Parasitism modifies functional diversity in cavity-nesting bee communities

Cavity-nesting bees occupy a unique ecological niche by utilizing pre-existing cavities in dead wood or plant stems for nesting. These species provide essential pollination services, driven by their extensive functional traits which connect bee behavior and morphology to pollination syndromes. Importantly, increased bee functional diversity is known to directly enhance the effectiveness of pollination. Parasitism is one factor influencing the abundances of cavity-nesting bee species. Insect parasites do not build or provision their nests, rather they deposit their eggs in nests that are built and provisioned by a host bee species. Such parasitism has a clear negative effect on host abundance, but may also play an important factor in maintaining healthy ecosystem function and stability by regulating competition among hosts and reducing the fecundity of dominant species, preventing competitive exclusion, and promoting additional niche space for less dominant individuals. We hypothesized that parasitic bees decrease the dominance of abundant species, thus impacting bee functional diversity. This, in turn, would theoretically increase the evenness and dispersion of functional traits related to pollination services. We tested this hypothesis using six years of bee block trap nesting data collected from over 500 sites in Minnesota. We found that parasitism decreased that abundance of commonly encountered species in our bee blocks and was higher when species dominance in the block was high. The distribution of bee functional traits were also shown to be impacted by parasitism, though had no significant effect of functional evenness nor dispersion.