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.

Using both natural and computer-generated food webs, the researchers ran analytical models to examine how often functional extinctions happen and to identify the circumstances in which they occur. The model food webs were generated based on roles that species tend to play in real food webs, and they followed general rules common to most ecosystems (such as large animals being less abundant than smaller animals). In each web, the relationships between each species—such as who eats what, how much they eat, and how often—was either known (in the case of the natural webs) or calculated (in the case of the model webs).

To investigate what happens in a food web when a particular population declines, the researchers focused on each species separately and increased its mortality rate until an extinction occurred. If the species they manipulated was the one that disappeared, the extinction could be considered a numerical extinction; if a different species in the ecosystem disappeared as a result of the manipulation, the extinction was functional. The researchers found that functional extinctions happened at a surprisingly high rate: the probability of a functional extinction, compared to a numerical one, was 0.49 in the natural food webs and 0.72 in the model webs. In the authors' words, "a species' ecological functionality is often lost long before its existence is threatened."

Furthermore, a species doesn’t have to be down to its last few straggling survivors in order to lose its ecological effectiveness. More than a quarter of the species in the natural food webs and more than half of those in the computer-generated webs became functionally extinct after losing just thirty percent of their population. That’s a pretty frightening statistic: a die-off of less than a third of a species’ members can unbalance an ecosystem enough to trigger the complete extinction of another species in the community.

Food webs are incredibly complicated and nuanced, and the results demonstrate the delicate balance of ecological communities. In the study, many of the species that disappeared as a result of a functional extinction weren’t even directly linked to the dying species in the food web. In other words, they were driven to extinction by a small decline in a species they don’t eat (and which doesn't eat them).

Of course, this study was a simplistic representation of what goes on in actual ecological communities. However, if anything, it’s probably a conservative estimate of the damage that increasing mortality rates and decreasing population sizes can cause. In all likelihood, the effects of a changing world—including climate change, decreasing forest cover, and human-wildlife conflict—can wreak even more havoc on ecosystems than this study suggests.

The bright side is that understanding functional extinctions can inform our decisions about managing and conserving species. For instance, the researchers found that there is an inverse relationship between a species’ body mass and its tendency to go numerically, rather than functionally, extinct; the bigger an animal is, the more likely it is that increasing its mortality rate will drive another species to extinction. This type of knowledge could help prioritize where conservation efforts and resources should go. However, to make this type of information useful, scientists must continue to investigate extinction in a more ecological sense, and policymakers must be willing to take the results into account.

Nature, 2013. DOI: 10.1038/nature12277 (About DOIs).