METAPOPULATION STRUCTURE AND MIGRATION IN THE BUTTERFLY MELITAEA-CINXIA

We describe a spatially realistic metapopulation model and parameterize it for a metapopulation of the butterfly Melitaea cinxia, residing in a network of 50 discrete habitat patches within an area of 15 km2. Parameter values that are difficult to estimate independently are obtained by fitting the model to patterns of patch occupancy and local density. There is no large ''mainland'' population, and the metapopulation appears to survive at an extinction-colonization equilibrium. Empty patches were smaller than the occupied ones, indicating the extinction proneness of especially small local populations; population turnover was 16% between 2 yr. Density in the occupied patches increased with decreasing isolation and with decreasing patch area, suggesting that migration plays an important role in local dynamics. Mark-recapture results confirmed that migration between local populations was common, in contrast to what is frequently assumed for butterflies with well-defined local populations in discrete habitat patches. The modelling results demonstrate that it is possible to have empty habitat patches in a metapopulation in spite of frequent migration, as we observed for M. cinxia. Colonization rate of empty patches may be low for several reasons, including difficulties in mate location at low density (not likely to be important here), conspecific attraction (possibly important), stepping-stone and other forms of nonrandom migration (likely to be important), and weak density dependence (likely to be important). Our results support the assumptions of structured metapopulation models, which demonstrate the possibility of alternative stable equilibria for metapopulations in which migration significantly affects local dynamics.