Determinants of distribution and abundance in the clouded apollo butterfly: a landscape ecological approach

Recent studies on the determinants of distribution and abundance of animals at landscape level have emphasized the usefulness of the metapopulation ap proach, in which patch area and habitat connectivity have often proved to e xplain satisfactorily existing patch occupancy patterns. A different approa ch is needed to study the common situation in which suitable habitat is dif ficult to determine or does not occur in well-defined habitat patches. We a pplied a landscape ecological approach to study the determinants of distrib ution and abundance of the threatened clouded apollo Parnassius mnemosyne b utterfly within an area of 6 km(2) of agricultural landscape in south-weste rn Finland. The relative role of 24 environmental variables potentially aff ecting the distribution and abundance of the butterfly was studied using a spatial grid system with 2408 grid squares of 0.25 ha, of which 349 were oc cupied by the clouded apollo. Both the probability of butterfly presence an d abundance in a 0.25 ha square increased with the presence of the larval h ost plant Corydalis solida, the cover of semi-natural grassland, the amount of solar radiation and spatial autocorrelation in butterfly occurrence. Ad ditionally, butterfly abundance increased with overall mean patch size and decreased with maximum slope angle and wind speed. Two advantages of the em ployment of a spatial grid system included the avoidance of a subjective de finition of suitable habitat patches and an evaluation of the relative sign ificance of different components of habitat quality at the same time with h abitat availability and connectivity. The large variation in habitat qualit y was influenced by the abundance of the larval host plant and adult nectar sources but also by climatological, topographical and structural factors. The application of a spatial grid system as used here has potential for a w ide use in studies on landscape-level distribution and abundance patterns i n species with complex habitat requirements and habitat availability patter ns.