Survival, reproduction and population growth of the bee pollinator, Osmia rufa ( Hymenoptera: Megachilidae), along gradients of heavy metal pollution

Bees are one of the most important groups of pollinators in the temperate zone. Although heavy metal pollution is recognised to be a problem affecting large parts of the European Union, we currently lack insights into the effects of heavy metals on wild bee survival and reproduction. We investigated the impact of heavy metal pollution on the wild bee Osmia rufa (Hymenoptera: Megachilidae) by comparing their survival, reproduction and population dynamics along two independent gradients of heavy metal pollution, one in Poland and the other in the United Kingdom. We used trap nests to evaluate the response of fitness and survival parameters of O. rufa. To quantify the levels of pollution, we directly measured the heavy metal concentration in provisions collected by O. rufa. We found that with increasing heavy metal concentration, there was a steady decrease in number of brood cells constructed by females and an increase in the proportion of dead offspring. In the most polluted site, there were typically 3-4 cells per female with 50-60% dead offspring, whereas in unpolluted sites there were 8 to 10 cells per female and only 10-30% dead offspring. Moreover, the bee population growth rate (R-0) decreased along the heavy metal pollution gradients. In unpolluted sites, R-0 was above 1, whereas in contaminated sites, the values tended to be below 1. Our findings reveal a negative relationship between heavy metal pollution and several fitness parameters of the wild bee O. rufa, and highlight a mechanism whereby the detrimental effects of heavy metal pollution may severely impact wild bee communities.