Pollen-, temperature-, and parasite-limitation of solitary bee reproduction and population growth

Background/Question/Methods

In high-elevation habitats, where low temperatures can limit foraging opportunities for ectotherms, warming temperatures could benefit many species, including bees. However, we know little about which factors limit population growth of wild bees, making it difficult to assess whether temperature increases ought to favor or disfavor bee populations. In this study, I asked what factors limit nesting progress and per capita reproductive output in subalpine populations of a pollen-specialist bee, Osmia iridis. I marked individual nesting females and monitored production of surviving (unparasitized) offspring in several populations over two summers. I simultaneously recorded site-specific data on temperatures, density of the floral host-plant, and rates of brood parasitism. I related individual daily nesting progress and overall reproductive output (at the population level) to these predictor variables using linear mixed models.

Results/Conclusions

Bee nesting progress (number of nest cells provisioned [=eggs laid] per day) was strongly predicted by both the ambient temperature during nesting (hours above a threshold temperature) and availability of floral resources (local density of host flowers, a good proxy for pollen abundance). However, mean per capita reproductive output at each site was predicted only by floral resources, and not by local temperatures. Site-specific rates of brood parasitism were a far stronger predictor of bee reproductive output than temperatures or floral resources. These results suggest strong top-down control of these bee populations. They also suggest that warming temperatures, despite being favorable for bee activity, will not provide a net benefit to these bee populations, likely because they simultaneously favor activity of the bees’ parasites.