Testing scale-dependent effects of semi-natural habitats on farmland biodiversity

The effectiveness of conservation interventions for maximizing biodiversity benefits from agri-environment schemes (AESs) is expected to depend on the quantity of semi-natural habitats in the surrounding landscape. To verify this hypothesis, we developed a hierarchical sampling design to assess the effects of field boundary type and cover of semi-natural habitats in the landscape at two nested spatial scales. We sampled three types of field boundaries with increasing structural complexity (grass margin - simple hedgerow - complex hedgerow) in paired landscapes with presence or absence of semi-natural habitats (radius 0.5 km), that in turn, were nested within 15 areas with different proportions of semi-natural habitats at a larger spatial scale (10 × 10 km). Overall, 90 field boundaries were sampled across a Mediterranean region (NE Italy). We considered species richness response across three different taxonomic groups: vascular plants, butterflies, and tachinid flies. No interactions between type of field boundary and surrounding landscape were found at either 0.5 and 10 km indicating that the quality of field boundary had the same effect irrespective of the cover of semi-natural habitats. At the local scale, extended-width grass margins yielded higher plant species richness, while hedgerows yielded higher species richness of butterflies and tachinids. At the 0.5 km landscape scale, the effect of the proportion of semi-natural habitats was neutral for plants and tachinids, while butterflies were positively related to the proportion of forest. At the 10 km landscape scale, only butterflies responded positively to the proportion of semi-natural habitats. Our study confirmed the importance of testing multiple scales when considering species from different taxa and with different mobility. We showed that the quality of field boundaries at the local scale was an important factor in enhancing farmland biodiversity. For butterflies, AESs should focus particular attention on preservation of forest patches in agricultural landscapes within 0.5 km as well as the conservation of semi-natural habitats at a wider landscape scale.