Small populations are vulnerable to local extinction, but a species has a greater likelihood of persistence where there are a number of local populations interconnected by occasional movements of individuals among them. Such a set of subdivided populations is often termed a “meta population” (Hanski 1999). Two main kinds of meta population have been described. A mainland-island model is where a large mainland population (such as a conservation reserve) provides a source of emigrants that disperse to nearby small populations.
The mainland population has a low likelihood of extinction, where as the small populations become extinct relatively frequently. Emigration from the mainland supplements the small populations, introduces new genetic material and allows recolonization should local extinction occur. A second kind of meta population is where the set of interacting populations are relatively similar in size and all have a likelihood of experiencing extinction. Although colonization and extinction may occur regularly, the overall population persists through time. The silver-spotted skipper (Hesperia comma), a rare butterfly in the UK, appears to function as a meta population (Hill et al. 1996). In 1982, butterflies occupied 48 of 69 patches of suitable grassland on the North Downs, Surrey. Over the next 9 years, 12 patches were colonized and seven populations went extinct. Those more susceptible to extinction were small isolated populations, whereas the patches more likely to be colonized were relatively large and close to other large occupied patches.
The conservation management of patchily distributed species is likely to be more effective by taking a metapopulation approach than by focusing on individual populations. However, “real world” populations differ from theoretical models. Factors such as the quality of habitat patches and the nature of the land mosaic through which movements occur are seldom considered in theoretical models, which emphasize spatial attributes (patch area, isolation). For example, in a meta population of the Bay checker spot butterfly (Euphydryas editha bayensis) in California, USA, populations in topographically heterogeneous fragments were less likely to go extinct than those that were in topographically uniform ones. The heterogeneity provided some areas of suitable topo climate each year over a wide range of local climates (Ehrlich and Hanski 2004). There also is much variation in the structure of subdivided populations depending on the frequency of movements between them. At one end of a gradient is a dysfunctional metapopulation where little or no movement occurs; while at the other extreme, movements are so frequent that it is essentially a single patchy population.
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