Not yet gone, but effectively extinct

US Fish and Wildlife Service

When we think of extinction, we often picture the last individual of its kind leading a solitary life until its death marks the species' disappearance from the face of the Earth. Called “numerical extinction,” this is the traditional concept of extinction, and it forms the basis for many conservation decisions.

But there is another type, called “functional extinction,” which takes a more ecological approach. Some scientists argue that the threshold for extinction should not be the complete disappearance of a species, but instead the point at which there aren’t enough individuals left in that species to perform whatever roles it was playing in the ecosystem. A species can be considered functionally extinct when its dwindling numbers cause another species in the same food web to disappear from the natural community first. These extinctions are important to understand since a species can go functionally extinct well before the population is small enough to put it in danger of a numerical extinction.

A recent theoretical paper in Nature demonstrates that functional extinctions occur at a surprisingly high frequency. The paper suggests that they should be considered when making conservation decisions and setting environmental policy.