How much does an individual habitat fragment contribute to metapopulation dynamics and persistence?

We derive measures for assessing the value of an individual habitat fragment for the dynamics and persistence of a metapopulation living in a network of many fragments. We demonstrate that the most appropriate measure of fragment value depends on the question asked. Specifically, we analyse four alternative measures: the contribution of a fragment to the metapopulation capacity of the network, to the equilibrium metapopulation size, to the expected time to metapopulation extinction and the long-term contribution of a fragment to colonization events in the network. The latter measure is comparable to density-dependent measures in general matrix population theory, though some differences are introduced by the fact that "density dependence" is spatially localized in the metapopulation context. We show that the value of a fragment depends not only on the properties of the landscape but also on the properties of the species. Most importantly, variation in fragment values between the habitat fragments is greatest in the case of rare species that occur close to the extinction threshold, as these species are likely to be restricted to the most favorable parts of the landscape. We expect that the measures of habitat fragment value described and analysed here have applications in landscape ecology and in conservation biology.