At what spatial scale do high‐quality habitats enhance the diversity of forbs and pollinators in intensively farmed landscapes?

Abstract

Summary: Over the last decades, biodiversity in agricultural landscapes has declined drastically. Initiatives to enhance biodiversity, such as agri‐environment schemes, often have little effect, especially in intensively farmed landscapes. The effectiveness of conservation management may be improved by scheme implementation near high‐quality habitats that can act as a source of species. We evaluated up to what distance high‐quality habitats (nature reserves and artificially created flower‐rich patches) affect the diversity of forbs and pollinators in intensively farmed landscapes of the Netherlands. We surveyed forbs, inflorescences, bees and hover flies and estimated pollination services in transects along ditch banks extending 300 m from four nature reserves forming small islands in landscapes dominated by agriculture. In a separate experiment, we surveyed inflorescences, bees and hover flies in 1500 m long transects on farmland adjacent to five newly introduced flower‐rich patches and in five control transects. Species density of forbs declined over the first 75 m and species density and abundance of hover flies declined over the first 125 m beyond the nature reserves. Beyond these distances, no further declines were observed. The effects of flower‐rich patches were spatially limited. The species density and abundance of bees and hover flies were significantly enhanced in the flower‐rich patch, but only the abundance of hover flies was enhanced up to 50 m beyond the patch. Synthesis and applications. In intensively farmed areas, remnant high‐quality habitats sustain more abundant and diverse pollinator and forb communities than the surrounding countryside. They do enhance biodiversity on nearby farmland but increases are spatially restricted (< 150 m) and relatively small. These habitats may therefore function only as dispersal sources for ecological restoration sites or agricultural fields under extensification schemes that are located in close proximity. Habitat restoration in intensively used farmland should therefore be implemented preferentially in the immediate vicinity of high‐quality habitats. In the short term, newly created flower‐rich habitats are no alternative to pre‐existing seminatural habitats for the promotion of pollinators on nearby farmland.