Specific habitat requirements, such as the quantity of wildflowers, must be taken into account in order to promote the long-term health of both wild bees and managed honeybee colonies. While having conflicting effects on bee health in general, boosting other habitat variables, such as the quantity of natural habitat surrounding croplands, may increase bee variety.
These are the main conclusions from a recent study of a large number of Michigan bees from 60 different species. The study examined the relationship between bee communities’ levels of common viral infections and the number and quality of bee habitat surrounding small agriculture areas.
According to University of Michigan biologist Michelle Fearon, lead author of a study published online Nov. 30 in the journal Ecology, “Future land management needs to consider that broadly improving habitat quality to benefit pollinator community diversity may not necessarily also benefit pollinator health.” The other writers are from the universities of Michigan and Washington.
According to Fearon, a postdoctoral associate in the Department of Ecology and Evolutionary Biology, “we need to focus on increasing certain habitat quality traits that are linked to reducing disease prevalence, such as planting increased densities of flowers to enhance pollinator health.”
Bees are essential pollinators that promote both global floral diversity and agricultural output. However, in recent years, both managed and native honeybee colonies have seen population decreases. This is attributed to a number of interrelated reasons, such as habitat degradation, parasites, disease, and pesticide use.
Fearon and her colleagues netted and caught more than 4,900 bees at 14 winter squash fields in southeast Michigan as part of the research for her doctoral dissertation at the University of Michigan. Both honeybees and wild native bees pollinate the squash blossoms.
Three typical viral pathogens were looked for in the bees. Species richness, or biodiversity, was highly correlated with decreased virus levels in the local bee groups. Each farm had seven to 49 different species of bee.
These results, which were reported in the February 2021 issue of Ecology, supported the concept of the diluting effect, according to ecologists. According to this contentious theory, higher biodiversity may reduce or attenuate the spread of infectious diseases.
However, once that study was published, there was still a lingering question: Was biodiversity actually to blame for the reported declines in viral levels, or was habitat quality driving changes in both bee biodiversity and viral pathogen prevalence?
Numerous studies have demonstrated that areas with high levels of biological diversity also have low rates of infectious disease. “But we also know that increased biodiversity frequently results from better habitat quality,” said University of Washington researcher and former University of Michigan Fellow Chelsea Wood.
“So, which—biodiversity or habitat—is actually lowering the risk of disease?” Do places with a lot of biodiversity have lower sickness rates? Or are the hosts in communities with high-quality habitats healthier and more able to fend off infections? Our findings suggest that some ostensible “dilution effects” may not actually be related to biodiversity at all.
Previous research has shown that environmental characteristics can directly affect an animal’s nutritional state and immune system strength, which in turn can determine how susceptible the animal is to viruses. For instance, Eurasian red squirrels in fragmented habitats had higher parasite burdens in their gastrointestinal tracts than those in continuous forest habitats.
Fearon and her co-authors developed models that allowed them to precisely untangle the impacts of habitat factors on patterns of disease prevalence in order to understand the underlying causes of their Michigan bee data.
They reexamined the data on bees that had already been gathered and added fresh data on local and landscape-level habitat. According to their definition for the study, high-quality bee habitats are those that offer an adequate variety and quantity of floral resources (both pollen and nectar) to support healthy pollinator nutrition.
The most important determinants of high-quality habitat at the local level were floral richness (meaning flower species variety) and floral density. Landscape richness (i.e., places with a greater variety of land cover types) and the proportion of “natural areas” around farm fields were the most important factors. Natural areas comprised grass pastures, wildflower meadows, herbaceous and woody wetlands, deciduous, evergreen, and mixed forests, shrubland, and shrubland.
The study’s findings revealed that habitat can affect pathogen levels in bee colonies in both favorable and unfavorable ways. This is proof of what the authors called a “habitat-disease link,” in which bee health is directly impacted by the quality of the habitat.
In general, larger levels of natural area and a broader variety of land cover types were linked to higher viral prevalence, but higher floral densities were linked to lower viral prevalence.
According to research co-author and Fearon’s dissertation advisor, Elizabeth Tibbetts, professor in the U-M Department of Ecology and Evolutionary Biology, “Areas with increased floral abundance could provide better pollen and nectar resources for bees to help them prevent or fight infection.” Additionally, increased floral abundance may lower pollinators’ effective foraging density and lessen the spread of pathogens.
Higher bee species diversity was also associated with more natural areas, which resulted in a lower or diluted virus prevalence.
Most crucially, Fearon stated, “We discovered that higher habitat quality in the nearby terrain was a key driver of the diluted impact that we previously reported.” This supports the hypothesis that disease and biodiversity are correlated and that habitat quality, which affects bee species variety, has an indirect effect on bee health.
“But many measurements of habitat quality had both positive and negative effects on virus prevalence patterns.” In light of the relative potency of the habitat-disease and biodiversity-disease pathways, habitat quality has the ability to either reduce or increase virus prevalence in pollinators.
Therefore, it’s crucial to think about how changing certain habitat quality metrics might have a variety of effects on bee diversity and bee health.
The National Science Foundation, the North American Pollinator Protection Campaign, the Pollinator Partnership, the Garden Club of America, U-Rackham M’s Graduate School, and the Department of Ecology and Evolutionary Biology all provided funding for the most recent study published in Ecology.
