Research shows that both habitat quality and biodiversity can affect bee health

European honeybee on a redbud flower. Honey bees were among the four most consistently common types of bees found during the University of Michigan study. Lower levels of three common viral pathogens were strongly linked to greater species richness among local bee communities. Credit: Michelle Fearon

Efforts to promote the future health of both wild bees and managed honey bee colonies must take into account specific habitat needs, such as wildflower density.

At the same time, improving other habitat measures, such as the amount of natural habitat around cropland, can increase bee diversity while having mixed effects on overall bee health.

Those are the key findings of a new analysis of several thousand Michigan bees from 60 species. The study looked at how the quality and quantity of bee habitat around small fields affects the levels of common viral pathogens in bee communities.

“Future land management should consider that broadly improving habitat quality to benefit pollinator community diversity does not necessarily also benefit pollinator health,” said University of Michigan biologist Michelle Fearon, lead author of a study published in the journal Ecology. The other authors come from UM and the University of Washington.

“To promote pollinator health, we need to focus on improving specific habitat quality traits associated with reducing the prevalence of pathogens, such as planting greater density of flowers,” said Fearon, a postdoctoral researcher at the Department of Ecology and Evolutionary Biology.

Bees are indispensable pollinators, supporting both agricultural productivity and the diversity of flowering plants worldwide. But in recent decades, both native bees and managed honey bee colonies have seen population declines, which have been attributed to multiple interacting factors, including habitat loss, parasites and disease, and pesticide use.

As part of work for her dissertation at U-M, Fearon and her colleagues captured and captured more than 4,900 bees on 14 winter squash farms in southeastern Michigan, where both honeybees and wild native bees pollinate the pumpkin’s flowers.

The 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 on each farm ranged from seven to 49.

Those findings, published in February 2021 in Ecology, supported what ecologists call the dilution effect. This controversial hypothesis argues that greater biodiversity can reduce or mitigate the transmission of infectious diseases.

But an unresolved question lingered after that study was published: Was biodiversity really responsible for the observed reduction in viral levels, or was there something about habitat quality that is driving changes in both bee biodiversity and the prevalence of viral pathogens? caused?

“Many studies have shown that high-biodiversity communities are low-infectious-disease communities. But we also know that better habitat quality often leads to greater biodiversity,” said study co-author Chelsea Wood of the University of Washington, a former Michigan fellow. at the UM.

“So which factor actually reduces disease risk: biodiversity or habitat? Do communities with high biodiversity dilute disease prevalence? Or do communities in high-quality habitat have healthier hosts, which are more resistant to infection? Our data shows that some apparent ‘dilution effects ’ could actually have nothing to do with biodiversity at all.”

Research shows that both habitat quality and biodiversity can affect bee health

The University of Michigan biodiversity study was conducted on 14 winter squash farms across the state. European honey bees and wild native bees help pollinate the pumpkin flowers. A diverse range of native bees were found in the fields and along the field edges. Credit: Michelle Fearon

Previous studies have shown that habitat factors can directly influence both an animal’s nutritional status and the strength of its immune system, which in turn can influence its susceptibility to pathogens. For example, Eurasian red squirrels living in fragmented habitats are more affected by gastrointestinal parasites than those living in contiguous forest habitats.

To get to the root of their Michigan bee observations, Fearon and her co-authors developed models that allowed them to rigorously disentangle the effects of habitat traits on patterns of pathogen prevalence.

They re-examined the 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 quantity and diversity of floral resources (both pollen and nectar) to maintain good nutrition for pollinators.

At the local level, floral richness (diversity of floral species) and floral density were the most important indicators of a high-quality habitat. At the landscape level, the proportion of “nature areas” around crop fields and landscape richness (ie areas with more land cover) were the main features. Natural areas include deciduous, evergreen, and mixed forests; herbaceous and wooded wetland; thickets; grass meadow; and flower meadow.

The researchers found that habitats can have both positive and negative effects on pathogen levels in bee communities. This is evidence for what the authors called a habitat-disease relationship, where habitat quality has a direct impact on bee health.

In general, a higher proportion of natural areas and a greater richness of land cover types were associated with an increased viral prevalence, while a greater floral density was associated with a decreased viral prevalence.

“Areas of greater floral abundance could provide bees with better pollen and nectar sources to help them resist or fight infections,” said study co-author Elizabeth Tibbetts, a professor in U-M’s Department of Ecology and Evolutionary Biology. who was Fearon’s dissertation advisor. “In addition, increased floral abundance may reduce effective pollinator foraging density and result in reduced pathogen transmission.”

More natural area was also associated with greater diversity of bee species, which in turn contributed to reduced or diluted viral prevalence.

“Most importantly, we found that greater habitat quality in the surrounding landscape was a major driver of the dilution effect we previously observed,” Fearon said. “This provides evidence for a habitat-driven relationship between biodiversity and disease, where habitat quality indirectly influences bee health by altering bee species diversity.

“But different measures of habitat quality had both positive and negative impacts on patterns of viral prevalence. This means that habitat quality has the potential to reduce or increase viral prevalence in pollinators, depending on the relative strength of the habitat disease and the biodiversity disease pathways.

“Thus, it is important to consider how improving specific habitat quality measures may impact bee diversity and bee health in different ways.”

More information:
Michelle L. Fearon et al, Habitat quality affects pollinator pathogen prevalence through both habitat disease and biodiversity disease pathways, Ecology (2022). DOI: 10.1002/ecy.3933

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