In a New Study, Spring Forest Bees Get Their Due
By Leslie Mertz, Ph.D.
Temperate forests in the eastern United States come to life in April and May with colorful blankets of wildflowers, birds singing from newly leafing tree branches, and plenty of insect activity. That includes one greatly understudied group of native insects: the forest-living bees. A new study, however, is shedding much-needed light on the ecology of these often small but busy bees that do much of the spring pollination work in woodlands.
“It’s not like spring-flying bees are a new discovery. Any good field insect naturalist knows that there are a lot of spring-flying bees in forests, but they had just never been studied ecologically like this,” says Rachael Winfree, Ph.D., professor of ecology, evolution, and natural resources at Rutgers University. Her research group’s study was published in August in the journal Biological Conservation. It combines five years of field work, much of it done by then-doctoral students Colleen Smith and Tina Harrison, with a meticulous review of thousands of museum specimens to understand more about the bees’ connection to the forest habitat and how a changing landscape has affected their diversity and abundance.
The study identified more than three dozen bee species that live within forests and, in some cases, rarely leave, Winfree says. Until this study, she adds, “It had never been shown that you don’t really find these bees outside of forest habitat.”
To determine the habitat preferences of various bee species, Winfree’s team set out to find large areas of each of three habitat types —agricultural land, urban/suburban areas, or forests. In all, they selected 12 of each type spread out over New Jersey, New York, and Pennsylvania. All had a radius of at least 1.5 kilometers in which the encompassed habitat matched the selected type, and the researchers then collected specimens from the center of those areas. This was important because individual bees are quite mobile and can often fly distances up to 1 km (0.6 miles), so sampling from the center of large areas helped ensure that the bees actually preferred that particular habitat and weren’t just passing through, Winfree says.
After carefully examining and classifying each collected bee to species—itself an arduous task—and comparing data from the different habitats within each geographic area, the researchers were able to separate out the true forest species. Among the 118 total species they collected in good number from across the three habitats, they were able to name more than three dozen as very clearly forest-associated, including seven species of mining bees (in the genus Andrena), nine sweat bees (Lasioglossum), nine cuckoo bees (Nomada), and eight mason bees (Osmia). Besides definitively—and finally—distinguishing these bees as forest-associated, the researchers also compared the diversity and population sizes of the forest bees in large versus small wooded tracts and established that both were greater in larger forest patches.
Following Bees Over Time
Going even further, the study’s historical review of museum specimens provided a long-term view of forest bees from the logging era of the late 1800s and early 1900s and through to the reforestation of some of those lands today. This relied on data gathered during a previous project in which American Museum of Natural History entomologist and bee specialist John Ascher, Ph.D., painstakingly went through tens of thousands of bees that had been placed in the museum’s collection from 1872 to 2011, recording location, date and other information from their individual (and tiny) labels and also verifying or correcting each identification. The specimens came from the eastern United States, ranging from West Virginia and Ohio to Maine.
“This museum data showed us which groups of bees were changing relative to other groups of bees over time,” Winfree says. “And what we found is that the number of forest species have been increasing, which makes sense because forests were at their lowest in the 1920s and have been regrowing here since.” Admittedly, the forests today are nowhere near as expansive as the woods of the pre-logging times, she says, but the data show that, as trees have returned, so have spring-flying forest species.
Why It Matters, and What’s Next
Understanding forest bees is important for many reasons, Winfree says. One is that they are integral parts of the native ecosystem, performing services such as pollination of many of the spring-blooming wildflowers and trees, possibly even some timber trees. “It’s amazing, but people don’t know what pollinates a lot of trees,” she says. “Maples, for example, are economically important timber trees, but it’s not clear whether sugar, red, and Norway maples are insect- or wind-pollinated. While the dogma is that maples are wind-pollinated, these trees make nectar and have a lot of bees on them in the spring, so, while it hasn’t been documented, I think they may well be bee-pollinated.”
Today, Winfree is looking forward to investigating the rare forest-bee species that weren’t included in the new study. “From a conservation point of view, it’s the rare species that need a focus, so I hope to learn more about things like their habitat needs and what plants they’re associated with,” she said.
Whether rare or common, forest bees need a closer look, she adds. “We should care about them in the same way that we care about other endemic species in eastern North American forests,” she says. “It’s just that we haven’t really thought of bees as being part of that habitat, but they absolutely are. And we’re continuing to work on that.”
Leslie Mertz, Ph.D., writes about science and runs an educational insect-identification website, www.knowyourinsects.org. She resides in northern Michigan.