Professor Rutgers University, New Jersey, United States
Background/Question/Methods
Population genetics may offer tools to address long-standing questions about bees. Of particular interest are effective population sizes (Ne) and intergenerational dispersal distances, which remain largely unknown for bees. These population parameters affect rates of genetic diversity loss and inbreeding depression, making them pertinent to the conservation of rare or declining species. More generally, however, they also represent fundamental knowledge gaps in bee ecology, even for common species. These gaps exist because Ne and dispersal are difficult to measure. Here, using published genotype data, we pilot a population genetic method to jointly estimate local Ne, effective density (i.e. Ne per area) and dispersal distances in populations of four bumble bee species. First, using observed rates of genetic isolation by distance (measured as Rousset’s a), we derive estimates of Wright’s effective neighborhood size. Neighborhood size represents the effective size of the local breeding pool (or local Ne), and is a function of effective density and dispersal distance. Second, we assumed a range of possible values for each density and dispersal in order to calculate corresponding values for the other. Altogether, these estimates help to put reasonable bounds on the population sizes and dispersal distances of these bumble bee species.
Results/Conclusions
We estimated neighborhood sizes of 101-349 reproductive individuals (queens + males), or 68-233 colonies. These neighborhood sizes correspond to surprisingly low effective densities and dispersal distances. Across species, a fairly low effective density of 20 colonies/km2 corresponds with a short average dispersal of < 1 km. Conversely, a more realistic dispersal of 2 km corresponds to effective densities of < 5 colonies/km2. This is much lower than estimates of census (i.e., not effective) bumble bee colony density we found in the literature, which ranged from tens to hundreds of colonies/km2. One reason for this discrepancy could be low ratios of effective to actual population size (i.e. Ne/N), which could occur due to reproductive variability among colonies such that only a small proportion of colonies contribute to successive generations. It is also possible that young queens typically disperse over shorter distances than previously thought, but this seems less likely. For species with broad, contiguous ranges, like those studied here, small neighborhood sizes and low Ne/N ratios may be important insights into bees’ population ecology, but of little consequence to those populations’ survival. For rare or patchily distributed species, however, small neighborhoods and low Ne/N could reduce long term population viability.
