Study suggests one-third of wild bee species in Pa. have declined in abundance
Researchers say monitoring bee biodiversity can help in conserving pollinators
Peer-Reviewed PublicationIMAGE: A GREEN SWEAT BEE (AUGOCHLORA PURA) FORAGES ON GOLDENROD FLOWERS. RESEARCHERS FOUND THAT MANY SWEAT BEE SPECIES WERE AMONG THOSE THAT DECLINED IN ABUNDANCE OVER SIX YEARS IN SOUTHCENTRAL PENNSYLVANIA. view more
CREDIT: NASH TURLEY/PENN STATE
Over a six-year period in southcentral Pennsylvania, measures of biodiversity among wild bee communities declined and one-third of species experienced decreases in abundance, according to a Penn State-led team of researchers.
Findings from their recently published study, the researchers contend, demonstrate the value of standardized, season-wide sampling across multiple years for identifying patterns in bee biodiversity and monitoring population trends among species.
“Pollinators facilitate the reproduction of more than 80% of flowering plants and increase the yield of about three-fourths of crop species,” said study lead author Nash Turley, postdoctoral scholar in entomology in Penn State’s College of Agricultural Sciences.
“Bees are one of the most important groups of pollinators, but previous research has found troubling declines among wild bees,” he said. “For example, the range and abundance of some species have shrunk substantially, especially bumble bees in North America and Europe. Tracking changes in bee biodiversity is important for developing pollinator management plans that can help sustain 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 abundance of specific species, both during individual years and from year to year, covering the period from 2014 to 2019. The study took place in and around Penn State’s 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 high diversity and abundance of native plants and pollinators,” said study co-author David Biddinger, tree fruit research entomologist and professor of entomology at the Fruit Research and Extension Center. “Only about 8% of the landscape is active orchards, and all of them are successfully pollinated only by wild pollinators.”
The researchers, who recently reported 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 that have been found in Pennsylvania,” Turley said.
Ten species had more than 1,000 individuals collected, while over half of the species had five or fewer individuals. “It is typical in nature for there to be 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 bumble bees, honey bees, carpenter bees and other commonly seen species — followed by Halictidae, often called sweat bees.
The scientists 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 pattern, with an average of nine species found per site in April, increasing to an average of 21 species in July.
The researchers spotted three general patterns by 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 the summer rather than in spring. The third group was composed mostly 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 of the early emerging bee species are of critical importance for early flowering plants such as spring ephemeral wildflowers, and these bee-plant interactions may be particularly sensitive to disruptions from climate change. And many crops, such as apples and blueberries, rely on pollination by early emerging wild bees.”
The evidence of changes in biodiversity over years also was strong, the researchers noted. For instance, the average abundance of bees captured declined by 48%, and the number of species detected fell 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 one-third of the species for which researchers had sufficient data, declined in abundance between 2014 and 2019. Many of the declining species were bumble bees and sweat bees, Turley said. By contrast, only one species increased in abundance during the study period.
The researchers pointed out that it will take more years of monitoring to determine if the changes they observed over time are part of a larger trend or a consequence of year-to-year fluctuations.
“Wild bee communities are diverse and dynamic, and little is known about what species or groups have the greatest conservation needs,” López-Uribe said. “Our findings could help to quantify the effects that different aspects of environmental change have on bee communities and to 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 U.S. Department of Agriculture’s National Institute of Food and Agriculture supported this work.
JOURNAL
Ecology and Evolution
METHOD OF RESEARCH
Observational study
SUBJECT OF RESEARCH
Animals
ARTICLE TITLE
Six years of wild bee monitoring shows changes in biodiversity within and across years and declines in abundance
Bee it known: Biodiversity is critical to
ecosystems
A Rutgers-led study on bees shows how different species pollinate the same plants over time
Peer-Reviewed PublicationRutgers has conducted the first study showing how many more species of bees are needed to maintain crop yields when a longer-term time frame is considered.
In the paper, which was recently published in Nature Ecology & Evolution, scientists said biodiversity of the bee population is critical to maintaining the ecosystem function of crop pollination, which is critical to humanity’s food supply.
“We found that biodiversity plays a key role in the stability of ecosystems over time,” said Natalie Lemanski, lead author on the study and a postdoctoral researcher in the Department of Ecology, Evolution and Natural Resources at the Rutgers School of Environmental and Biological Sciences (SEBS). “You do actually need more bee species in order to get stable pollination services over a growing season and over years.”
The team on the study focused on various populations of bees at dozens of farms in New Jersey, Pennsylvania and California and found that many more bee species were not only needed for pollination than expected over an entire flowering season, but even more were needed over multiple years.
The researchers said they discovered different bee species pollinated the same types of plants at different times of the year. They also found that different bee species were the dominant pollinators on the same kind of plants in different years. Because of natural fluctuations in bee populations, researchers said, all bee species present were needed to maintain a minimum threshold of pollination during lean years.
“This research shows that abundance [of a species] matters, but bee diversity matters even more,” said Michelle Elekonich, the deputy division director of the National Science Foundation’s Directorate for Biological Sciences, which funded the study. “It’s not the same bees that are abundant at a given point in time, and variety is necessary to provide balance during a growing season – and from year to year.”
Lemanski said the study offers substantiation to a long-standing concept ecologists refer to as the “insurance hypothesis.” The idea is that ecosystems probably benefit when nature “diversifies the portfolio,” supporting multiple species of a category of a plant or animal, rather than relying on one dominant species.
“We found that two to three times as many bee species were needed to meet a target level of crop pollination over the course of a growing season compared to a single date,” Lemanski said. “Similarly, twice as many species were needed to provide pollination over the course of six years compared to a single year.”
The researchers based their analysis on their own extensive observations of bee visits to flowers and measurements of the quantity of pollen grains deposited on individual flowers over weeks and months within a given calendar year and then over multiple years. They collected the data, with permission of farmers, at 16 blueberry farms in South Jersey, at 25 watermelon farms in Central Jersey and eastern Pennsylvania and at 36 watermelon farms in the Northern Central Valley of California.
“The magnitude of increase in species needed over multiple years was remarkably consistent among crop systems when considered over the same interval of time,” Lemanski said. “In addition, the fact that the relationship between timescale and the number of species needed did not level off suggests that even longer time series, spanning multiple seasons, may further bolster the need for biodiversity to ensure reliable ecosystem service.”
Rachael Winfree, a professor in the Rutgers Department of Ecology, Evolution and Natural Resources at SEBS, was the senior author on the paper, which was also written in collaboration with Neal Williams of the University of California-Davis. Funding was provided by the National Science Foundation (NSF) and U.S. Department of Agriculture, National Institute of Food and Agriculture, Agriculture and Food Research Initiative.
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