Impact of urbanization on wild bees underestimated

The impacts of urbanization on wild bees, indispensable pollinators supporting both agricultural productivity and the diversity of flowering plants worldwide, may have been underestimated, a study posted on the website of the University of Michigan (UM) on Wednesday showed.

To better understand how urbanization affects wild bee populations, the UM-led researcher 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 researchers caught more than 3,300 bees from 143 species, with 74 percent of the captured bees belonging to species that nest underground, and the rest belonging to species that nest above ground in cavities or hollow tree stems.

They found that the sex ratio of the wild bees became more male-dominated as urbanization increased, mainly driven by a decline in medium- and large-bodied ground-nesting female bees.

In urban landscapes, where floral resources are scattered and patchy, larger-bodied male bees are more likely than females to disperse sufficiently far from their home nest to reach food sources and to survive, the researchers explained.

An alternative explanation for the observed sex ratio shift has to do with urbanization-induced changes in sex allocation by bees.

In most bee species, the production of reproductive females requires a greater investment of food resources than the production of males. As a result, a scarcity of pollen and nectar could result in a shift toward production of more male bees.

"While multiple studies have found reductions in ground-nesting bee populations in urban areas, the magnitude of these reductions may be greater than what total abundance measures indicate if, as we suggest in this study, urban ground-nesting bee populations are subsidized by males dispersing from less urban areas," said Paul Glaum, one of the study's first authors and a postdoctoral researcher in UM's Department of Ecology and Evolutionary Biology.

Populations of many wild bee species are in widespread decline worldwide due to multiple interacting factors.

Habitat loss, parasites and disease, pesticide use and climate change have all been blamed. Urbanization contributes to habitat loss, and that trend is expected to accelerate in coming decades.

The study is published Wednesday in the journal Scientific Reports.