Over a six-year period in south-central Pennsylvania, measures of biodiversity among wild bee communities declined and a third of species experienced a decrease in abundance, according to a team of researchers led by the Penn State.
According to the researchers, the results of their recently published study demonstrate the value of standardized sampling at the seasonal scale over several years to identify patterns of bee biodiversity and monitor population trends among species.
“Pollinators facilitate the reproduction of more than 80% of flowering plants and increase the yield of about three-quarters of cultivated species,” said the study’s lead author, Nash Turley, postdoctoral researcher in entomology at Penn State’s College of Agricultural Sciences.
“Bees are one of the most important pollinator groups, but previous research has revealed troubling declines among wild bees,” he said. “For example, the range and abundance of some species have drastically decreased, especially bumblebees in North America and Europe. Tracking changes in bee biodiversity is important for developing pollinator management plans that can help maintain wild plant communities and maximize crop yields.
In this study, the research team set out to characterize changes in bee community biodiversity and changes in the abundance of specific species, both within individual years and from year to year. covering the period from 2014 to 2019. The study took place in and around the Penn State Fruit Research and Extension Center, near Biglerville in Adams County.
The researchers sampled bees at eight locations adjacent to four active apple orchards, collecting bees continuously from April through October each year and removing specimens from traps weekly for species identification.
“These orchards are in a landscape that has a great diversity and abundance of native plants and pollinators,” said study co-author David Biddinger, a fruit tree research entomologist and professor of entomology. at the Fruit Research and Extension Center. “Only about 8% of the landscape is active orchard, and all of it is successfully pollinated only by wild pollinators.”
The researchers, who recently published their results in Ecology and evolution, examined more than 26,700 individual bees representing five bee families, 30 genera, and 144 species. “We collected 33% of the total number of bee species found in Pennsylvania,” Turley said.
Ten species had more than 1,000 individuals collected, while more than half of the species had five or fewer individuals. “It’s typical in nature that there are a few very abundant species and many rare species,” Turley explained.
The largest number of specimens and species collected came from the family Apidae – which includes bumblebees, honey bees, carpenter bees and other commonly seen species – followed by Halictidae, often called sweat bees.
Scientists have found strong evidence for seasonal changes in all measures of biodiversity, indicating that bee communities are completely different almost every month. When measuring abundance, for example, they counted an average of 21 bees per site in April, compared to 168 bees per site in July. Species richness, or the number of species present, showed a similar trend, with an average of nine species found per site in April, dropping to an average of 21 species in July.
The researchers spotted three general trends per month. Some solitary species emerged early in the year and had a short period of activity. Other solitary ground-nesting species also had short periods of activity, but in summer rather than spring. The third group consisted mainly of social species with much longer periods of seasonal activity.
Such seasonal variation is an important target for monitoring, according to study co-author Margarita López-Uribe, associate professor of entomology and Lorenzo L. Langstroth Early Career Professor.
“These groups of bees provide unique ecological functions,” she said. “For example, many early-emerging bee species are critically important to early-flowering plants such as ephemeral spring wildflowers, and these bee-plant interactions may be particularly susceptible to disturbances from climate change. And many crops, such as apples and blueberries, depend on pollination by early emerging wild bees.”
Evidence for changes in biodiversity over the years was also strong, the researchers noted. For example, the average abundance of bees captured decreased by 48% and the number of species detected dropped by 41%.
At the species level, monitoring suggested that 26 species were stable over time, with no detectable change in abundance. However, 13 species, or about a third of the species for which researchers had sufficient data, declined in abundance between 2014 and 2019. Most of the declining species were bumblebees and sweat bees, Turley said. In contrast, only one species increased in abundance over the study period.
The researchers stressed that it will take more years of monitoring to determine whether the changes they have observed over time are part of a larger trend or are the consequence of year-to-year fluctuations. .
“Wild bee communities are diverse and dynamic, and little is known about which species or groups have the greatest conservation needs,” López-Uribe said. “Our results could help quantify the effects that different aspects of environmental change have on bee communities and identify species of conservation concern.”
Neelendra Joshi, Associate Professor, Department of Entomology and Plant Pathology, University of Arkansas, also contributed to this research.
The Pennsylvania Department of Agriculture and the National Institute of Food and Agriculture of the United States Department of Agriculture supported this work.