Spatial variation in relative abundances of two butterfly species sharing both host plant and natural enemies

The decline of insect populations is of great concern because they play an essential part in several services that are key for ecosystem functioning and human well-being. Therefore, full understanding of the processes and factors shaping spatial variation in insects is required for their effective conservation. Here, we study a system comprising two congeneric butterfly species (Brimstone Gonepteryx rhamni and Cleopatra G. cleopatra) that share both host plants and natural enemies and analyse whether biotic and/or abiotic factors explain their relative abundances. The two species coexist in continental Spain but not on a nearby archipelago, where only the Cleopatra occurs. The hypotheses tested were based on (H1) dispersal behaviour; (H2) apparent competition mediated via shared parasitoids; and (H3) environmental conditions (overwintering habitat availability, abundance of host plants and temperature). H1 explained differences in Brimstone abundance between climate regions on the mainland since in warmer summers populations increased in cooler areas but decreased in warmer areas. Cleopatra did not show the same pattern but was found to have twice the number of summer adults on one island than on the mainland. It is unlikely that H2 can explain this result because, although richer parasitoid communities were found on the mainland, larval mortality rates were similar. H3 was important in explaining variation in abundances between sites within each climate region even though similar environmental conditions were found on the mainland and on the islands. Our study demonstrates the complexity of any attempt to understand insect population dynamics in space due to the number of factors that are potentially involved. We argue thus that a more comprehensive approach taking into account landscape topography and resource connectivity on a broader scale is required to unravel the factors shaping the relative abundance of insects in island systems.