Concepts: Population interactions: predation and competition

3.1 A world without single species

No habitat on earth holds just one species. The minimum a species needs is one more species to feed on. Even a plant that makes its own food cannot manage alone, because it needs soil microbes to break down organic matter and release nutrients, and it needs an animal for pollination. Organisms therefore live in webs of interaction.

Interspecific interactions involve populations of two different species and may help, harm or do nothing to each partner. Writing plus for benefit, minus for harm and zero for no effect gives the full set: mutualism (+, +), competition (-, -), predation (+, -), parasitism (+, -), commensalism (+, 0) and amensalism (-, 0). In mutualism both partners gain; in competition both lose; in predation and parasitism one gains and the other loses; in commensalism one gains and the other is unaffected; amensalism harms one partner and leaves the other untouched. In predation, parasitism and commensalism the two species also live in close contact.

3.2 Predation

Predation is nature's way of moving the energy that plants fix to higher trophic levels. The picture that comes to mind is a tiger and a deer, but a sparrow eating a seed is a predator too. Animals that eat plants are listed separately as herbivores, yet in a broad ecological sense they act like predators.

Predators do more than transfer energy. They keep prey numbers under control, and without them a prey species could grow dense enough to destabilise the ecosystem. Exotic species often run wild in a new area because their natural predators are missing. The prickly pear cactus, brought to Australia in the early 1920s, spread over millions of hectares of rangeland and was only controlled after a cactus-feeding moth was introduced from its native home. Biological pest control rests on the same idea, using a predator's ability to regulate prey numbers. Predators also help species diversity by reducing competition among prey species. In the rocky intertidal zone of the American Pacific coast, removing the starfish Pisaster from an enclosed area led to the extinction of more than ten invertebrate species within a year through interspecific competition.

A predator that overexploits its prey can drive the prey extinct, and then starve itself. That is why predators behave prudently in nature. Prey, in turn, have evolved defences. Some insects and frogs are cryptically coloured so they are hard to spot. Some are poisonous and are avoided. During the caterpillar stage the Monarch feeds on a poisonous weed and stores the chemical, which makes birds avoid it.

On the plant side, herbivores play the predator. Roughly one insect species in four feeds on plant sap and tissue, and a plant cannot flee. They have evolved many defences instead. Thorns, as in Acacia and cactus, are the most familiar morphological defence. Many plants store chemicals that make an herbivore sick or disturb its feeding, digestion or reproduction. Calotropis produces highly poisonous cardiac glycosides, and cattle and goats leave it alone. Nicotine, caffeine, quinine, strychnine and opium are commercial plant products that the plants originally made as defences against grazers.

3.3 Competition

Darwin treated interspecific competition as a strong force in evolution. Competition is often described as a contest between closely related species for the same limited resource, but that description is incomplete. Unrelated species compete too: flamingoes visiting certain shallow South American lakes and the resident fish there both eat the same zooplankton. Resources also need not be limiting. In interference competition, one species reduces the feeding efficiency of another simply by being present and interfering, even when food and space are plentiful. So competition is better defined as a fall in one species' fitness, read through its r, when a second species shares the habitat.

Lab experiments show the pattern clearly. Gause and others found in the laboratory that with limited resources the stronger competitor eventually removes the weaker one. Evidence from nature is less direct but exists. The Abingdon tortoise on the Galapagos Islands became extinct within a decade of goats being introduced, apparently because the goats browsed more efficiently. Competitive release is another kind of evidence: a species restricted to a small area by a superior competitor expands dramatically when that competitor is removed. Connell's field experiments showed that on the rocky coasts of Scotland the larger barnacle Balanus dominates the intertidal zone and keeps out the smaller barnacle Chathamalus. Herbivores and plants seem to suffer more from competition than carnivores do.

Gause's competitive exclusion principle says that two closely related species using the same resources cannot coexist forever, and the weaker one will eventually disappear. That holds when resources are limiting, not otherwise. Later work does not support the principle as a sweeping rule. Species facing competition may evolve ways to coexist. One such way is resource partitioning: two species may divide the resource by feeding at different hours or searching in different ways. MacArthur showed five closely related warblers sharing one tree coexisting through differences in their search behaviour.