Interacting effects of farming practice and landscape context on bumble bees

Abstract

Organic farming has been suggested to counteract declines in farmland biodiversity, but comparisons to conventional farming have produced variable outcomes. To examine whether this is due to the landscape context farms are situated in and traits of the studied organisms, we surveyed bumble bees in cereal field borders and margins at 12 pairs of matched organic and conventional farms, with half the pairs located in heterogeneous farmland and the remaining in homogeneous plains. Species richness and abundance of bumble bees were significantly positively related to both organic farming and landscape heterogeneity. However, there was an interaction effect between farming practice and landscape context so that species richness and abundance were only significantly higher on organic farms in homogeneous landscapes. The higher abundance of bumble bees on organic farms was partly related to higher flower abundance on these sites. The effect of landscape context on bumble bee abundance was stronger for species with medium sized colonies than for those with smaller and larger colony sizes. These patterns may reflect that species with medium sized foraging ranges are most affected by fragmentation of foraging habitat, because colony size reflects the spatial scale at which bumble bees utilize resources. We conclude that both organic farming and landscape heterogeneity can be used to increase bumble bee species richness and abundance, but that organic farming has a larger effect in homogeneous landscapes and landscape heterogeneity a larger effect on conventional farms. The effects differed between species, suggesting that a single prescription to increase pollinator abundance may not be valid.