Recognition that many populations are in fact metapopulations was firmly established around 1970, but there was a delay of around 20 years before that recognition was translated into action and an increasing number of studies placed metapopulation dynamics prominently on the ecological stage. Now, the danger is not so much one of neglect, but that all populations are thought of as metapopulations, simply because the world is patchy.
Central to the concept of a metapopulation is the idea, emphasized by Andrewartha and Birch back in 1954, that habitable patches might be uninhabited simply because individuals have failed to disperse into them. To establish that this is so, we need to be able to identify habitable sites that are not inhabited. Only very rarely has this been attempted. One method involves identifying characteristics of habitat patches to which a species is restricted and then determining the distribution and abundance of similar patches in which the species might be expected to occur. The water vole (Arvicola terrestris) lives in river banks, and in a survey of 39 sections of river bank in North Yorkshire, UK, 10 contained breeding colonies of voles (core sites), 15 were visited by voles but they did not breed there (peripheral sites) and 14 were apparently never used or visited. A 'principle component' analysis was used to characterize the core sites, and on the basis of these characteristics a further 12 unoccupied or peripheral sites were identified that should have been suitable for breeding voles (i.e. habitable sites). Apparently, about 30% of habitable sites were uninhabited by voles because they were too isolated to be colonized or in some cases suffered high levels of predation by mink (Lawton & Woodroffe, 1991).
Habitable patches can also be identified for a number of rare butterfly species because the larvae feed on only one or a few patchily distributed plant species. Thomas et al. (1992) found that the patches that remained uninhabited were small and isolated from the sources of dispersal: the butterfly Phlebejus argus was able to colonize virtually all habitable sites less than 1 km from existing populations. Indeed, the habitability of some of the isolated (previously uninhabited) sites was established when the butterfly was successfully introduced (Thomas & Harrison,
1992). This is the crucial test of whether a site is really habitable or not.
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