As part of a study on the diversity of wild bee species involved in pollinating fruit crops over multi-year periods, a group of researchers mapped the timing of peak abundance for each species. One of the more than six dozen species of wild bees to visit eastern watermelon fields was Agapostemon texanus, sometimes known as the Texas sweat bee (male shown here). (Photo by Thomas Langhans via Flickr, republished with permission)

By Leslie Mertz, Ph.D.

Leslie Mertz, Ph.D.

Greater biodiversity yields greater ecosystem resilience. Despite the overwhelming acceptance of this concept, called the “insurance hypothesis,” validation for it has been sparse. A detailed study of wild bee species in fruit crops, however, has provided clear data showing that diversity in these vital pollinators is necessary for consistent flower cross-fertilization over multiple years.

“It’s a little surprising to me: This is one of the more talked-about ideas dealing with sustainability, biodiversity, and ecology,” says Rachael Winfree, Ph.D., professor of ecology, evolution and natural resources at Rutgers University. “But people haven’t really tested it empirically all that often—actually collected the data to see how it plays out—and that is exactly what this paper did.”

Winfree is a co-author of the study, published in August 2022 in Nature Ecology & Evolution. It tracked which wild bee species were doing the pollinating in two common fruit crops and found that species not only rotated during a single season but also varied from year to year.

Blueberry is a native perennial, and Habropoda laboriosa (female shown here) is one of the wild bee species that specializes in pollinating blueberry blossoms. (Photo by USGS Bee Inventory and Monitoring Lab via Flickr, public domain)

Watermelon is planted in a staggered fashion, so the crop’s overall bloom time is longer than that for blueberry. One of the wild bees that is important in watermelon pollination is the Triepeolus lunatus (female shown here). (Photo by USGS Bee Inventory and Monitoring Lab via Flickr, public domain)

“This is actually evidence that, yes, biodiversity really does matter. You do see fluctuations from year to year, so diversity does provide insurance for the pollination services the bees provide,” says Natalie Lemanski, Ph.D., who conducted the analytics side of the research as part of Winfree’s group and was the study’s lead author. Lemanski is now assistant professor of biology at Ramapo College of New Jersey.

For the study, the researchers identified which wild bee species were visiting blossoms over a three-year span on 16 blueberry farms in the eastern U.S. and 36 watermelon farms in the western U.S. as well as over a six-year span on 25 watermelon farms in the eastern U.S. That involved a great deal of meticulous identification work, but they took it one step further. They also wanted to know how much pollination each species was supplying, so they collected individual bees, allowed each to pollinate a virgin flower, and then counted the number of pollen grains deposited.

“So, if a single bee of this species deposits five pollen grains on average, then we can multiply that by how many visits this species is making in a given time period and use that to estimate the amount of pollen being delivered by a species in the field,” Lemanski says.

“That was quite time-consuming work to do and probably a big part of why we don’t have more of these types of longer-term datasets,” she says. “But, if you don’t look over the long term, you might be missing the fact that different bees might be important in different years or even in different parts of one year.”

The three- and six-year datasets provided a clear picture of the changeover in wild-bee pollinators. On the blueberry farms, the number of species needed to maintain a threshold level of pollination was 47 percent  higher over a three-year span versus a single year. On watermelon farms, the number of needed species was 62 percent higher over three years versus one year, and 219 percent over a six-year span versus a single year. Lemanski speculates that longer-span datasets would likely reveal that even more species engage in pollination.

Natalie Lemanski, Ph.D., now an assistant professor of biology at Ramapo College of New Jersey, reviewed years of wild bee data as part of a study while working with Rachael Winfree, Ph.D., at Rutgers University. The analysis found that many species participated in pollination of two fruit crops over the multi-year spans, with different species being important at different times during the season or from one year to the next. (Photo courtesy of Natalie Lemanski, Ph.D.)

Researchers Rachael Winfree, Ph.D., of Rutgers University, and Neal Williams, Ph.D., of the University of California, Davis, are two co-authors of a study providing evidence of the long-held “biodiversity insurance hypothesis.” The hypothesis suggests that greater biodiversity yields greater ecosystem resilience. (Photo courtesy of Rachael Winfree, Ph.D.)

Although the study didn’t investigate the specific causes of species variability, many things can account for differences over a season or over years, and that includes climate change. “Climate change can make a difference in a variety of ways. One way is just through more extreme weather events that may favor certain species over others,” Winfree says. “Whenever the environment you’re living in gets more variable, it tends to be the case that you benefit from having a lot of different species, because chances are you’ll have some that are okay with the current environment, and some that aren’t.”

This study underlines why detailed, multi-year studies, as well as careful analytics, are critical to understanding ecosystem function, Lemanski says: “Going out into the field in your cargo shorts and boots is an important part of ecology, but finding patterns in data is also a huge part of ecology work, and looking at the data over years and in new ways can give you new insights. Absolutely.”

Leslie Mertz, Ph.D., writes about science and runs an educational insect-identification website, www.knowyourinsects.org. She resides in northern Michigan.