Dispersal among populations is a key component of population biology affecting the dynamics and genetic structure of populations. Within spatial population networks (metapopulations sensu lato), dispersal among populations can enhance network persistence. Despite its importance for metapopulations and for predicting their persistence, dispersal parameters are usually the least well understood. Here, using the Virtual Migration model and 18 years of mark-recapture data for males, females, and the total population, we directly estimated the yearly metapopulation capacity for the alpine butterfly, Parnassius smintheus, residing within an 18 subpopulation network. This Virtual Migration model is particularly useful in that its parameters can be used to directly calculate connectivity, and along with local population colonization and extinction rates, can be used to calculate metapopulation capacity (λM). Metapopulation capacity is an estimate of whether a particular species can persist within a given landscape or network configuration relative to its rates of local population extinction and recolonization. Due to forest encroachment in this alpine system, habitat area has decreased and isolation (forest habitat) has increased over the course of the study, providing an opportunity to determine how these changes affect estimates of metapopulation persistence.
Results/Conclusions
Over the study, we captured 18,234 butterflies 25,439 times and observed 1,020 among subpopulation dispersal events. Estimated dispersal parameters varied over the years, but the only consistent response to forest encroachment was an increase in male dispersal distance within meadow habitat. Estimates of metapopulation capacity varied tremendously among years and sexes, but the network was predicted to persist based on all estimates, except for female butterflies in one year. Surprisingly, metapopulation capacity actually increased over the course of the study. The increase in emigration and dispersal distance in meadow habitat resulting from forest encroachment have more than offset the increase in extinction risk due to habitat loss and decrease in dispersal due to more intervening forest habitat. Thus, over the period time studied, the network is less susceptible to extinction, despite forest encroachment.