Butterfly Diversity and Dispersal in Fragmented Grasslands

Sammanfattning: During the last decades, the distribution and abundance of many European species of birds, plants and insects in agricultural landscapes have declined due to agricultural intensification. To be able to mitigate these declines, it is essential to know the relative importance on local and regional ecological processes on population persistence and patterns of species richness. I have studied the effect of landscape composition on local species richness, abundance and dispersal patterns of butterflies, and in one of the papers also on bumblebees. In the first study, I showed that the species richness of butterflies in southern Sweden had declined significantly during the last decades, and that the observed declines could be related to changes in local habitat quality rather that to landscape composition. However, a larger turnover of species also in sites where species richness was constant indicated that metapopulation dynamics might be important. In the second study, I studied butterfly species richness in similar grasslands situated in contrasting landscapes. A high proportion of semi-natural grassland at the landscape scale increased species richness at the local scale. In the third study, I found that the effect of landscape composition on species richness was dependent on the mobility of the species, with intermediately mobile species most strongly affected. By studying species richness and population densities at different distances from semi-natural grasslands and using mark-recapture techniques to study butterfly movements, I found that in intensively farmed agricultural landscapes, semi-natural grassland can act as population sources, contributing to higher species richness and population densities in their surroundings. So called ecological corridors have been suggested to increase dispersal between local populations. I found no evidence of this when studying the dispersal of three grassland butterflies, but there was some evidence of a diffusion of butterflies over shorter distances along the corridors. In conclusion, my results clearly show that population processes at the landscape scale are important for population persistence and species richness. However, they also show that different landscape ecological processes may be important, which implies that both thorough studies and explicit objectives are needed in order to be able to suggest measures to counteract biodiversity loss.