In an ecosystem, predation is a biological interaction where a predator (an organism that is hunting) feeds on its prey (the organism that is attacked). Predators may or may not kill their prey prior to feeding on them, but the act of predation often results in the death of the prey and the eventual absorption of the prey's tissue through consumption. Thus predation is often, though not always, carnivory. Other categories of consumption are herbivory (eating parts of plants), fungivory (eating parts of fungi), and detritivory (the consumption of dead organic material (detritus)). All these consumption categories fall under the rubric of consumer-resource systems. It can often be difficult to separate various types of feeding behaviors. For example, some parasitic species prey on a host organism and then lay their eggs on it for their offspring to feed on it while it continues to live in or on its decaying corpse after it has died. The key characteristic of predation however is the predator's direct impact on the prey population. On the other hand, detritivores simply eat dead organic material arising from the decay of dead individuals and have no direct impact on the "donor" organism(s).
Selective pressures imposed on one another often leads to an evolutionary arms race between prey and predator, resulting in various antipredator adaptations. Ways of classifying predation surveyed here include grouping by trophic level or diet, by specialization, and by the nature of the predator's interaction with prey.
- 1 Functional classification
- 2 Ecological role
- 3 Adaptations and behavior
- 4 Population dynamics
- 5 Evolution of predation
- 6 Humans and predation
- 7 See also
- 8 References
- 9 Further reading
- 10 External links
Classification of predators by the extent to which they feed on and interact with their prey is one way ecologists may wish to categorize the different types of predation. Instead of focusing on what they eat, this system classifies predators by the way in which they eat, and the general nature of the interaction between predator and prey species. Two factors are considered here: How close the predator and prey are (in the latter two cases the term prey may be replaced with host) and whether or not the prey are directly killed by the predator is considered, with true predation and parasitoidism involving certain death.
A true predator can commonly be known as one that kills and eats another living thing. Whereas other types of predator all harm their prey in some way, this form kills them. Predators may hunt actively for prey in pursuit predation, or sit and wait for prey to approach within striking distance, as in ambush predators. Some predators kill large prey and dismember or chew it prior to eating it, such as a jaguar or a human; others may eat their (usually much smaller) prey whole, as does a bottlenose dolphin swallowing a fish, or a snake, duck or stork swallowing a frog. Some animals that kill both large and small prey for their size (domestic cats and dogs are prime examples) may do either depending upon the circumstances; either would devour a large insect whole but dismember a rabbit. Some predation entails venom that subdues a prey creature before the predator ingests the prey by killing, which the box jellyfish does, or disabling it, found in the behavior of the cone shell. In some cases, the venom, as in rattlesnakes and some spiders, contributes to the digestion of the prey item even before the predator begins eating. In other cases, the prey organism may die in the mouth or digestive system of the predator. Baleen whales, for example, eat millions of microscopic plankton at once, the prey being broken down well after entering the whale. Seed predation and egg predation are other forms of true predation, as seeds and eggs represent potential organisms. Predators of this classification need not eat prey entirely. For example, some predators cannot digest bones, while others can. Some may eat only part of an organism, as in grazing (see below), but still consistently cause its direct death.
Grazing organisms may also kill their prey species, but this is seldom the case. While some herbivores like zooplankton live on unicellular phytoplankton and therefore, by the individualized nature of the organism, kill their prey, many only eat a small part of the plant. Grazing livestock may pull some grass out at the roots, but most is simply grazed upon, allowing the plant to regrow once again. Kelp is frequently grazed in subtidal kelp forests, but regrows at the base of the blade continuously to cope with browsing pressure. Animals may also be 'grazed' upon; female mosquitos land on hosts briefly to gain sufficient proteins for the development of their offspring. Starfish may be grazed on, being capable of regenerating lost arms.
Parasites can at times be difficult to distinguish from grazers. Their feeding behavior is similar in many ways, however they are noted for their close association with their host species. While a grazing species such as an elephant may travel many kilometers in a single day, grazing on many plants in the process, parasites form very close associations with their hosts, usually having only one or at most a few in their lifetime. This close living arrangement may be described by the term symbiosis, "living together", but unlike mutualism the association significantly reduces the fitness of the host. Parasitic organisms range from the macroscopic mistletoe, a parasitic plant, to microscopic internal parasites such as cholera. Some species however have more loose associations with their hosts. Lepidoptera (butterfly and moth) larvae may feed parasitically on only a single plant, or they may graze on several nearby plants. It is therefore wise to treat this classification system as a continuum rather than four isolated forms.
Parasitoids are organisms living in or on their host and feeding directly upon it, eventually leading to its death. They are much like parasites in their close symbiotic relationship with their host or hosts. Like the previous two classifications parasitoid predators do not kill their hosts instantly. However, unlike parasites, they are very similar to true predators in that the fate of their prey is quite inevitably death. A well-known example of a parasitoids are the ichneumon wasps, solitary insects living a free life as an adult, then laying eggs on or in another species such as a caterpillar. Its larva(e) feed on the growing host causing it little harm at first, but soon devouring the internal organs until finally destroying the nervous system resulting in prey death. By this stage the young wasp(s) are developed sufficiently to move to the next stage in their life cycle. Though limited mainly to the insect order Hymenoptera, Diptera and Coleoptera parasitoids make up as much as 10% of all insect species.
Degree of specialization
Among predators there is a large degree of specialization. Many predators specialize in hunting only one species of prey. Others are more opportunistic and will kill and eat almost anything (examples: humans, leopards, dogs and alligators). The specialists are usually particularly well suited to capturing their preferred prey. The prey in turn, are often equally suited to escape that predator. This is called an evolutionary arms race and tends to keep the populations of both species in equilibrium. Some predators specialize in certain classes of prey, not just single species. Some will switch to other prey (with varying degrees of success) when the preferred target is extremely scarce, and they may also resort to scavenging or a herbivorous diet if possible.
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. Organisms that feed on autotrophs, the producers of the trophic pyramid, are known as herbivores or primary consumers; those that feed on heterotrophs such as animals are known as secondary consumers. Secondary consumers are a type of carnivore, but there are also tertiary consumers eating these carnivores, quartary consumers eating them, 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, and may go only as high as three trophic levels (for example, a lion that preys upon large herbivores such as wildebeest, which in turn eat grasses). A predator at the top of any food chain (that is, one that is preyed upon by no organism) is called an apex predator; examples include the orca, sperm whale, anaconda, Komodo dragon, tiger, lion, tiger shark, Nile crocodile, and most eagles and owls—and even omnivorous humans and grizzly bears. An apex predator in one environment may not retain this position as a top predator if introduced to another habitat, such as a dog among alligators, a skunk in the presence of the great horned owl immune to skunk spray, or a snapping turtle among jaguars; a predatory species introduced into an area where it faces no predators, such as a domestic cat or a dog in some insular environments, can become an apex predator by default.
Many organisms (of which humans are prime examples) eat from multiple levels of the food chain and, thus, make this classification problematic. A carnivore may eat both secondary and tertiary consumers, and its prey may itself be difficult to classify for similar reasons. Organisms showing both carnivory and herbivory are known as omnivores. Even herbivores such as the giant panda may supplement their diet with meat. Scavenging of carrion provides a significant part of the diet of some of the most fearsome predators. Carnivorous plants would be very difficult to fit into this classification, producing their own food but also digesting anything that they may trap. Organisms that eat detritivores or parasites would also be difficult to classify by such a scheme.
Predation as competition
An alternative view offered by Richard Dawkins is of predation as a form of competition: the genes of both the predator and prey are competing for the body (or 'survival machine') of the prey organism. This is best understood in the context of the gene centered view of evolution. Another manner in which predation and competition are connected is throughout intraguild predation. Intraguild predators are those that kill and eat other predators of different species at the same trophic level, and thus that are potential competitors.
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 in riparian areas, which protected beavers from having their food sources encroached upon. The removal of wolves had a direct effect on beaver populations, as their habitat became territory for grazing. Furthermore, predation keeps hydrological features such as creeks and streams in normal working order. Increased browsing on willows and conifers along Blacktail Creek due to a lack of predation caused channel incision because they helped slow the water down and hold the soil in place.
Adaptations and behavior
The act of predation can be broken down into a maximum of four stages: Detection of prey, attack, capture and finally consumption. The relationship between predator and prey is one that is typically beneficial to the predator, and detrimental to the prey species. Sometimes, however, predation has indirect benefits to the prey species, though the individuals preyed upon themselves do not benefit. This means that, at each applicable stage, predator and prey species are in an evolutionary arms race to maximize their respective abilities to obtain food or avoid being eaten. This interaction has resulted in a vast array of adaptations in both groups.
One adaptation helping both predators and prey avoid detection is camouflage, a form of crypsis where species have an appearance that helps them blend into the background. Camouflage consists of not only color but also shape and pattern. The background upon which the organism is seen can be both its environment (e.g., the praying mantis to the right resembling dead leaves) or other organisms (e.g., zebras' stripes blend in with each other in a herd, making it difficult for lions to focus on a single target). The more convincing camouflage is, the more likely it is that the organism will go unseen.
Mimicry is a related phenomenon where an organism has a similar appearance to another species. One such example is the drone fly, which looks a lot like a bee, yet is completely harmless as it cannot sting at all. Another example of batesian mimicry is the io moth, (Automeris io), which has markings on its wings that resemble an owl's eyes. When an insectivorous predator disturbs the moth, it reveals its hind wings, temporarily startling the predator and giving it time to escape. Predators may also use mimicry to lure their prey, however. Female fireflies of the genus Photuris, for example, copy the light signals of other species, thereby attracting male fireflies, which are then captured and eaten (see aggressive mimicry).
While successful predation results in a gain of energy, hunting invariably involves energetic costs as well. When hunger is not an issue, in general most predators will not seek to attack prey since the costs outweigh the benefits. For instance, a large predatory fish like a shark that is well fed in an aquarium will typically ignore the smaller fish swimming around it (while the prey fish take advantage of the fact that the apex predator is apparently uninterested). Surplus killing represents a deviation from this type of behaviour. The treatment of consumption in terms of cost-benefit analysis is known as optimal foraging theory, and has been quite successful in the study of animal behavior. Some adaptations of successful predators include speed- built for speed, weapons- sharp teeth and claws, camouflage- to avoid being seen by prey, and depth perception- eyes to the front of the head to judge size and distance.
Social predation allows predators to kill creatures larger than those that members of the species could overpower singly. Lions, hyenas, wolves, dholes, African wild dogs, and piranhas can kill large herbivores that single animals of the same species usually don't dispatch. Social predation allows some animals to organize hunts of creatures that would easily escape a single predator; thus chimpanzees can prey upon colobus monkeys, and Harris's hawks can cut off all possible escapes for a doomed rabbit. Social predation is often complex behavior, and not all social creatures perform it. Even without complex intelligence, some ant species can destroy much larger creatures.
Size-selective predation involves predators preferring 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, in general, safe from predation by lions, but juveniles are vulnerable.
Many antipredator adaptations have evolved in prey populations due to the selective pressures of predation over long periods of time. Some species mob predators cooperatively. Others such as Thomson's gazelle stot to signal to predators such as cheetahs that they will have an unprofitable chase. Many prey animals are aposematically colored 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.
It is fairly clear that predators tend to lower the survival and fecundity of their prey, but on a higher level of organization, populations of predator and prey species also interact. It is obvious that predators depend on prey for survival, and this is reflected in predator populations being affected by changes in prey populations. It is not so obvious, however, that predators affect prey populations. Eating a prey organism may simply make room for another if the prey population is approaching its carrying capacity.
The population dynamics of predator–prey interactions can be modelled 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.
Evolution of predation
Predation appears to have become a major selection pressure shortly before the Cambrian period—around —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 .
Humans and predation
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 from a pest's natural range are introduced to control populations, at the risk of causing unforeseen problems. Besides their use in conservation biology, predators are also important for controlling pests in agriculture. Natural predators are an environmentally friendly and sustainable way of reducing damage to crops, and are one alternative to the use of chemical agents such as pesticides.
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- Wolfram Demonstrations Project: Predator-Prey Equations by Eric W. Weisstein
- Predators, three articles by Olivia Judson, NY Times, Sept. & Oct., 2009