Understanding Community- and Colony-Level Bee Responses to Resource Availability in Bioenergy Grasslands.

Conversion of marginal agricultural lands to perennial grasslands for bioenergy production may help satisfy renewable energy needs while helping to preserve biodiversity and a suite of valuable ecosystem services, including pollination. The diversity of bees depends on the availability of flower resources both locally and in the surrounding landscape, and may therefore be affected by the disturbance created by production-scale grassland harvesting. We therefore conducted a large-scale experiment to determine the relative importance of grassland harvesting and multi-scale resource availability on wild bee communities and on the performance of colonies of the bumble bee Bombus impatiens.

Our study was conducted at 32 semi-natural grassland sites across southern Wisconsin and Michigan USA in 2014. Half of the sites were harvested and half were left as unharvested controls. We sampled wild bees to estimate species richness and relative abundance. At the same sites we also deployed colonies of B. impatiens, allowing them to forage and grow naturally for two months, then measured colony growth and reproductive potential. Local resource availability was estimates as the diversity of flowering plants within sites and landscape-scale resource availability was the area of semi-natural habitat within a 2000 m radius.

We found that grassland harvesting had little effect on wild bee species richness or bumble bee colony performance. However, bees at community- and colony-levels were strongly dependent on resource availability at local and landscape scales. Wild bee species richness increased with multi-scale resource availability, but, unexpectedly, colony growth and reproductive potential decreased with resource availability. Interspecific interactions, such as competition for food resources or the transmission of pathogens may explain the negative association between resource availability and bumble bee colony performance. After two years of harvesting, bee diversity and colony performance appear to be much more dependent on resource availability than the disturbance created by grassland harvesting.