METAPOPULATION STRUCTURE AND MIGRATION IN THE BUTTERFLY MELITAEA-CINXIA

We describe a spatially realistic metapopulation model and parameteriz e it for a metapopulation of the butterfly Melitaea cinxia, residing i n a network of 50 discrete habitat patches within an area of 15 km2. P arameter values that are difficult to estimate independently are obtai ned by fitting the model to patterns of patch occupancy and local dens ity. There is no large ''mainland'' population, and the metapopulation appears to survive at an extinction-colonization equilibrium. Empty p atches were smaller than the occupied ones, indicating the extinction proneness of especially small local populations; population turnover w as 16% between 2 yr. Density in the occupied patches increased with de creasing isolation and with decreasing patch area, suggesting that mig ration plays an important role in local dynamics. Mark-recapture resul ts confirmed that migration between local populations was common, in c ontrast to what is frequently assumed for butterflies with well-define d local populations in discrete habitat patches. The modelling results demonstrate that it is possible to have empty habitat patches in a me tapopulation in spite of frequent migration, as we observed for M. cin xia. Colonization rate of empty patches may be low for several reasons , including difficulties in mate location at low density (not likely t o be important here), conspecific attraction (possibly important), ste pping-stone and other forms of nonrandom migration (likely to be impor tant), and weak density dependence (likely to be important). Our resul ts support the assumptions of structured metapopulation models, which demonstrate the possibility of alternative stable equilibria for metap opulations in which migration significantly affects local dynamics.