The Environmental Role of Keystone Species
Does Ecosystem Balance Always Depend on One Definitive Organism?
Oct 8, 2009
Keystone species is the phrase often used to indicate any animal or plant that plays an important part in balancing its environment. In architecture, the keystone supports the structure of an arch, thus a comparison is made with species that ‘support’ their environment.
The classic example used when discussing keystone species is that of sea otters (Enhydra lutris) whose presence is crucial to kelp beds as they eat sea urchins (Strongylocentrotus sp), who in turn prey on the kelp. But a recent study has shown that the return of the sea otter to some coastal habitat has not had the expected impact on the environment. A clearer picture of what truly defines a keystone species may be needed. Likewise, species may be keystone in one environment and not in another.
A More Exact Definition of the Term Keystone Species
The original intent of the concept of keystone species, introduced by Robert T. Paine a professor of zoology at the University of Washington, in 1969, was to highlight those organisms whose presence or absence caused significant changes to the ecosystem.
The definition of a keystone species, according to a 1993 paper by Mills, Soule and Doak, is that they are
- organisms whose presence is crucial to maintaining the organization and diversity of their ecological communities
- exceptional, relative to the rest of the community in their importance
These criteria are what determines the keystones species designation. If you remove any species from an ecosystem, there will be changes to that ecosystem. But when a keystone species is removed, the changes are dramatic.
When is the Impact Dramatic Enough to Declare a Keystone Species?
In some cases, there may be universal agreement. The changes that occurred in Yellowstone National Park when the wolf was reintroduced were significant. Trees, elk, deer, fox and coyote in that ecosystem were all affected, reducing the need for some of the human intervention that controlled or protected their numbers.
In other situations it is not so clear. For example, the impact of sea otters on the ecosystem around Vancouver Island, Canada in a short, six month period was significant enough that scientists agree they are keystone species there. In a study of the waters around the Semichi Islands in the Aleutian chain, however, it was shown that their return has not resulted in the same level of change, even after five years.
Thus a species may be considered keystone in one area but in another they may be simply part of the interdependent ecosystem. But whether a species is keystone or one part of the complex ecosystem web, the words of Aldo Leopold should resonate with anyone involved in ecosystem conservation, “The first rule of intelligent tinkering is to save all the parts.”
By preventing any further loss of species ecosystem balance is more likely to be maintained.
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