by Efforts to improve the future health of both wild bees and managed honeybee colonies must take into account specific habitat needs, such as the density of wildflowers.
At the same time, improving other habitat measures, such as the amount of natural habitat surrounding farmlands, can increase bee diversity while having mixed effects on overall bee health.
These are key findings from a new analysis of several thousand Michigan bees from 60 species. The study looked at how the quality and quantity of bee habitats surrounding small farm areas affect levels of common viral pathogens in bee communities.
“Future land management needs to take into account that drastically improving habitat quality to benefit pollinator community diversity may not also benefit pollinator health,” said University of Michigan biologist Michelle Fearon, lead author of a study published online Nov. Ecology. Other authors are from UM and the University of Washington.
“To improve pollinator health, we need to focus on improving certain habitat quality traits associated with reducing pathogen prevalence, such as planting more densely,” said Fearon, a postdoctoral researcher in the Department of Ecology and Evolutionary Biology.
Bees are indispensable pollinators that support both agricultural productivity and the diversity of flowering plants worldwide. But in recent years, both native bees and managed honeybee colonies have witnessed population declines that have been blamed on many interacting factors, including habitat loss, parasites and disease and pesticide use.
As part of their UM doctoral thesis work, Fearon and colleagues swarmed and trapped more than 4,900 bees at 14 winter squash farms in southeast Michigan, where both honeybees and wild domestic bees pollinate squash flowers.
Bees were analyzed for the presence of three common viral pathogens. Consistently, lower virus levels were strongly linked to greater species richness or biodiversity among local bee communities. The number of bee species in each farm ranges from seven to 49.
These findings, published in Ecology in February 2021, provided support for what ecologists call the dilution effect. This controversial hypothesis proposes that increased biodiversity may reduce or dilute infectious disease transmission.
But after this study was published, an unsolved question remained: Was biodiversity really responsible for the observed reductions in viral levels, or was something in habitat quality causing changes in both bee biodiversity and viral pathogen prevalence?
“Most studies have shown that high biodiversity communities have lower rates of infectious disease. But we also know that better habitat quality often leads to greater biodiversity,” said study co-author Chelsea Wood, a former Michigan University of Washington. Friend at UM.
“So which factor actually reduces disease risk: biodiversity or habitat? Are communities with high biodiversity reducing disease prevalence? dilution effects may actually have nothing to do with biodiversity.”
Previous studies have shown that habitat factors can directly affect both an animal’s nutritional status and the strength of its immune system, which in turn can affect its susceptibility to pathogens. For example, Eurasian red squirrels living in fragmented habitats host a greater load of gastrointestinal parasites than those living in continuous forest habitats.
To get to the root cause of the Michigan bee sightings, Fearon and his co-authors built models that allowed them to rigorously decipher the effects of habitat characteristics on pathogen prevalence patterns.
They reexamined previously collected bee data and added new information about the habitat at the local and landscape level. For the study, the researchers defined high-quality bee habitat as areas that provide sufficient and varied flower resources (both pollen and nectar) to maintain good pollinator nutrition.
At the local level, flower richness (meaning flower species diversity) and flower density were key indicators of high-quality habitat. At the landscape level, the proportion of “natural areas” surrounding farmlands and landscape richness (meaning areas with more land cover types) were key features. Natural areas include deciduous, evergreen and mixed forests; herbaceous and woody wetland; bushes; pasture; and wildflower meadow.
Researchers have found that habitat can have both positive and negative effects on pathogen levels in bee communities. This is evidence of what the authors call the habitat-disease relationship, where habitat quality has a direct impact on bee health.
Overall, a higher proportion of natural areas and greater land cover species richness were associated with increased viral prevalence, while greater flower density was associated with decreased viral prevalence.
“Areas with greater flower abundance can provide better sources of pollen and nectar to help bees resist or fight infection,” said study co-author Elizabeth Tibbetts, a professor in the UM Department of Ecology and Evolutionary Biology, who is Fearon’s thesis advisor. “Furthermore, greater flower abundance can reduce the effective foraging intensity of pollinators, resulting in reduced pathogen transmission.”
The more natural area was also associated with higher bee species diversity, contributing to a reduction or dilution of viral prevalence.
“Most importantly, we found that higher habitat quality in the surrounding land was the main driver of the dilution effect we observed earlier,” Fearon said. Said. “This provides evidence for a habitat-driven biodiversity-disease relationship in which habitat quality indirectly affects bee health by changing bee species diversity.
“But different measures of habitat quality have affected viral prevalence patterns both positively and negatively. This means that habitat quality has the potential to reduce or increase viral prevalence in pollinators, depending on the relative strengths of habitat-disease and biodiversity-disease pathways.
“It is therefore important to consider how improving certain habitat quality measures may affect bee diversity and bee health in different ways.”
The most recent research, reported in Ecology, was funded by the National Science Foundation, the North American Pollinator Conservation Campaign, the Pollinator Partnership, the Garden Club of America, and UM’s Rackham Institute and Division of Ecology and Evolutionary Biology.
Fearon is supported in part by the Dow Chemical Company Foundation through the Dow Sustainability Fellows Program at UM’s Graham Institute for Sustainability. Wood was supported by the Alfred P. Sloan Foundation, the National Science Foundation, the University of Washington Innovation Award, and the UW Royalty Research Fund.
