Measures of species biodiversity, such as richness (i.e., the number of species present) and evenness (i.e., their relative abundance), describe an ecological community’s structure. Many factors affect community structure, including abiotic factors (e.g., sunlight and nutrients), disturbances (e.g., fire or flood), species interactions (e.g., predation or competition), and chance events (e.g., foreign species invasion). Certain species—such as keystone species—also play a pivotal role in the structure of a community.

Relative to their abundance, keystone species have a disproportionately large impact on community structure. Keystone species exert top-down control over lower trophic level organisms and reduce those organisms’ exploitation of the ecosystem’s resources. The intertidal sea star (Pisaster ochraceus) is a keystone species that influences the biodiversity of the Pacific coast’s kelp forest ecosystem. If the sea star is removed, the population of their prey species (mussels) increases. Left unchecked, mussels overrun the community and displace other organisms—changing the community’s species composition and reducing biodiversity.

Recognizing keystone species is important for the maintenance and restoration of ecosystems. The North American gray wolf is a keystone species that affects the biodiversity of the Greater Yellowstone Ecosystem (GYE). In the early 1900s, humans hunted the gray wolf into near extinction because ranchers feared the wolf would also target livestock. Since humans eliminated the elk’s primary predator, its population soared. Overgrazing led to the destruction of other organisms’ habitats and altered abiotic factors, such as stream bank stability and nutrient cycling. When gray wolves were reintroduced into the GYE, the ecosystem largely recovered.

Keystone species preserve the balance and often safeguard the existence of a community. However, other ecological roles exist that also impact community structure. For example, foundation species (e.g., kelp) are habitat-forming organisms that support an ecosystem, while dominant species (e.g., mussels) are the most abundant organisms in a community. Ecologists’ knowledge of the roles of the various organisms in an ecosystem allows for more effective conservation and restoration efforts.