So far in this course we've mentioned only deterministic threats to the persistence of species.1 Deterministic forces are guaranteed to drive a population to extincition in a relatively short period of time. Overexploitation, development, deforestation, and disease are examples of deterministic threats to persistence. Deterministic threats to persistence are traditionally where conservationists have focused most of their efforts. We eliminate the use of DDT, we set aside reserves or national parks, we use captive breeding to increase the number of individuals. More generally, first we try to identify the factors that are causing a species to decline, and then we try to find ways to mitigate or eliminate those factors.
One of the important insights from population genetics and population ecology that was incorporated into conservation thinking in the 1980s was the idea that random events in small populations may have a large impact on population dynamics and population persistence. In other words, in addition to any deterministic threats they may face, small populations may also face stochastic threats to their persistence.2 If the rate of population growth varies from one generation to the next, a series of unlucky generations in which there are successive declines in population size can lead to extinction even if the population is growing, on average.