Predation

Predation is the interaction in which one organism, the predator, kills and consumes another, the prey. The classic image is a tiger hunting a deer, but a sparrow eating a seed is, in ecological terms, no less an act of predation. Grazing herbivores that eat plants are, in this sense, not very different from predators either -- from the plant's point of view, the herbivore is its predator.

Predators play several important ecological roles. They keep prey populations under control -- without them, prey species can build up to very high population densities and destabilise an ecosystem, as happens when the loss of frogs allows locust populations to explode and destroy large areas of agricultural crop. This is exactly the principle behind biological pest control in agriculture, which relies on a predator's ability to regulate a pest's population. Predators also help maintain overall species diversity within a community, by moderating the intensity of competition among the various prey species that would otherwise compete more fiercely with each other. When an exotic species is introduced -- accidentally or deliberately -- into a new geographical area without its natural predators, it can turn invasive and spread rapidly and uncontrollably, as has happened with zebra mussels in the intertidal zone of North America. If a predator over-exploits its prey to the point of driving it extinct, the predator itself will then starve and go extinct for lack of food -- which is why predators in nature tend, in effect, to be 'prudent'.

Prey species have, in turn, evolved a wide range of defences to reduce the impact of predation, in an ongoing 'evolutionary arms race': better camouflage for concealment and greater speed for escape are typical prey adaptations, which the predator must then counter with adaptations of its own. Many insects and frogs are cryptically coloured to avoid being spotted at all. Other species take the opposite approach and advertise their unpalatability with bold warning (aposematic) colouration -- the Monarch butterfly (Fig. 13.11) is highly distasteful to bird predators because of a special chemical present in its body, a chemical it acquires during its caterpillar stage by feeding on a poisonous weed. …