Landscape heterogeneity
Landscape heterogeneity should have a positive effect on pollinators, as heterogeneous landscapes provide more niches with a higher diversity of food resources and nesting sites (Fahrig et al. 2011, Hopfenmüller et al. 2014). We predicted that generalist hoverflies benefit more from landscape heterogeneity compared to wild bees, as they forage across a wider range of habitats (H2). In our study, wild bees were positively affected by landscape heterogeneity at intermediate spatial scales (340-780m). Similar, hoverfly abundance was positively affect at small scales (100-140m). However, at large scales (<750m), landscape heterogeneity had continuously a negative effect on hoverflies. We expect that the observed negative relationship of hoverflies to landscape heterogeneity is primarily driven by hoverfly responses to a limitation of other resources in the landscape (see above), since both landscape heterogeneity and arable field cover are negatively related to each other at large scales. Hence, positive effects of landscape heterogeneity may only be important for hoverflies, if landscape heterogeneity is uncorrelated to arable field cover (as in our study for small spatial scales). In conclusion, we found no support for the hypothesis 2, which may be reasoned by specifics of our landscape, though negative correlations between arable field cover and landscape heterogeneity should be present in many areas worldwide (Tscharntke et al. 2012).