation refers to a situation in which prey is extremely abundant during a short or unpredictable period of time, so that the capability of predators to catch and eat the prey is overwhelmed. For example, to reduce the impact of predation of their fruits, many species of plants flower and seed prolifically at unpredictable times, so herbivores cannot collect and consume all of the fruits, and many seeds survive. There are also many animal-prey examples of predator satiation. For example, metamorphosis of the larval stages of many species of frogs and salamanders is often closely synchronized, so that most individuals transform and leave the breeding pond at about the same time. This is a very risky stage of the life history of these animals, and although many of the individuals are predated upon, the ability of the predators to catch and process this superabundant prey is limited. Consequently, many of the recently transformed frogs and salamanders manage to survive.