Predation occurs when one animal (the predator) eats another living animal (the prey) to utilize the energy and nutrients from the body of the prey for growth, maintenance, or reproduction. In the special case in which both predator and prey are from the same species, predation is called cannibalism. Sometimes the prey is actually consumed by the predator's offspring. This is particularly prevalent in the insect world. Insect predators that follow this type of lifestyle are called parasitoids, since the offspring grow parasitically on the prey provided by their mother.
Predation is often distinguished from herbivory by requiring that the prey be an animal rather than a plant or other type of organism (bacteria). To distinguish predation from decomposition, the prey animal must be killed by the predator. Some organisms occupy a gray area between predator and parasite. Finally, the requirement that both energy and nutrients be assimilated by the predator excludes carnivorous plants from being predators, since they assimilate only nutrients from the animals they consume. See Food web
Population dynamics refers to changes in the sizes of populations of organisms through time, and predator-prey interactions may play an important role in explaining the population dynamics of many species. They are a type of antagonistic interaction, in which the population of one species (predators) has a negative effect on the population of a second (prey), while the second has a positive effect on the first. For population dynamics, predator-prey interactions are similar to other types of antagonistic interactions, such as pathogen-host and herbivore-plant interactions.
Community structure refers generally to how species within an ecological community interact. The simplest conception of a community is as a food chain, with plants or other photosynthetic organisms at the bottom, followed by herbivores, predators that eat herbivores, and predators that eat other predators. This simple conception works well for some communities. Nonetheless, the role of predator species in communities is often not clear. Many predators change their ecological roles over their lifetime. Many insect predators that share the same prey species are also quite likely to kill and devour each other. This is called intraguild predation, since it is predation within the guild of predators. Furthermore, many species are omnivores, feeding at different times as either predators or herbivores. Therefore, the role of particular predator species in a community is often complex.
Predator-prey interactions may have a large impact on the overall properties of a community. For example, most terrestrial communities are green, suggesting that predation on herbivores is great enough to stop them from consuming the majority of plant material. In contrast, the biomass of herbivorous zooplankton in many aquatic communities is greater than the biomass of the photosynthetic phytoplankton, suggesting that predation on zooplankton is not enough to keep these communities green. See Population ecology