Category Archives: Ecology


Climate change is an important contributor to insect declines around the world, according to a new study published in the scientific journal PNAS that examined continuous long-term monitoring of butterfly and other insect populations.

“Specifically, we looked at long-term data sets from relatively protected areas – areas where the impacts of other stressors are weaker, such as in the mountains,” study lead author Chris Halsch, a doctoral candidate in the Ecology, Evolution, and Conservation Biology program of the University of Nevada, Reno, said. “We specifically didn’t look at agriculture or urban areas for this study, for the most part, our sites are in undisturbed natural areas.”

Hesperia comma, a widespread skipper butterfly known for many local forms and ecological races, is among the top 50 most severely declining species in a recent study of western butterflies.  Photo credit: Jeffrey Glassberg, North American Butterfly Association 

Few insect monitoring programs encompass extensive elevational gradients, but one exception is the Shapiro Transect across Northern California with 10 sites and 163 species of butterflies from sea level to 8,200 feet of elevation. The sites have a wide variety of land use, from the intensely modified Central Valley of California to above tree line in the Sierra Nevada mountains, which this study encompasses. Observations at these sites have been taken every two weeks during the butterfly flight season for anywhere from 32 to 48 years, depending on the site.

“The issue is that there are multi-faceted reasons for these declines: agriculture, pesticides, urban growth causing habitat loss, pollution, forest fires and extreme weather events – and in most cases it is not just one thing,” Halsch said. “And none of these factors are as geographically pervasive or as likely to interact with all other factors as climate change.”

In the Donner Pass and Castle Peak areas of northern California’s Sierra Nevada, researchers (including Halsch) found butterfly populations that had been relatively stable until the 2000s. Then, the extreme drought from 2011 to 2015 hit and populations haven’t recovered.

Research team members from University of California, Davis, James Thorne and David Waetjen, provided climate modeling to go with the population data for the northern California transect. They found warming minimum temperatures and nights that are not cooling off as much, which all impacts biological processes.

The butterflies’ nectar sources are getting harder to find as plants wither in the heat, especially towards the end of summer and into early fall. Heat is responsible for dry vegetation, which was a catalyst for last year’s devastating wildfires, which have consumed butterfly habitat across the West.

“An important aspect of climate change impacting insects is extreme events,” Halsch said. “Six or so of the studies we reviewed looked at extreme events (flood, drought, etc.) and these events are more likely to be negative across the board. The negative impacts of extreme events are shown in mountain drought – the butterflies in the mountains did poorly; and the butterflies in California’s Central Valley did relatively well during the drought, but since the drought ended the declines are happening again, numbers have plummeted back down.”

Art Shapiro, who began the northern California butterfly research in the 1970s, said generalists – butterflies that can survive in a variety of environments – are in many cases doing poorly, relative to those that only thrive in a narrow range of environmental conditions. 

“Generalists tend to be upslope colonizers in the mountains,” Shapiro, a co-author in the study, said. “Their decline in the mountains probably reflects the fact that the main populations down here [in the Central Valley] are doing poorly.”

Halsch and his co-authors reviewed studies from around the world looking for relevant information about butterflies, moths, ants and flies. By far, butterflies are the most monitored across all insects, with the bulk of the studies from North American and northern Europe.

In all, for their research, the team used 60 studies from around the world, 11 studies that used the Shapiro Transect data, and 12 studies based on United Kingdom butterfly monitoring – monitoring programs that are known as some of the best in the world.

“Butterflies are, if anything, in worse trouble in Europe than here in North America,” Shapiro said. “The declines themselves are similar, but the reasons for the declines are different.”

Robust insect populations are vital for a variety of reasons, ranging from how they support the world’s food supply to how they support backyard flowers through pollination. Biologists are particularly interested to see how insects will respond to contemporary climate change because they are the most diverse lineage of multicellular organisms on the planet and are of fundamental importance to the functioning of freshwater and terrestrial ecosystems.

Colleague Lee Dyer in the College of Science’s biology department and EECB program at the University of Nevada, Reno, recently completed one of the few studies on insect populations in tropical forests and found strong signals of climate related to declines in population.

Matt Forister, Halsch’s graduate school advisor and co-author of the study that is a part of the published PNAS special issue about insect declines, is an expert in insect and butterfly populations. He has taken the lead on maintaining and expanding the Shapiro Transect monitoring in the Sierra Nevada. He said butterflies, like the rest of the natural world, are in decline, but they can rebound from their grim situation.

“We’re likely on the verge of losing some butterflies locally or regionally,” he said. “While those at-risk butterflies are not about to become extinct worldwide, that could change over the next 30 to 50 years. Insects really are survivors. We’ll lose some more, but if we can smarten up our agricultural practices and rein in climate change a bit, then there’s a lot of hope.”

Biodiversity Key to Protect Bees

A bumble bee (Bombax impatiens) feeding on a calendula flower.

A new analysis of thousands of native and nonnative Michigan bees shows that the most diverse bee communities have the lowest levels of three common viral pathogens.

University of Michigan researchers netted and trapped more than 4,000 bees from 60 species. The bees were collected at winter squash farms across Michigan, where both managed honeybee colonies and wild native bees pollinate the squash flowers.

All but one species—Apis mellifera, the common European honeybee—are native bees. The number of bee species found at each farm ranged from seven to 49.

Consistently, lower virus levels were strongly linked to greater species richness among the local bee communities. The study was published online Feb. 11 in the journal Ecology.

An eastern bumblebee on a wingstem flower. European honeybees and eastern bumblebees showed the highest levels of three common viral pathogens in the University of Michigan study. Image credit: Michelle Fearon, University of Michigan.

“This result is exciting because it suggests that promoting diverse bee communities may be a win-win strategy to simultaneously reduce viral infections in managed honeybee colonies while helping to maintain native bee biodiversity,” said study lead author Michelle Fearon, a postdoctoral researcher in the University of Michigan Department of Ecology and Evolutionary Biology.

“In light of recent global pollinator population declines that are due in part to the spread of pathogens, these results offer hope that conservation efforts could also broadly benefit pollinator health,” said Fearon, who conducted the study for her doctoral dissertation. She is now pursuing a follow-up study that explores how natural areas keep pollinator communities healthy.

The Ecology study is the first to show that high levels of biodiversity within bee communities can help dilute the harmful effects of viral pathogens. Support for this “dilution effect” has been reported in other host­—pathogen systems—such as tick-borne Lyme disease—but this is the first time it’s been seen with pollinator viruses. The idea of a dilution effect remains controversial among ecologists, however.

Fearon and her colleagues collected 4,349 bees at 14 Michigan winter squash farms over two summers. Michigan winter squashes include acorn squash, butternut squash, spaghetti squash and pumpkins.

Honeybees were found at all of the sites, and a diverse array of native bees were also present in the squash fields and along field edges. In fact, native pollinators were much more common visitors to the squash flowers than honeybees at most locations.

Four types of bees—the European honeybee, the eastern bumblebee (Bombus impatiens), the squash bee (Eucera pruinosa) and several species of sweat bee (genus Lasioglossum)—were the most consistently abundant species among the bee communities that were sampled.

Those four groups were tested for the presence of three viruses that commonly infect managed honeybee colonies: deformed wing virus, black queen cell virus and sacbrood virus.

These pathogens contribute to high rates of colony loss among honeybees, and there are no widely available treatments that beekeepers can use to control them. Previous studies suggested that native bees are less commonly infected and may be less likely to transmit the pathogens to other bees.

The viruses spread as bees move from flower to flower, gathering pollen and nectar and pollinating the plants in the process. Consumption of virus-contaminated pollen is believed to be a primary mode of transmission.

For each of the four target bee groups in the U-M study, researchers found that lower viral prevalence was strongly linked to greater biodiversity of the local bee community: the more bee species present, the lower the percentage of bees infected.

Species-rich communities included many native bee species, which apparently helped to dilute the impact of the pathogens.

“Native bees likely reduce the viral prevalence in pollinator communities because they are poorer viral hosts than honeybees. This means that some native bees don’t get as sick as honeybees and are less likely to spread the virus to other bees,” said study co-author Elizabeth Tibbetts, a professor in the U-M Department of Ecology and Evolutionary Biology who was Fearon’s dissertation adviser.

A male eastern bumblebee (Bombus impatiens) and a sweat bee on a cone flower. Bumblebees and sweat bees were part of a University of Michigan study showing that the most diverse bee communities have the lowest levels of three common viral pathogens. Image credit: Michelle Fearon, University of Michigan.

“So, bees from pollinator communities with lots of species are less likely to get sick because they are sharing flowers with many bee species that are less likely to spread the virus, while bees from communities dominated by honeybees are more likely to share flowers with honeybees that are good at spreading the virus,” Tibbetts said.

Bees are indispensable pollinators, supporting both agricultural productivity and the diversity of flowering plants worldwide. In recent decades, both native bees and managed honeybee colonies have seen population declines blamed on multiple interacting factors including habitat loss, parasites and disease, and pesticide use.

“We found encouraging evidence that pollinator conservation efforts can broadly benefit the health of both managed honeybee colonies and native bees,” Fearon said. “This management strategy could be especially crucial in agricultural areas where crop flowers are visited by both honeybees and native bees—places that may be hot spots for viral transmission among bee species.”

Source: Credit: Jim Erickson

Study Link (Ecology)


Most pathogens are embedded in complex communities composed of multiple interacting hosts, but we are still learning how community‐level factors, such as host diversity, abundance, and composition, contribute to pathogen spread for many host–pathogen systems. Evaluating relationships among multiple pathogens and hosts may clarify whether particular host or pathogen traits consistently drive links between community factors and pathogen prevalence. Pollinators are a good system to test how community composition influences pathogen spread because pollinator communities are extremely variable and contain several multi‐host pathogens transmitted on shared floral resources. We conducted a field survey of four pollinator species to test the prevalence of three RNA viruses (deformed wing virus, black queen cell virus, and sacbrood virus) among pollinator communities with variable species richness, abundance, and composition. All three viruses showed a similar pattern of prevalence among hosts. Apis mellifera and Bombus impatiens had significantly higher viral prevalence than Lasioglossum spp. and Eucera pruinosa. In each species, lower virus prevalence was most strongly linked with greater pollinator community species richness. In contrast, pollinator abundance, species‐specific pollinator abundance, and community composition were not associated with virus prevalence. Our results support a consistent dilution effect for multiple viruses and host species. Pollinators in species‐rich communities had lower viral prevalence than pollinators from species‐poor communities, when accounting for differences in pollinator abundance. Species‐rich communities likely had lower viral prevalence because species‐rich communities contained more native bee species likely to be poor viral hosts than species‐poor communities, and all communities contained the highly competent hosts A. mellifera and B. impatiens. Interestingly, the strength of the dilution effect was not consistent among hosts. Instead, host species with low viral prevalence exhibited weaker dilution effects compared to hosts with high viral prevalence. Therefore, host species susceptibility and competence for each virus may contribute to variation in the strength of dilution effects. This study expands biodiversity–disease studies to the pollinator–virus system, finding consistent evidence of the dilution effect among multiple similar pathogens that infect ‘replicate’ host communities.