Allee effect and population dynamics in the Glanville fritillary butterfly

The Allee effect - a decrease in population growth rate at low density - may be produced by several mechanisms including difficulty in finding mates at low densities. We present evidence for the Allee effect in natural populations of an endangered butterfly, the Glanville fritillary (Melitaea cinxia) and for two mechanisms that cause it. Specifically, we show that emigration rate increases and the fraction of mated females decreases with decreasing local density. We then demonstrate a connection between mating success and population growth rate and more generally, slightly reduced reproductive success in the smallest butterfly populations in an extensive data set gathered over four years. The Allee effect increases the significance of the rescue effect in metapopulations, and thereby the potential for alternative stable states in metapopulation as well as in local dynamics.