Urbanization Stresses Wild Bees, but Green Spaces Can Help

Urban environments can be stressful places for wild animals. They are not only exposed to loud noises, vehicles and humans, but to other environmental stressors like lack of adequate food sources and fragmented habitats.

In a new study, researchers from Canada’s York University looked at the increased environmental stressors — such as difficulty finding mates and optimal nesting areas — facing wild bees living in cities like Toronto compared with those in suburban and rural areas.

“Rapid expansion of human populations continues to reshape landscapes and poses one of the largest threats to natural ecosystems and biodiversity. To compensate, it is expected that urbanization will drive over two-thirds of the human population to reside in cities by 2050,” the scientists wrote in the study. “Urbanization fragments and degrades natural landscapes, threatening wildlife including economically important species such as bees.”

The researchers found that wild bees living in Toronto and other densely populated urban areas and fragmented habitats faced increased parasites and pathogens, as well as changes in their microbiomes, a York University press release said.

There will likely be an increase in the environmental stressors plaguing wild bees in the future, as cities continue to expand and threaten bees’ biodiversity, as well as the natural ecosystems they call home.

“Having less connected habitats in dense urban areas not only leads to more inbreeding, so less genetic diversity, but it also creates higher pathogen diversity leaving city bees exposed to more pathogens,” said Sandra Rehan, corresponding author and associate professor in the Department of Biology at York University, in the press release.

The study, “Integrative population genetics and metagenomics reveals urbanization increases pathogen loads and decreases connectivity in a wild bee,” was published in the journal Global Change Biology.

For the study, the research team examined the population genetics, microbiomes and metagenome of 180 common carpenter bees — Caratina calcarata — using whole genome sequencing. They also looked at the impact environmental stressors had on the bees across Greater Toronto.

The small carpenter bees the team looked at for the study were not managed, non-native populations like honeybees, but wild, native bees.

In their research, the scientists found that, even when there were no genetic differences between bees, there was marked variation in the nutritional resources available to them, as well as their microbiomes.

Lead author of the study Katherine D. Chau, a York Ph.D. student, said there are measures cities can take to bridge the habitat gap for wild bees.

“Parasite and pathogen infections in bees are a major driver in global bee population declines and this is further exacerbated by urbanization and a loss of habitat and degraded habitat. There are things, though, that cities could do to help wild bees,” Chau said in the press release. “We found the best way to connect bee habitats and create conditions for more genetic diversity is through green spaces, shrubs and scrub. Conservation efforts focused on retaining and creating these habitat connectors could go a long way toward helping wild bee health.”

While bees are the most prominent pollinators, these measures by cities could help all insect pollinators, which are responsible for pollinating more than 75 percent of global food crops and 87 percent of flowering plants.

The urban heat island effect of higher temperatures can also cause flowers to bloom at times that are not optimal for bees, such as before or after they have gone out to forage.

The researchers said that, because there are concentrations of bees in certain urban areas, it is more likely that infected bees will contaminate flowers and spread them to other bees — even those of other species — causing more parasite and pathogen infections.

“Our research is the first known whole genome sequencing, population genomic and metagenomic study of a wild, solitary bee in an urban context, which looks at the complex relationship between bees, metagenomic interactions and dense urban landscapes,” Rehan said in the press release. “This approach provides a tool to assess not only the overall health of wild bees in urban settings but could also be applied across a broad range of wildlife and landscapes.”

The research team said their identification of known plant and bee pathogens in urban areas paves the way for other studies to monitor threats to wildlife in cities and detect them early. “Future studies should explore the link between reduced genetic diversity and the fitness of wild bees in cities,” Chau said in the press release.