Communities and Ecosystems

Positive Ecological Interactions

Many positive interactions take place in an ecosystem in which organisms live in symbiosis, a beneficial ecological relationship between two or more organisms living in direct contact with each other.
Different species living in a community interact with one another in different ways. Many different types of interactions occur. Food chains and food webs are used to describe interactions between organisms according to what they eat. However, there are other interspecific interactions that affect populations within a community. Symbiosis is the ecological relationship between two or more organisms that live in direct contact. There are many types of symbiosis. One type, in which at least one of the interacting species benefits from the interaction, is called a positive interaction.


In mutualism, all species interacting with one another will benefit.
Mutualism is an interaction in which both organisms benefit from the relationship, a positive interaction form of symbiosis. There are two common types of mutualism: obligate mutualists and facultative mutualist. An obligate mutualist is an organism that must have its partner in order to survive. A facultative mutualist is an organism that can survive on its own but for which there is an added benefit if it remains with another organism.

An example of complex obligate mutualism is leaf cutter ants, fungus, and bacteria. The ants cut and harvest leaves from different species of plants. These leaves are carried back to their colony, where the ants grind up the plant matter. The ants inoculate the leaves with a fungus that produces fruiting bodies, which are used as a food source for the ant colony. The ants use bacteria that lives on their body to protect this fungus garden against pathogens that would destroy their fungal partner. The relationship between the ant, fungus, and bacteria is one of mutualism.

An example of facultative mutualism is a plant that produces a fruit that is eaten by a bird. The fruit helps the bird survive, and the bird helps with dispersing seeds when they move through its digestive system and are deposited in locations that the plant could not reach on its own. Both organisms could survive on their own, but there is a benefit to each if they remain in the other's environment.


In commensalism, one species benefits, but the other is not affected either positively or negatively by the interaction.
Commensalism, a positive interaction form of symbiosis, is an interaction in which one species benefits but the other neither benefits nor is harmed. Most often this occurs between a larger organism and a smaller one. The smaller organism often receives the benefit of the relationship. An example of commensalism occurs with cowbirds and cattle. Cowbirds eat insects that become available as cattle wander through the grasslands to feed. Cowbirds follow the cattle, eating the insects that are disturbed by the cattle's movement. In this case, the cattle do not benefit, but they also not harmed by the cowbirds as they follow the cattle to eat insects.