Wild bees are indispensable pollinators, supporting both agricultural productivity and the diversity of flowering plants worldwide.
But wild bees are experiencing widespread declines resulting from multiple interacting factors. A new University of Michigan-led study suggests that the effects of one of those factors—urbanization—may have been underestimated.
The study, led by a group of current and former U-M students and conducted at sites across southeast Michigan, looks at one aspect of this topic they say has received scant attention from bee researchers: the sex ratio of wild bees and how it changes across a rural-to-urban land-use gradient.
The team found that the sex ratio of wild bees became more male-dominated as urbanization increased, mainly driven by a decline in medium- and large-bodied ground-nesting female bees. The study, published March 6 in the journal Scientific Reports, is believed to be the first investigation of observed sex ratio in a complete wild bee community along a rural-to-urban gradient.
“These findings have potential implications on bee population health and pollination services, since male and female bees often have different pollination behaviors,” said Paul Glaum, one of the study’s first authors and a postdoctoral researcher in U-M’s Department of Ecology and Evolutionary Biology.
Female and male bees of the same species often pollinate different plant species. As a result, a decline in female bees has the potential to limit pollination services for part of the plant community, he said.
Additionally, a declining female population can mean fewer mates for male bees. This threatens ground-nesting bees’ reproduction rates and their ability to maintain future generations of pollinating bees. It may even threaten the genetic diversity of these species, Glaum said.
“Our results suggest that research may be underestimating the negative impacts of urbanization on ground-nesting bees and highlight the importance of considering sex-specific differences in bee behavior when analyzing the effects of environmental change on bee populations,” he said.
To better understand how urbanization affects wild bee populations, the U-M-led team sampled wild bees at community gardens, nature reserves and farms across southeast Michigan. Sampling was done at 26 sites spanning nearly 70 miles.
The other co-first authors of the Scientific Reports paper are U-M doctoral students Gordon Fitch and Chatura Vaidya of the Department of Ecology and Evolutionary Biology; and Maria-Carolina Simao, a recent doctoral graduate of the U-M School for Environment and Sustainability, now at Rutgers University. Contributing authors are Jill Matthijs, a former U-M Environmental Science undergraduate; Benjamin Iuliano, a former undergraduate in the U-M Department of Ecology and Evolutionary Biology; and Ivette Perfecto, a professor at the School for Environment and Sustainability.
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The story was on Michigan Radio