“Intraspecific variation in worker body size makes North American bumble bees (Bombus spp.) less susceptible to decline”
Matthew W. Austin and Aimee S. Dunlap (Sep 2019)
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Intraspecific variation in worker body size makes North American bumble bees less susceptible to population declines
Population declines of bees are of broad interest, as bees are important pollinators of much of our wild and agricultural crops. While bee declines are likely caused by many factors, human-induced environmental changes are thought to be key culprits of these declines. A puzzle, however, is that not all bee species are declining – some are in fact thriving – which suggests differences between species in traits that enable responses to rapidly changed environments.
In a new paper in The American Naturalist, M. W. Austin and A. S. Dunlap investigate traits in North American bumble bees (Bombus spp.) that may make certain species of bumble bees more susceptible to decline. They study two traits that may be particularly important for bees when encountering rapidly changed environments: 1) the amount of body size variation within a species (i.e. how close in size individual bees are) and 2) head size, a proxy for brain size and behavioral plasticity. High body size variation is likely adaptive within colonies; larger bees are more efficient workers, while smaller bees can withstand starvation for longer periods of time. Behavioral plasticity is thought to benefit species in changed environments, by allowing individuals to plastically change their behavior to novel environmental conditions.
Using natural history collections from the Smithsonian, the American Museum of Natural History, the Field Museum, and the Illinois Natural History Survey, along with measures of bumble bee decline from the International Union for Conservation of Nature, the authors find that bumble bee species with low body size variation are more susceptible to decline, while species with higher body size variation are more likely to be thriving. Head size does not appear to affect a species’ likelihood of decline. These results suggest that high variation in body size enables bumble bees to successfully respond to environments altered by human activity, perhaps due to the benefits of body size variation within colonies.
This study is part of Austin’s Ph.D. dissertation at the University of Missouri–St. Louis, where he became interested in this topic while considering why closely related species experience divergent population trends.
Population declines have been documented in approximately one-third of bumble bee species. Certain drivers of these declines are known, however less is known about the interspecific trait differences that make certain species more susceptible to decline. Two traits, which have implications for responding to rapidly changed environments, may be particularly consequential for bumble bee populations: intraspecific body size variation and brain size. Bumble bee body size is highly variable and is likely adaptive at the colony level, and brain size correlates with cognitive traits (e.g. behavioral plasticity) in many groups. Trait variation and plasticity may buffer species against negative effects of rapidly changed environments. Using phylogenetically controlled analyses of 31 North American bumble bee species, we find higher intraspecific body size variation is associated with species having increased their relative abundance over time. However, this variation does not significantly interact with tongue length, another trait thought to influence bees’ decline susceptibility. Head size, a proxy for brain size, is not correlated with change in relative abundance. Our results support the hypothesis that variation in body size makes species less susceptible to decline in rapidly altered environments and suggests that this variation is important to the success of bumble bee populations.