For example, for a population with a birth rate of 0.5 per year, which is reasonable for terrestrial vertebrates, Table 14.8 shows the probabilities for various population sizes and periods of time. The table shows that the chance of extinction after a fixed period increases greatly as the size of the population decreases. For example, a population of 100 has a 14% chance of disappearing over the next century, whereas a population of 10 has an 82% chance.
This effect has been observed in nature in island ecosystems. On average the lost species are balanced by immigration or by evolution of new species. However, the role of humans is disturbing this balance. Urban and agricultural development may destroy only part of an ecosystem. Yet, even if a portion is preserved out of concern for the wildlife, the chances are increased that the species contained within will be lost. Simply putting a road through a wilderness divides it into two smaller parts. If movement of organisms of a particular species is inhibited from crossing, the overall chances of survival will have been decreased.
These concepts have led to the idea of minimum viable population, which is a level sufficient to protect a species not only from stochastic extinction but also from calamities such as fire or extreme weather. Furthermore, protection of a species requires that a species have multiple populations for two reasons: First, if one population is wiped out, it could be reintroduced from another. Second, having multiple centers of population maintains genetic diversity, which may be necessary to develop protection against evolving diseases.
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