Influence of habitat quality and patch size on occupancy and persistence in two populations of the Apollo butterfly (Parnassius apollo)

Recent studies on butterflies emphasize habitat characteristics together with metapopulation parameters (patch area and isolation) giving a more thorough understanding of processes influencing population persistence and patch occupancy, than either of them alone. We studied a coastal and an archipelago population of the Apollo butterfly (Parnassius apollo) in SW Finland. Larvae were surveyed for four years in both populations. Counting larvae on three consecutive days and temporarily removing them tested the survey accuracy. The removals showed four times higher larval abundance in the archipelago than on the coast. Survey methods were reliable, provided that empty patch status was not based on single visits only, if larval abundance was low. On the coast, large patches, and patches with high host-plant abundance were often occupied. In the archipelago, patches rich in host-plant were often occupied whereas patch area did not affect patch occupancy. In both populations, the probability of patches being occupied for three consecutive years increased with increasing host-plant abundance and patch area. Conservation of P. apollo depends on securing host-plant abundance on large enough patches in both study systems. In these systems, even crude habitat measures prove useful for understanding ecological processes behind observed patterns.