The Community Effect

Species can interact directly through food chains and in other ways, such as symbiosis and competition. One species can also affect other species indirectly, by influencing members of the community that, in turn, affect another set of community. One species can also affect the environment, which then affects a group of species in the community. When the interaction between two species leads to changes in the presence or absence of other species or in a large change in abundance of other species, then a community-level interaction is said to have occurred. Here, a two-species interaction affects the entire community.

Example:  Interactions at the community level are illustrated by the sea otters of the Pacific Ocean. One of the preferred food of sea otters is sea urchins. Sea urchins feed on kelps, the large brown algae that form undersea forests. Kelp beds are important habitat for many species and are the location for the reproduction of some of these species. Sea urchins do not eat the entire kelp. Instead, they graze along the bottom of the beds, feeding on the base of the kelp, called holdfasts. When holdfasts, which attach kelp to the bottom, are eaten through, the kelp float free and die.

Where sea otters are abundant, kelp beds are abundant and there are few urchins. Where sea otters lack, sea urchins are abundant and there is little kelp. Experimental removal of sea urchins led to an increase in kelp.

Otters affect the abundance of kelp indirectly. Sea otters neither feed on kelp nor protect individual kelp plants from attack by sea urchins. Sea otters reduce the number of sea urchins. With fewer sea urchins, there is less destruction of kelp. With more kelp, there is a larger habitat for many other species, so, indirectly, sea otters increase diversity. Thus sea otters have a community-level effect.

This example demonstrates that community-level effects can alter the distribution and abundance of individual species. When a species such as the sea otters has a large effect on its community or ecosystem, it is called a keystone species, or a key species. The balance of the entire system is keyed to the activities of this species; its removal or alteration of its role within the ecosystem would change the basic nature of the community.

                 

Fig.: Otters, Purple Urchins, Kelp

The existence of community-level effects tells us that there are processes that can take place only because of the existence of a set of species interacting along a food chain.