Landscape composition, connectivity and fragment size drive effects of grassland fragmentation on insect communities.
Calcareous grasslands are among the most species-rich habitats in Europe, but are increasingly threatened due to abandonment and fragmentation. Little is known about how the surrounding landscape influences fragmentation effects. Here, we focus on the interaction between habitat fragmentation and landscape composition on leafhoppers, a highly diverse group of insects, including many species that are likely to be vulnerable to changes in their environment. We selected 14 small and 14 large fragments of calcareous grassland in Central Germany, differing in isolation from other calcareous grasslands and in composition of the surrounding landscape. Leafhoppers, sampled by sweep netting, were either specialists that depended on calcareous grasslands or generalists that could use the landscape matrix, but still required low-productivity habitats. Increasing habitat isolation reduced leafhopper species richness in simple (dominated by arable crops), but not in complex landscapes. This effect was driven by the generalist species. In simple landscapes, leafhoppers may find it more difficult to reach the next suitable fragment due to a lack of alternative resources during dispersal. Moreover, we found that generalist species richness increased with increasing connectivity on small fragments, whereas it remained stable with increasing connectivity on large fragments. In small, isolated fragments, a higher extinction rate combined with a lower probability of recolonization is thought to cause the reduced species richness. Synthesis and applications. Our results show for the first time that insect species richness can be negatively affected by increasing habitat isolation in simplified but not in complex landscapes and in small but not in large fragments. We provide evidence that mitigating the negative effects of habitat fragmentation needs to take the surrounding landscape into account. Management efforts should prioritize (i) an increase in connectivity of small, isolated fragments, (ii) an increase in connectivity of fragments in simple landscapes and (iii) enhanced dispersal by increasing heterogeneity of both landscape composition and configuration. Moreover, extensive management of fragments by grazing or mowing to increase local habitat quality for leafhoppers would benefit other insect groups as well.