M m

-3 -2.5 -2 -1.5 -1 -0.5 0 log10 Metapopulation capacity (am)

Figure 3 Metapopulation size of the Glanville fritillary butterfly (Melitaea cinxia) as a function of the metapopulation capacity in 25 habitat patch networks. The vertical axis shows the size of the butterfly metapopulation based on a survey of habitat patch occupancy in 1 year. The empirical data have been fitted by a spatically realistic model. The result provides a clear empirical example of the extinction threshold. From Hanski I and Ovaskainen O (2000) The metapopulation capacity of a fragmented landscape. Nature 404: 755-758.

influence the patch-specific extinction and colonization rates, but it takes time before the metapopulation has reached a new quasi-equilibrium following environmental change. The length of the transient period is longer when the change in landscape structure is greater, when the rates of extinction and colonization are lower, and when the new quasi-equilibrium following environmental change is located close to the extinction threshold. The latter result has important implications for conservation. Species that have become endangered due to recent changes in landscape structure are located, by definition, close to their extinction threshold, and hence the length of the transient period in their response to environmental change is predicted to be long. This means that we are likely to underestimate the level of threat to endangered species, because many of them do not occur at quasi-equilibrium with respect to the present landscape structure but are only slowly declining due to past habitat loss and fragmentation.

Considering a community of species, the term extinction debt refers to situations in which, following habitat loss, the threshold condition is not met for some species, but these species have not yet had time to go extinct. More precisely, the extinction debt at a given point in time is the number of extant species that are predicted to go extinct in the future because the threshold condition is not satisfied for them.

Reserve Selection

Setting aside a sufficient amount of habitat as reserves is essential for conservation of biodiversity. Reserve selection should be made in such a manner that a given amount of resources for conservation makes a maximal

contribution toward maintaining biodiversity. In the past, making the optimal choice of reserves out of a larger number of potential reserves was typically done by selecting reserves that together would include the largest number of species, without any consideration for the long-term viability of the species. More appropriately, we should ask the question which choice of reserves maintains the largest number of species to the future, taking into account spatiotemporal dynamics of species and possibly and preferably also predicted changes in climate and land use. Metapopulation models can be incorporated into analyses that aim at providing solutions to such questions.

extinction threshold, a critical amount and configuration of habitat that is necessary for long-term metapopulation persistence. The models thus predict that with increasing habitat loss and fragmentation, species will go extinct before all habitat has been lost. Spatially realistic metapopulation models combine a description of landscape structure with a model of the extinction and colonization processes. These models can be parametrized with empirical data and applied to real metapopulations for the purposes of research, management, and conservation.

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