The Berkeley Science Review: Articles

Hive Minds
Researchers uncover reasons to care about California’s native bees (view PDF)
by Jacqueline Chretien

Times are dire for many farmers who depend on the European honeybee to pollinate their crops. An estimated $18 billion worth of US fruits and vegetables, including kiwi, avocado, cucumber, almonds, blueberries, cherries, watermelon, and even chocolate depends on honeybees for pollination, which is required for robust fruit development. Until recently, farmers have been able to rent hives of these social insects for $50–150 apiece, unleashing one to five hives, or up to 250,000 bees, per acre. Unfortunately—you may have read the headlines—these managed colonies of honeybees have been dying off in record numbers. This mysterious “colony collapse disorder” has cast US agriculture’s dependence on a single delicate species for its pollinating needs into sharp relief, causing many to ask whether a new approach might be necessary.

Environmental science, policy, and management professor Claire Kremen has advocated a new approach for some time. Since 1998, her group has studied the habits of native bees, which evolved in the Americas and carried out the majority of pollination on local farms long before the practice of renting hives of European honeybees (introduced to this country in the 1600s) became widespread. The group has been especially interested in the impact of modern farming practices on the health and activity of native pollinators.

Industrial farming has not been kind to native bees. The transformation of wilderness into farmland has drastically limited nesting space, while the trend toward monoculture (growth of a single crop over a large area) means that the land only provides nutrition for a small fraction of the bees’ active foraging season. Most native bee species actively collect pollen and nectar for three months or more, but the average crop is only in bloom for a few weeks to a month. Managed honeybee colonies also suffer from a lack of diverse food sources; in fact, some researchers have suggested that malnutrition could render them particularly susceptible to the virus that seems to cause colony collapse. Meanwhile, the overall increase in crop production means that more bees are needed to pollinate ever larger areas of farmed land. The rise of industrial farming has created two problems, Kremen says; “it has demanded a source of pollinator[s] and eliminated the ones that were there.”

Habitat for Humani-bee
As an assistant professor at Princeton in 2002, Kremen found that for crops with heavy pollination requirements (they studied watermelon as an example), only organic farms with significant natural habitat nearby had native bee communities large enough to service the entire crop. All other farms studied—including large industrial farms and organic farms without natural habitat nearby—supported far less bee diversity and abundance, both of which are key to maintaining healthy and productive farms. (Kremen won a 2007 MacArthur “genius” Award for this work and her conservation research in Madagascar).

Bee diversity is like an insurance policy for crop yield. “If we have more species, we definitely have more stability,” Kremen explains, “because different species peak in abundance in different years.” When crops are pollinated by many different types of bee, species that are in an upswing make up for species that aren’t doing as well, allowing production to remain relatively consistent from year to year. In contrast, when a single species is responsible for all pollination, crop yield is exquisitely sensitive to fluctuations in that species’ health. Unfortunately, “most bees are responding to agricultural intensification in the same way. They’re all declining,” Kremen says. This has led farms to rely on rented honeybees, and, it is feared, will lead to more inconsistent harvests in the future.

Currently, Kremen and her group at UC Berkeley are exploring ways to reverse this trend. In collaboration with the Xerces Society, Audubon Society of California, the Farm on Putah Creek, and the Center for Land-Based Learning, they are working to encourage farmers to restore native pollinators to their farms. In some cases, patches of farmland will be allowed to return to their wild state, while other farms are introducing blocks of land filled with bee-friendly plants and nesting materials. The next step will be to quantify these restoration efforts by measuring whether the changes enhance the abundance and diversity of the bee community. The group also hopes to assess changes in crop yield, though this is trickier to measure. The common practice of moving crops to different plots in each growing season makes it difficult to separate the effects of increased bee diversity from the effects of a new crop position (e.g. better sunlight, better drainage, better soil, or proximity to other crops). Instead, the group will chart the health of potted plants that will remain in the same location over the course of several growing seasons.

Though Kremen’s research suggests that restoration of native habitats on or near farms will be the most effective way to enhance populations of native pollinators, there are less complicated ways for farmers to boost their local bee species. Most of the suggested measures focus on providing nutrition for bees over a longer portion of their flight season. One simple fix is for farmers to be less zealous about the eradication of weeds. When weeds on the borders of plots are allowed to flower, they can provide sustenance for pollinators when farm crops aren’t in season. Similarly, allowing cover crops (non-edible crops planted to prevent soil erosion) to bloom before plowing them under could attract and support a larger number of bees. “The culture now is clean farming, fencerow to fencerow, and in a sense what we need is to go back to an earlier model,” Kremen says.

As a bonus, she adds, “things we do to help native [bees] would also help honeybees.” The introduction of more wild food sources would help keep honeybees healthy. But the mere presence of native bees can also help honeybees, at least indirectly. In a paper published in 2006 in the Proceedings of the National Academies of Sciences, Kremen found that interactions between native bees and honeybees caused a five-fold increase in the honeybees’ pollination efficiency, as bumblebees searching for mates drove honeybees to vacate flowers—and therefore visit new ones—faster than they would have on their own.

Country bee, city bee
The value of bee diversity on the farm is apparent, but what about bee diversity in the city? According to Professor Gordon Frankie, also of the Department of Environmental Science, Policy and Management, “they’re part of the heritage of the land... they remind us that relationships have developed over the years between all of the fabulous plants that we have in California and the bees that pollinate them. There’s a historic evolutionary value in knowing that.” The benefits of city bees go well beyond the historic. A healthy population of native bees can increase the bounty from fruit trees and vegetable gardens, boost the seed yield of ornamental plants, and even induce flowers to bloom more plentifully.

As on the farm, development in the city has seriously reduced the available habitat for native bees. Only about 250 of California’s 1600 species of native bees have been observed in urban environments. But it’s likely that more could survive in city parks and gardens, as long as those parks and gardens contain the plants that bees enjoy the most. “[In] urban environments you can [even] get specialists (bees that feed on only one or two flowers), if you have the right plants,” Frankie says.

A major tenet of Frankie’s research has been identifying the “right” plants to lure native bees to city gardens. By counting the number of bees that land in a particular flower patch in the space of three minutes, his group has come up with a list of plants that are most attractive, at least from a bee’s-eye view. Frankie’s group has found that while native bees are attracted to both native and exotic (non-native) plants, they are extremely picky about the exotics they choose to visit. Only nine percent of all exotic plants tested attracted California native bees, compared to almost 80 percent of native plants tested. Within plant families, bees seem to prefer plant varieties that occur naturally in California over similar plants that hail from other areas.

Of 1,000 total plant varieties tested by the group, only 129—39 of 50 native plants, and 90 of 950 exotic plants—attracted a significant number of native bees. Fortunately for botanical aesthetes, many of these plants are attractive to humans as well. California poppies, borage, sunflowers and gold marie are all on the list of top bee attractors. Furthermore, many plants that bees like to land on are also enticing to more conventionally pretty insects (e.g. butterflies), as well as hummingbirds.

The type of plant isn’t the only important factor in bee preference, either. Just like humans looking for prime real estate, bees searching for a patch of plants are all about location, location, location: The group found the most bee diversity and abundance in gardens with many different bee-attracting flowers in bloom, and found that more native bees are likely to make their nests in gardens that had high plant diversity overall.

“If you plant it, they will come”
The group began putting their findings to the test in 2003 by planting a bee-attracting garden in the Oxford tract just northwest of the main Berkeley campus. After planting clusters of the most highly attractive plants, graduate students Jennifer Hernandez and Vicky Wojcik found that it didn’t take long for native bees to arrive. Within just one month of planting, they found holes in their rose leaves, a tell-tale sign that native leaf cutter bees (Megachilidae) had built nests nearby. Many other bees followed. “In the [entire] city of Berkeley we had already recorded 82 species of bees, and in that garden alone…we’ve had 40 species since we set it up,” Frankie says. The bees identified in the Oxford garden included four species that had not been found elsewhere in Berkeley, and more new species are expected as the project becomes better established.

Another key to attracting native bees is to plant a large enough patch of their preferred flowers. “We like a meter square, even two meters is better, if you have that kind of space,” Frankie says. “Then you can see [the bees] at work; they seem less likely to be disturbed, and you can get up fairly close to them. It’s when you have little tiny patches of plants that they don’t stay very long and tend to move around very quickly. But if you have these bigger patches, they stay there.” It’s also necessary to choose flowers that will bloom at different times—most urban bees feed on the nectar and pollen of several different plants and need diverse sources of food to cover their entire active season.

Nesting sites are another consideration. A giant hive buzzing with thousands of insects may be the first thing to come to mind when you think of a “bee’s nest”, but most bees that are native to California are solitary, and only need burrows large enough to lay 10-20 eggs. However, their preference for underground nests makes good sites hard to find. Few gardeners are tolerant of the bare patches of dirt and uneven slopes that California’s native digger bees find most hospitable, and Frankie cites the resulting “mulch madness” as a major threat to urban bee habitats. Garbage bag-like weed barriers laid just under the topsoil are another scourge, creatively referred to as “BPI”—short for “Black Plastic Insanity”.

Perhaps the most important step toward restoring native bee habitats in the city is outreach and public education. “Our job really is to make these things as cool as we can,” Frankie says. “It’s a matter of getting information out there so people can use it.” To that end, he and his group have summarized their research and, naturally, posted it on the Internet. Their Urban Bee Gardens website (nature.berkeley.edu/urbanbeegardens), authored largely by former Berkeley undergraduate Mary Schindler, discusses the importance of native bees, offers tips for creating bee-friendly gardens, and is easily accessible to the average California gardener.

Frankie says he believes that the website has played a huge role in increasing the visibility of his research, which seems to have hit critical mass: “We can’t keep up with the invitations we’re getting, from garden clubs, from museums, botanical gardens, school garden efforts, it goes on and on...we’ve had three or four TV stations want to do things with us. We had options to go on the [MacNeil-]Lehrer News Hour, Martha Stewart. But I didn’t want to do that,” he laughs. Despite declining Martha’s invitation—politely, one imagines—the group has expanded their public efforts considerably over the last few years. In addition to the research garden on Oxford Street, Frankie has helped develop bee gardens at the Middle Creek Middle School in Marin (with teacher Sue Holland), at the Randall Museum in San Francisco, and most recently, at Emerson Park Garden in downtown San Luis Obispo.

The return of the natives?
Despite the challenges faced by native bees, both Kremen and Frankie seem optimistic. In fact, with increased awareness of the important roles native pollinators play both on the farm and in the city, a new era of bee-friendly conservation may be arriving. “The more you tell people about it, the more they get interested, to the point where some of them want to become citizen-scientists,” Frankie says (which is why the Urban Bee Gardens website offers instructions on how to put the group’s bee count protocol into practice in a home garden). And, he notes, many human practices that threaten native bees can be combated simply by telling people about the impact they are having. “I just got an e-mail from [the head horticulturist] at Descanso Gardens [a large botanical garden in Flintridge CA],” Frankie says. “He was telling me, ‘by the way, we’re not going to mulch as heavily as we did before. We’re actually clearing dirt in places where we had a lot of mulch before, to help the bees.’ And I said, ‘well that’s good, then—it’s working.’”

Change is happening beyond the individual level, too. In part due to the effects of colony collapse disorder, Kremen and her group have had the opportunity to interact with state and federal governments to suggest policy changes based on their research. In particular, she has offered advice on how to include pollinator research in the 2007 Farm Bill. “It’s compatible with existing programs; the Farm Bill conservation programs just need to be tweaked,” Kremen says. “We’re actively seeking for pollinators to be recognized in the Farm Bill for research and habitat conservation...In general, there needs to be more funding for the conservation provisions of the Farm Bill, and we need more outreach.” California Senator Barbara Boxer introduced one major piece of Farm Bill legislation, the Pollinator Protection Act, which will allocate 89 million dollars over five years for USDA research on bees and other native pollinators. (The provision made it into the Farm Bill passed by the US House of Representatives this spring, but has yet to be approved by the Senate or President Bush.)

“It’s interesting to know how we depend on nature for things we take for granted,” Kremen says. The attention that has been paid to colony collapse disorder in the government and in the media suggests that we’re no longer taking pollinators for granted. And, thanks to the efforts of researchers like Kremen and Frankie, we may soon be better able to understand our complicated relationship with native bees.

Jacqueline Chretien is a graduate student in molecular and cell biology.

Want to know more?:
Check out the Frankie lab outreach page: nature.berkeley.edu/urbanbeegardens

Xerces Society:xerces.org/home.htm

Center for Land-Based Learning and Farm on Putah Creek: landbasedlearning.org

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