Immigration and emigration in the Sinai Baton Blue butterfly: estimates from a single patch
The movement of individuals among patches of suitable habitat is a key process in metapopulation biology, but is very difficult to observe under natural conditions. Thus, many estimates of rates of movement are indirect and incomplete, and there is little empirical knowledge of the factors affecting immigration and emigration. I studied intensively a local population of Sinai Baton Blue butterflies in a discrete habitat patch. The study lasted the entire adult flight period, and involved almost every individual in the local population. Using these data I attempted to estimate the number of immigrants and emigrants, and identified an important factor affecting inter-patch movement. Early in the season, individuals were resident in the study patch for only a very short time; I assumed this was because they were emigrating, and thus estimated that at least 14% of the population emerging in the study patch emigrated. I assumed that butterflies not caught until they were relatively old, had immigrated to the patch (rather than being missed at a young age), and thus estimated that 13% of the population caught in the patch was composed of immigrants, most of which were females. Individuals assumed to be immigrants arrived in the patch throughout the adult flight period, but older individuals generally arrived later in the season. Timing of migration was almost certainly linked to phenology of the butterfly’s only hostplant, Sinai Thyme. Most butterflies remained in their natal patch. The degree of inter-patch movement estimated using this method was consistent with other metapopulation studies, and would suffice for this species to exist as a metapopulation.