Just as all individuals must die, all species eventually become extinct. However, the life spans of species are not random: some groups of species tend to have high rates of extinction and short life spans, whereas other groups have low rates of extinction. The risk of extinction is higher for some taxonomic groups than others. For example, within North America a high proportion of aquatic species are endangered. The factor most clearly related to endangerment is body size, where species with larger body size are more likely to become extinct. For example, the megafauna of North America, which included such species as the woolly mammoth, sabertooth cat, and the giant ground sloth, were lost approximately 8000 years ago. Analyses of extinction risk among families of birds showed that larger body size and lower fecundity were related to higher chance of extinction, and this remains the case today.
Another clear pattern is that species with small geographic ranges tend to be more prone to extinction. This is why many conservation efforts focus on endemic species - these are species whose distribution is limited to only one defined area (e.g., a country, ecoregion, or island). Depending on how small the geographic distribution is, an endemic species can be quite vulnerable to human perturbations because disruption over a limited area can lead to their extinction globally. Lastly, species with very specialized requirements are most likely to be endangered - largely because if something goes wrong with their diet, or habitat, they are unable to thrive in altered conditions. In sum, species with large body size, narrow geographic range, and specialized habitat or food requirements are more prone to extinction. The upshot of this is that as we accelerate the extinction rate, we will increasingly find ourselves in a world filled with small bodied, widely distributed, generalist species - science fiction projections of a world populated by rats, cockroaches, and gulls may not be far off the mark.
Discussion of extinction and endangered species often create an impression of hopelessness and despair. Fortunately, by paying attention to the science of endangered species, it is possible to take measures to recover species. Approximately 10% of the populations of threatened and endangered species in the USA are actually now increasing, and 30% are stable. Also globally, among species with a known trend, approximately 15% of the threatened bird species and 40% of threatened amphibian species have stable or increasing populations. In addition, several prominent species that were at one time endangered have in fact been recovered. The gray whale off the coast of California has now been restored to historical population levels thanks to a cessation of harvest. The American alligator may be an even more dramatic success story. This species was near extinction in the mid-twentieth century and now has a population of nearly a million in the US. Its source of endangerment was hunting for the skins and loss of habitat due to the draining of wetlands. With the abatement of these threats, the alligator population quickly recovered. In some cases Herculean captive breeding programs have rescued species from the brink of extinction. The Peregrine falcon had totally disappeared from eastern North America by 1964. A massive breeding program that drew on falcons from Europe, Australia, and South America yielded the release of 1300+ birds between 1974 and 2000. The reintroduction was so successful that these falcons are no longer considered endangered. If threats are abated and a species can be bred in captivity, then endangered species need not remain at risk forever.
There are also other reasons for optimism. Because habitat loss is understood to be the primary threat to species, many conservation-minded organizations and government agencies have shifted from a species to a habitat conservation focus. In the process they have secured larger areas where key ecological processes are more likely to remain intact, and larger populations of endangered species can potentially persist or develop. Recently, the rediscovery of the ivory-billed woodpecker in the Big
Woods of Arkansas, USA was thought to be partially the result ofprevious efforts to conserve bottomland hardwood forests in the region. By preserving large habitat landscapes, we can overcome our lack of species knowledge, and thereby protect species we think we lost or do not even know exist. Beyond a habitat-based approach, our greatest scientific need when studying endangered species is likely to be more attention to evolutionary adaptability. Human impacts are so great and so rapid, that the best insurance against extinction will often be adaptability. It is unclear to what extent we can actually manage adaptability in endangered species, as opposed to simply having to work with the situation as it stands without any proactive efforts. Endangered species with ample genetic variability and behavioral plasticity may well be able to get through bottlenecks of low population size and recover from their at-risk status.
See also: Biodiversity; Island Biogeography; Population Viability Analysis.
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