Predation

Predation is nature's way of passing the energy fixed by plants to higher trophic levels. A sparrow eating a seed is as much a predator as a tiger eating a deer; herbivores are, broadly, predators too.

Predators keep prey populations in check; without them prey could reach very high densities and destabilise the ecosystem.

Exotic species often turn invasive because the new land lacks their natural predators. Prickly pear cactus brought to Australia in the early 1920s spread over millions of hectares of rangeland until a cactus-feeding moth from its native range was introduced.

Biological control of crop pests is based on this ability of a predator to regulate its prey.

Predators also maintain species diversity by easing competition among prey species. When the starfish Pisaster was removed from an enclosed intertidal area on the American Pacific coast, more than 10 invertebrate species went extinct within a year through interspecific competition.

A predator that overexploits its prey drives the prey, and then itself, to extinction; so predators in nature are 'prudent'.

Prey defences: some insects and frogs are camouflaged (cryptically coloured); some are poisonous. The Monarch butterfly tastes very bad to birds because of a chemical it takes up as a caterpillar from a poisonous weed.

Nearly 25 per cent of all insects are phytophagous (feed on plant sap and other parts). Plants cannot run away, so they defend themselves.

Morphological defence: thorns, as in Acacia and cactus, are the commonest.

Chemical defence: Calotropis makes highly poisonous cardiac glycosides, so cattle and goats never browse it. Nicotine, caffeine, quinine, strychnine and opium are also made by plants as defences against grazers and browsers.