Bumblebee colony performance increases with local or landscape resource availability, but not both

Pollinators, especially bumblebees, are in decline. Bumblebees are highly effective crop pollinators and an integral component of natural systems but are often the most vulnerable to habitat loss and the homogenization of resources. Grasslands can be an important source of native bees for crop pollination in agricultural landscapes. Thus, the goal of our study was to examine how resource availability locally (flowering plant diversity) and in the surrounding landscape (proportion of seminatural habitat area) affects the growth and reproductive output of bumblebee colonies within grasslands.

Our study was conducted at 32 grassland sites across southern Wisconsin and Michigan USA in the summer of 2014. We deployed managed colonies of Bombus impatiens, allowing them to forage and grow naturally for approximately 7 weeks and then measured a number of colony growth and reproductive output metrics.

We found that bumblebee colony growth (change in colony mass) was strongly correlated with the number of queen cells produced (reproductive output). There was a strong interaction between local resource availability and the availability of resources in the surrounding landscape in their effect on colony performance. When local resource availability was low, colony growth increased with the availability of resources in the surrounding landscape. However, when local resource availability was high, colony growth decreased with resource availability in the surrounding landscape. Colony performance was therefore greatest when the availability of resources was high locally or in the surrounding landscape, but unexpectedly, not when resources were highly available at both scales. Resource competition may explain this pattern as the total abundance of bees captured at research sites increased with both local and landscape scale resource availability. Our study suggests that the effect of resource availability on the performance of individual bumblebee colonies is scale dependent and may be mediated by competitors.