Habitat loss reduces parasitoid diversity only in highly fragmented landscapes

Habitat fragmentation is considered one of the major threats to biodiversity. Although many empirical and theoretical studies have elucidated the effects of habitat fragmentation on populations and communities of primary producers and consumers, little attention has been paid to the impacts of this driver on the third trophic level. In this study, the parasitoid flies in the family Tachinidae (Diptera) were used as a model system to test the effects of habitat fragmentation on the third trophic level. A pan-trap sampling was conducted in 18 semi-natural grasslands along statistically orthogonal gradients of area, connectivity, and habitat heterogeneity. The sampling was conducted every two weeks between March and November 2012 with a number of traps proportional to patch area. A total of 1606 specimens were collected and 129 species were identified. Tachinid parasitoids showed a clear species−area relationship with a linear increase in species richness as the area of the patches increased. However, species richness was negatively affected by habitat loss in highly fragmented landscapes, but not in landscapes with relatively high habitat connectivity. We did not find a clear relationship between habitat diversity and overall parasitoid species richness at the landscape scale, but at the local scale we found that habitat diversity differently affected species diversity depending on the host specialization. Specifically, the richness of tachinid parasitoid taxa attacking coleopterans was positively related to habitat diversity, while the richness of taxa using hemipteran and lepidopteran hosts was negatively affected. These results have important implications for biodiversity conservation as any attempts to mitigate the negative effects of habitat loss need to take the general level of habitat connectivity in the landscape into account.