A predator is an animal or other organism that hunts and kills other organisms for food in an act called predation. Most predators are carnivores. Some predators are omnivores. The difference between a predator and a parasite is that for a predator killing the prey is necessary for consuming it, but for parasites it is not even desirable because a parasite lives on or in its host. Some might consider herbivores to be predators as well, but this is arguable as most herbivores only consume parts of their food species, leaving the remainder alive. However, where the "prey" consists of single-celled algae, the activities of the herbivorous grazer is generally of the same nature as that of a carnivore. As usual in ecology as most fields of study, there is seldom consensus on the distinctions; some ecologists prefer functional definitions like the one outlined above, others rather look at the ecological dynamics the relationships between the species create.
There may be hierarchies of predators; for example, though small birds prey on insects, they may in turn be prey for snakes, which may in turn be prey for hawks. A predator at the top of its food chain (that is, one that is preyed upon by no organism) is called an apex predator. Sometimes an apex predator 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, can have drastic cascading effects on the equilibrium of many other populations in the ecosystem. In this instance the organism may be described as a keystone predator.
The Volterra-Lotka equations describe a simple mathematical model of the interaction between predators and their prey.
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