Biotic Interactions

Biotic Interactions Between Species

Organisms sharing an environment interact with one another in ways that range from mutually beneficial to one-sidedly harmful. These interactions are a major force shaping community structure and the evolution of the species involved.

Mutualism

In mutualism, both interacting species benefit. A textbook example is the fig–wasp relationship: certain fig trees are pollinated exclusively by a specific species of tiny wasp. The female wasp enters the fig's enclosed inflorescence to lay her eggs, and in doing so pollinates the fig flowers inside; her larvae develop and feed within some of the developing fig fruit, while the fig gets pollinated and produces viable seeds. Neither partner can complete its life cycle without the other, making this an obligate, highly specific mutualism.

Commensalism

In commensalism, one species benefits while the other is neither helped nor harmed. A classic plant example is an orchid growing epiphytically on the branch of a mango tree: the orchid gains physical support and better access to sunlight by perching on the mango's branches, while the mango tree itself derives no benefit and suffers no harm from the orchid's presence (the orchid is not parasitic and draws no nutrients from the mango's tissues).

Parasitism

In parasitism, one species (the parasite) benefits at the direct expense of the other (the host), typically living in or on the host and deriving nourishment from it, often over an extended period, while usually not killing the host outright (since the parasite's own survival depends on the host remaining alive, at least for a time).

Brood parasitism is a specialised and striking form of parasitism, best known in birds such as the cuckoo. The parasitic bird lays its eggs in the nest of another (host) species and leaves all parental care — incubation, feeding, protection — to the unwitting host parent. The cuckoo egg is often remarkably similar in size and colour to the host's own eggs, an adaptation that reduces the chance the host will recognise and reject it, and the resulting cuckoo chick may even eject the host's own eggs or chicks from the nest to monopolise the host parents' care.

Predation

In predation, one organism (the predator) kills and consumes another (the prey). Beyond its obvious role in energy transfer between trophic levels, predation performs two important ecological functions:

• Biological control: predators (and parasites) can be deliberately used to keep the population of a pest species in check without chemical pesticides — a natural, self-sustaining check on population growth.
• Maintenance of species diversity: by preferentially feeding on the most abundant competing prey species, a predator can prevent any single prey species from competitively excluding others, thereby keeping several prey species coexisting in the same habitat rather than one dominant competitor driving the rest out.

Competition

Interspecific competition occurs when two or more species require the same limited resource (food, space, light, nesting sites) and each suffers reduced growth, survival or reproduction because of the other's presence. When two competing species are experimentally or naturally separated — for instance, if a superior competitor is removed from a habitat — the remaining, previously suppressed species often expands rapidly to use the resources and space no longer contested; this expansion of a species' realised niche following the removal of a competitor is called competitive release. It demonstrates that many species occupy a narrower niche in the presence of competitors than they are physiologically capable of occupying alone.

Amensalism …