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.
Evolutionary strategies in parasitism and predation
(Intensity-dependent: green, roman;
Intensity-independent: purple, italics)
||Single host, stays alive
||Single host, dies
(fitness > 0)
|Trophically transmitted parasite[a]
Trophically transmitted pathogen
(fitness = 0)
|Trophically transmitted parasitic castrator
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.