Landscape‐level synergistic and antagonistic effects among conservation measures drive wild bee densities and species richness

Abstract

Pollinators face significant global decline due to agricultural intensification. Local conservation measures (CMs), such as an annual flower field, an organic crop field, or a perennial semi-natural habitat (SNH), are implemented to counteract this negative trend, with variable success, as local CMs may not support ecological processes at spatially larger landscape scales. This can be achieved by planning CMs at the landscape level (landscape CMs), for example multiple fields with a specific CM or combinations of different types of CMs. However, interactive effects between combined landscape CMs may limit their efficacy. It remains unclear whether multiple combined landscape CMs can be more efficient than single landscape CMs (synergistic effect), reduce each other's effectiveness (antagonistic effect) or sum together (additive effect) to promote biodiversity. We assessed the interactive effects of three landscape CMs: organic crops, annual flower fields and perennial SNH, on wild bee species richness and densities at the landscape scale. We surveyed wild bees within multiple transects in 32 landscapes and upscaled bee densities to the landscape scale. We observed a synergistic effect between landscape-scale organic crops and perennial SNH. Specifically, non-Bombus wild bee densities increased with higher area shares of organic crops in landscapes with high area shares of perennial SNH. This is likely due to their provision of complementary resources. For bumblebees, we found an additive effect of organic crops and perennial SNH, suggesting that bumblebees benefit from both landscape CMs regardless of their respective availability. However, antagonistic effects were more common, for example between landscape-scale annual flower fields and organic farming, both providing similar floral resources and disturbance regimes. Only in landscapes with a low area of annual flower fields did bee densities and species richness increase with area shares of organic crops. Synthesis and applications. Interactive effects of combined landscape CMs determine landscape-scale bee densities and species richness. In particular, functionally different and complementary resources of landscape CMs can create synergistic effects while antagonistic effects occur when similar resources are provided in different landscape CMs. Hence, we recommend that future bee conservation schemes should use smart mixing of landscape CMs, based on judicious evaluation to maximize complementary benefits and reduce redundancy with respect to landscape-scale floral resources and provision of nesting habitat, while considering the habitat requirements of different bee taxonomic groups.