In some taxa, such as butterflies, annuals, and amphibians, population size varies dramatically from generation to generation. In recent years, populations may be so large that they seem in little danger of extinction.
However, years of abundance can be misleading when followed by subsequent years of low abundance. Typically, in a population of individuals experiencing extreme dimensional fluctuations, the size of the population required to ensure continued persistence (i.e., la population minimal viable (MVP)) is actually much closer to the smallest than the largest number of individuals in any given year .
However, during years of low abundance, a phenomenon known as a bottleneck in the population may occur, i.e. the small size of the population may result in the loss of rare alleles from one generation to the next. Population bottlenecks can lead to greater inbreeding depression which in turn reduces reproductive success and increases susceptibility to disease.
Low genetic diversity in the great white sharks (Carcharodon carcharias, VU), in the South African Indian Ocean is thought to be the result of a bottleneck of inhabitants. New populations founded by only a few individuals are vulnerable to a special type of demographic bottleneck, the founder effect.
Individual founders of a new population by definition start with low genetic diversity, far less than the original population left behind by the founders. This low genetic diversity defines the new population at risk of a further decrease in genetic diversity, which have long-lasting effects.
This situation can occur naturally when only a small number of individuals disperse to establish a new population or when to found individuals come from a small population that suffers from low genetic diversity.
Being aware of these concerns is especially important for transfer or captive breeding. For example, to prevent the extinction of the world’s smallest gazelle, Speke’s gazelle (Gazella spekei, EN), a captive population of this species, almost entirely confined to Somalia, was established in the United States.
The founding population for this captive breeding project consisted of a single male and three females, resulting in severe levels of inbreeding depression and high mortality rates in offspring. Understanding the importance of managing genetic diversity can help avoid these and other challenges that can threaten the success of relocation projects.