1253
Despite gaps in our understanding of biodiversity, methods exist for estimating the extinction probability of species, taking into account their population sizes, genetic variation, life history traits, and ecology. The IUCN has published categories that define the risk of extinction of species. Species in imminent danger of extinction in all or most of their range are classified as “critically endangered” or “endangered”; those believed to be susceptible to extinction in the near future are classified as “vulnerable.” Biologists consider species in any one of these three categories to be “threatened” (Figure 58.3B).
Rarity in and of itself is not always a cause for concern. Some species may be specialized for rare and unusual habitats and not be particularly abundant by nature. But as you saw earlier, “newly rare” species are of concern because large and rapid reductions in population sizes can lead to smaller effective population sizes through a variety of mechanisms (see Figure 58.2).
Certain aspects of species’ life histories can be important in predicting the ability of species to recover from declines (see Key Concept 54.3). In fishes and mammals, for example, one of the best predictors of extinction risk is age at maturity, a life history trait that influences the rate of reproduction. Ecological requirements can also influence the ability of species to recover from rapid declines. Species with specialized habitat or dietary requirements, for example, are more likely to become extinct than species with more generalized requirements.
Ecologists can apply the principles of the species–