Quantifying the impacts of bioenergy crops on pollinating insect abundance and diversity: a field-scale evaluation reveals taxon-specific responses.
Global declines in pollinating insects have been linked with agricultural intensification and land-use change. Increased production of novel crops for bioenergy is causing changes in agricultural practice, but the effects on different pollinating taxa have not yet been quantified. However, the major pollinating groups (social bees, solitary bees, hoverflies and butterflies) are likely to respond differently to changes in land use and shifts in crop cultivation patterns. We assessed the impacts of two bioenergy crops, oilseed rape Brassica napus L. and Miscanthus Miscanthus × giganteus on (i) the diversity and abundance of four taxonomic groups of pollinating insects, (ii) insect community composition, (iii) floral resources and (iv) nesting sites for bumblebees, by comparing bioenergy crops with conventional arable and grass crops in a large network of commercial fields. We found that although there were more bumblebees in oilseed rape fields, conventional crops and bioenergy crops did not differ greatly in species richness of most pollinator groups, although more floral resources were found in the bioenergy crops. However, solitary bees were more abundant and species rich in energy crops than conventional wheat, and more trap-nesting bees and wasps were found in Miscanthus than oilseed rape. In addition, novel assemblages of solitary bees were found in the bioenergy crops. All pollinator groups, and flowering plants, were more abundant in field margins than in the centre of crop fields. Although there were no differences among crop types, nest-searching bumblebees were confined to field margins. This highlights the importance of field margins as forage and nesting habitat for pollinators in farmland. Synthesis and applications. Differential responses to field-scale growth of bioenergy crops were displayed by the taxonomic groups studied, suggesting that effects on biodiversity cannot be generalized from studies based on a small suite of taxonomic groups and must include a range of taxa for overarching results. To optimize habitat for pollinators in agricultural areas, our data suggest that a diversity of crop types at small scales may have positive impacts and supports appropriate management and conservation of field margins and hedgerows during conversion to bioenergy production.