Predation is a biological interaction where a predator (a hunting animal) kills and eats its prey (the organism that is attacked). Predators are adapted and often highly specialized for hunting, with acute vision, hearing, and sense of smell. Many have sharp claws and jaws to grip, kill, and cut up their prey.
In ecology, predators are heterotrophic, getting all their energy from other organisms. This places them at high trophic levels in food webs. Many predators are carnivores; others include egg predators. Predation is one of a family of common feeding behaviours that includes parasitism and micropredation which usually do not kill the host, and parasitoidism which always does, eventually. All these are evolutionarily stable strategies.
Predator and prey adapt to each other in an evolutionary arms race, coevolving under natural selection to develop antipredator adaptations in the prey and adaptations such as stealth and aggressive mimicry that improve hunting efficiency in the predator.
Heterotrophic animals such as predators that derive their energy from other organisms (prey or hosts) can be classified by their consumer-resource interactions with those organisms. A perspective on the evolutionary options available to predators and parasites can be gained by considering four questions: the effect on the fitness of the prey or host; the number of prey or hosts they have per life stage; whether the prey or host is prevented from reproducing (by being killed, or by being castrated), reducing its evolutionary fitness to zero; and whether the effect depends on intensity. From this analysis, the major evolutionary strategies of predation and micropredation emerge, alongside parasitism and parasitoidism; social predators such as lion and wolf are distinguished from solitary predators like the cheetah.
A conventional predator is one that kills and eats another living thing. Predators may hunt actively for prey in pursuit predation, or sit and wait for prey to approach within striking distance in ambush predation. Some predators kill large prey and dismember or chew it prior to eating it, as do humans; others may eat their prey whole, as do bottlenose dolphins swallowing fish, white storks swallowing frogs, or baleen whales swallowing thousands of krill or small fish at once. Some predators use venom to subdue their prey before the predator ingests it, as in the box jellyfish, while the venom of rattlesnakes and some spiders also helps to digest the prey. Seed and egg predation are true predation, as seeds and eggs are potential organisms.
Grazing and micropredation
Grazing animals generally do not kill their prey, but like predators, they live by feeding on other organisms. While some herbivores like zooplankton live on unicellular phytoplankton and therefore inevitably kill what they eat, in a relationship sometimes called predation, many others including cattle and sheep only eat a part of the plants that they graze. Many species of plant are adapted to regrow after grazing damage. For example, the growing meristems of grasses are not at the tips as they are in most flowering plants, but at the base of the leaves. Similarly, kelp is grazed in subtidal kelp forests, but continuously regrows from a meristem at the base of the blade where it joins the stipe. Herbivore-plant interactions, as with predator-prey interactions, have driven plants to evolve defences such as thorns and chemicals to dissuade grazing.
Animals may also be 'grazed' upon by blood-feeding micropredators. These include annelids such as leeches, crustaceans such as branchiurans and gnathiid isopods, dipterans such as mosquitoes and tsetse flies, other arthropods such as fleas and ticks, fish such as lampreys, and mammals such as vampire bats.
Parasites, like predators, live by feeding on another organism, but differ in that they often do not kill their hosts. The entomologist E. O. Wilson has characterised parasites as "predators that eat prey in units of less than one".
Parasitoids are insects living in or on their host and feeding directly upon it, eventually leading to its death, making their strategy comparable with predation. They are, however, much like parasites in their close associations with their hosts. Unlike typical parasites, they always kill their hosts, but often not instantly. Parasitoid wasps are solitary insects that live a free life as adults, laying eggs on or in other insects such as lepidopteran caterpillars. The wasp larvae feed on the growing host, eventually killing it. Parasitoids make up as much as 10% of all insect species.
In social predation, a group of predators cooperates to kill creatures larger than those they could overpower singly. Social predators such as lions, hyenas, and wolves collaborate to catch and kill large herbivores. By hunting socially chimpanzees can catch colobus monkeys that would readily escape an individual hunter, while cooperating Harris hawks can trap rabbits.
Each predation-like strategy is illustrated for comparison.
Under the pressure of natural selection, predators have evolved a variety of physical adaptations for detecting, catching, killing, and digesting prey. The senses they use to hunt with, including vision, smell, and hearing, are well developed. 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 (61 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.
Predators including big cats, birds of prey, and ants share powerful jaws 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; 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.
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.
Diet and behaviour
Predators are often highly specialized in their diet and hunting behaviour; for example, the Eurasian lynx only hunts small ungulates. Others such as leopards are more opportunistic generalists, preying on at least 100 species. 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 to be more specialized as the prey are more conspicuous and can be found more quickly.
In size-selective predation, predators select prey of a certain size. 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.
Coevolution with prey
An alternative view considers predation as a form of competition: the predator's and prey's genes are competing for the prey's body. Competition is directly evident in intraguild predation, where predators kill and eat predators of competing species at the same trophic level. For example, coyotes compete with and sometimes kill gray foxes and bobcats.
A hunting predator is attempting to obtain its next meal; its prey is attempting to save its own life. This sets up an evolutionary arms race, causing many antipredator adaptations to evolve in prey populations due to the selective pressures of predation over long periods of time.
Many prey animals are aposematically coloured or patterned as a warning to predators that they are distasteful or able to defend themselves. Such distastefulness or toxicity is brought about by chemical defenses, found in a wide range of prey, especially insects, but the skunk is a dramatic mammalian example. Chemical defences include toxins, such as bitter compounds in leaves absorbed by leaf-eating insects and used to dissuade potential predators.
Camouflage makes use of coloration, shape, and pattern to misdirect the visual sensory mechanisms of predators, enabling prey to remain unrecognized. Among the many mechanisms of camouflage are countershading and disruptive coloration. 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 the drone fly, which resembles a bee, yet has no sting. Many butterflies and moths have wing markings that resemble eyes. When a predator disturbs the insect, it reveals its hind wings, startling the predator and giving it time to escape.
As prey evolves to become harder to catch, on their side of the evolutionary arms race predators adapt to use stealth, camouflage and aggressive mimicry to improve their hunting efficiency. 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) have coloration and patterns suiting their habitats. Female Photuris fireflies, for example, copy the light signals of other species, thereby attracting male fireflies, which they capture and eat. Flower mantises are ambush predators; camouflaged as flowers, such as orchids, they attract prey and seize it when it is close enough. 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.
Role in ecosystems
Predators are often another organism's prey, and likewise prey are often predators. Though blue jays prey on insects, they may in turn be prey for cats and snakes, and snakes may be the prey of hawks. One way of classifying predators is by trophic level. Carnivores that feed on heterotrophs are secondary consumers; their predators are tertiary consumers, and so forth. Because only a fraction of energy is passed on to the next level, this hierarchy of predation must end somewhere, and very seldom goes higher than five or six levels. For example, a lion, an apex predator (at the top of its food chain, if parasites are not considered) that preys upon large herbivores such as wildebeest, which in turn eat grasses, is only a secondary consumer. Other apex predators include the sperm whale, Komodo dragon, tiger, and most eagles and owls. Many predators eat from multiple levels of the food chain. A carnivore may eat both secondary and tertiary consumers, and its prey may itself be difficult to classify for similar reasons.
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. 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.
The elimination of wolves from Yellowstone National Park had profound impacts on the trophic pyramid. 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.
Predators tend to lower the survival and fecundity of their prey, but also depend on prey for their survival, so predator populations are affected by changes in prey populations and vice versa. The population dynamics of predator–prey interactions can be modeled using the Lotka–Volterra equations. These provide a mathematical model for the cycling of predator and prey populations. Predators tend to select young, weak, and ill individuals, leaving prey populations able to regrow. When prey numbers are low, the predators find little food, produce few young, and may starve, so their population tends to fall. When predator numbers are low, few prey are killed, and they can reproduce freely, so their population grows. Once prey numbers are high, the predators catch food more readily and raise more young, so their population grows. When predator and prey numbers are high, the predators kill many prey, depleting their population, and their numbers fall. The cycles then repeat.
Predation appears to have become a major selection pressure shortly before the Cambrian period—around 550 million years ago—as evidenced by the almost simultaneous development of calcification in animals and algae, and predation-avoiding burrowing. However, predators had been grazing on micro-organisms since at least 1,000 million years ago.
In human society
Humans are to some extent predatory, fishing, hunting and trapping animals using weapons and tools. They also use other predatory species, such as dogs, cormorants, and falcons to catch prey for food or for sport.
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 not do harm to non-pest species, are an environmentally friendly and sustainable way of reducing damage to crops, and are an alternative to the use of chemical agents such as pesticides.
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