AREA-DEPENDENT MIGRATION BY RINGLET BUTTERFLIES GENERATES A MIXTURE OF PATCHY POPULATION AND METAPOPULATION ATTRIBUTES

Interpretation of spatially structured population systems is criticall y dependent on levels of migration between habitat patches. If there i s considerable movement, with each individual visiting several patches , there is one ''patchy population''; if there is intermediate movemen t, with most individuals staying within their natal patch, there is a metapopulation; and if (virtually) no movement occurs, then the popula tions are separate (Harrison 1991, 1994). These population types actua lly represent points along a continuum of much to no mobility in relat ion to patch structure. Therefore, interpretation of the effects of sp atial structure on the dynamics of a population system must be accompa nied by information on mobility. We use empirical data on movements by ringlet butterflies, Aphantopus hyperantus, to investigate two key is sues that need to be resolved in spatially-structured population syste ms. First, do local habitat patches contain largely independent local populations (the unit of a metapopulation), or merely aggregations of adult butterflies (as in patchy populations)? Second, what are the eff ects of patch area on migration in and out of the patches, since patch area varies considerably within most real population systems, and bec ause human landscape modification usually results in changes in habita t patch sizes? Mark-release-recapture (MRR) data from two spatially st ructured study systems showed that 63% and 79% of recaptures remained in the same patch, and thus it seems reasonable to call both systems m etapopulations, with some capacity for separate local dynamics to take place in different local patches. Per capita immigration and emigrati on rates declined with increasing patch area, while the resident fract ion increased. Actual numbers of emigrants either stayed the same or i ncreased with area. The effect of patch area on movement of individual s in the system are exactly what we would have expected if A. hyperant us were responding to habitat geometry. Large patches acted as local p opulations (metapopulation units) and small patches simply as location s with aggregations (units of patchy populations), all within 0.5 km(2 ). Perhaps not unusually, our study system appears to contain a mixtur e of metapopulation and patchy-population attributes.