A mesoscale approach to extinction risk in fragmented habitats

Assessing the fate of species endangered by habitat framentation(1-3) using spatially explicit and individual-based models(4-7) can be cumbersome and requires detailed ecological information that is often unavailable. Convers ely, Levins-like(8) macroscale models(9,10) neglect data on the distributio n of local numbers, which are frequently collected by field ecologists(11-1 3). Here we present an alternative, mesoscale approach for metapopulations that are subject to demographic stochasticity, environmental catastrophes a nd habitat loss. Starting from a model that accounts for discrete individua ls in each patch and assumes a birth-death stochastic process with global d ispersal(14,15), we use a negative-binomial approximation(16) to derive equ ations for the probability of patch occupancy and the mean and variance of abundance in each occupied patch(17). A simple bifurcation analysis(18) can be run to assess extinction risk. Comparison with both the original model and a spatially explicit model with local dispersal proves that our approxi mation is very satisfactory. We determine the sensitivity of metapopulation persistence to patch size, catastrophe frequency and habitat loss, and sho w that good dispersers are affected more by habitat destruction than by env ironmental disasters.