Niche complementarity drives increases in pollinator functional diversity in diversified agroforestry systems

Abstract

Rising demand for food production poses a major threat to biodiversity by placing competing pressures on land. Diversified farming systems are one widely promoted nature-based solution to this challenge, which aim to integrate biodiversity-based ecosystem services into agricultural production. The underlying theory behind this approach is that diverse communities enhance ecosystem service provision, although the evidence to support this theory is often inconsistent for reasons that are not always clear. Here we investigate the contribution of pollinators to ecosystem function in a model example of a diversified farming system, silvoarable agroforestry comprising apple trees intercropped within arable fields. We assess pollinator species richness, species diversity, and functional trait diversity, between agroforestry fields and paired monoculture arable controls, and within agroforestry fields at set distances from tree rows, to quantify their potential contributions to pollination service. Species richness and diversity, and functional richness and dispersion, of wild bees were found to be significantly higher in agroforestry systems, despite weak effects on mean trait values. No significant effects were found for hoverflies. Supplemental bee species found in agroforestry systems were shown to increase functional diversity primarily by enhancing niche complementarity, effectively filling in gaps in niche space for traits, which could be partly attributed to a higher abundance and diversity of floral resources in the associated understorey. Nationally rarer bee species also contributed substantially to functional richness but not consistently to functional dispersion, suggesting that while they provide a unique functional role, their contributions to ecosystem services remain limited by low local abundances. These mechanistic insights reveal how the relationship between biodiversity and ecosystem functioning can be influenced by farm management practices through their effect on the spatial and temporal availability of habitat resources.