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| What Factors Determine Extinction Probability? |
Other factors being equal, species that have high population sizes are more likely to persist than those with low population sizes. Very small populations are likely to go extinct just by chance in a process called demographic stochasticity. As an extreme case, consider a sexual species that has just two individuals. If both members of the pair are the same gender, it is doomed. Even if the pair does include a male and a female, the species cannot persist unless it produces offspring that are of both genders. The risk of demographic stochasticity leading to extinction is most severe for species with population sizes below about 10 but still is a hazard up until a population size of around 50 to 100, especially for species with low birth rates. Compared with sexual species, demographic stochasticity would be less of a factor for asexuals such as dandelions because a single individual can reproduce without the need for others.
Species with population sizes that number in the hundreds to a few thousand, while not at risk for extinction due to demographic stochasticity, still face other risks. The random evolutionary force of genetic drift reduces genetic variation every generation. The strength of genetic drift is inversely proportional to population size. Thus, species with lower population sizes generally have less genetic variation than their more numerous counterparts. Species that have little genetic variation are at risk of being wiped out by disease. They are also less able to respond to other changes such as global warming. Although there is some disagreement, the consensus is that species with populations above 5,000 are probably safe from extinction because of these genetic factors.
Even species with very large population sizes can go extinct. For instance, a species faces extinction if its habitat is lost and it cannot find a suitable replacement. One striking example is that of the passenger pigeon. During the early 1800s the passenger pigeon (Fig. 2) had a population size in the billions, on the order of the current human population. Overexploitation by hunters and habitat degradation caused its numbers to rapidly dwindle. As its numbers decreased, the species became vulnerable to the genetic factors listed above and then demographic stochasticity. In September 1914, as World War I was beginning, the last passenger pigeon died in captivity. This species went from very abundant to extinct in a century.