8-4 Species Interactions: Paracitism, Mutualism, and Commensalism

What Are Parasites, and Why Are They Important? Living on or in Another Species

Parasitism occurs when one species (the parasite) feeds on part of another organism (the host), usually by living on or in the host. In this relationship, the parasite benefits and the host is harmed. Unlike a conventional predator, a parasite usually is much smaller than its host (prey) and rarely kills its host.

Tapeworms, disease-causing microorganisms, and other endoparasites live inside their hosts. Ectoparasites attach themselves to the outside of their hosts. Examples are ticks, fleas and mosquitoes.

How Do Species Interact so That Both Species Benefit? Win-Win Relationship

In mutualism, two species interact in a way that benefits both. The pollination mutualism between flowering plants and animals such as insects, birds, and bats is one of the most common forms of mutualism.

Some species benefit from nutritional mutualism. Lichens, hardy species that can grow on trees or barren rocks, consist of colourful photosynthetic algae and chlorophyll-lacking fungi living together.

In gut inhabitant mutualism, vast armies of organisms such as bacteria live in an animal’s digestive tract. The bacteria receive a sheltered habitat and food from their host. In turn, they help break down (digest) their host’s food.

It is tempting to think of mutualism as an example of cooperation between species, but actually it involves each species benefiting by exploiting the other.

How Do Species Interact so That One Benefits but the Other Is Not Harmed? Do No Harm

Commensalism is a species interaction that benefits one species but has little, if any, effect on the other species. One example is a redwood sorrel, a small herb. It benefits from growing in the shade of tall redwood trees, with no known negative effects on the redwood trees.