Ecologists have used information on metapopulation dynamics to conserve endangered species. We know from mathematical models that the risk of extinction for a species can depend on the number and size of its populations within a metapopulation, and on the rates of dispersal among them. In some metapopulations there are source populations, which serve as a net source of individuals for sink populations, which receive more immigrants than they produce. Thus, although some individual populations may undergo severe decline as a consequence of natural or human-caused factors, source populations within a metapopulation can serve to rescue them.

A dramatic example of such metapopulation dynamics is provided by Edith’s checkerspot butterfly (Euphydryas editha bayensis). This endangered subspecies feeds on only two species of annual plants (California plantain and purple owl’s clover) that are endemic to outcrops of serpentine rock on hills south of San Francisco. In 1960 Paul Ehrlich and his colleagues at Stanford University began studying a population of this butterfly in the nearby Jasper Ridge Biological Preserve and discovered that it was one of several populations within a large, very fragmented metapopulation (Figure 54.17). They followed all the populations over several years and found that they varied in size. During drought years, the butterfly populations declined in size because most of the host plants that the caterpillars relied on died early in the spring, before the caterpillars had a chance to eat them. A severe drought in 1975–1977 led to extinctions of some of the populations. One of the empty habitat patches was repopulated a few years later, most likely by individuals from the largest single population, Morgan Hill, which as late as 1989 contained several hundred thousand butterflies. In 1998, however, the Morgan Hill population, which had historically been the largest in the metapopulation, went extinct. Ehrlich and his colleagues examined 70 years of climate data for the region and concluded that increasing climate variation accounted for the extinction. They also concluded that without a large, stable source population to provide emigrants for recolonization, as the Morgan Hill population did during the 1970s drought, it is unlikely that any of the other populations, which are sink populations, will persist without human intervention.