Interspecies Interaction

Competition

In interspecific competition, members of two different species use the same limited resource and therefore compete for it. Competition negatively affects both participants (-/- interaction), as either species would have higher survival and reproduction if the other was absent.

Species compete when they have overlapping niches, that is, overlapping ecological roles and requirements for survival and reproduction. Competition can be minimized if two species with overlapping niches evolve by natural selection to utilize less similar resources, resulting in resource partitioning.

Predation

In predation, a member of one species—the predator—eats part or all of the living, or recently living, body of another organism—the prey. This interaction is beneficial for the predator, but harmful for the prey (+/- interaction). Predation may involve two animal species, but it can also involve an animal or insect consuming part of a plant, a special case of predation known as herbivory.

Predators and prey regulate each other's population dynamics. Also, many species in predator-prey relationships have evolved adaptations—beneficial features arising by natural selection—related to their interaction. On the prey end, these include mechanical, chemical, and behavioral defenses. Some species also have warning coloration that alerts potential predators to their defenses; other harmless species may mimic this warning coloration.

Symbiosis

Symbiosis is a general term for interspecific interactions in which two species live together in a long-term, intimate association. In everyday life, we sometimes use the term symbiosis to mean a relationship that benefits both parties. However, in ecologist-speak, symbiosis is a broader concept and can include close, lasting relationships with a variety of positive or negative effects on the participants.

Mutualism

In a mutualism, two species have a long-term interaction that is beneficial to both of them (+/+ interaction). For example, some types of fungi form mutualistic associations with plant roots. The plant can photosynthesize, and it provides the fungus with fixed carbon in the form of sugars and other organic molecules. The fungus has a network of threadlike structures called hyphae, which allow it to capture water and nutrients from the soil and provide them to the plant.

Commensalism

In a commensalism, two species have a long-term interaction that is beneficial to one and has no positive or negative effect on the other (+/0 interaction). For instance, many of the bacteria that inhabit our bodies seem to have a commensal relationship with us. They benefit by getting shelter and nutrients and have no obvious helpful or harmful effect on us.

It's worth noting that many apparent commensalisms actually turn out to be slightly mutualistic or slightly parasitic (harmful to one party, see below) when we look at them more closely. For instance, biologists are finding more and more evidence that our normal microbial inhabitants play a key role in health.

Parasitism

In a parasitism, two species have a close, lasting interaction that is beneficial to one, the parasite, and harmful to the other, the host (+/- interaction).

Some parasites cause familiar human diseases. For instance, if there is a tapeworm living in your intestine, you are the host and the tapeworm is the parasite—your presence enhances the tapeworm's quality of life, but not vice versa.