City Gardens May Drive Bee Diversity, Study Says

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Sure, they boost garden health and productivity. But bees and other pollinators form a nearly invisible force behind what appears on dinner plates, pollinating 15 to 30 percent of U.S.-grown food.

It's thought that pollinators, which also include beetles and other insects, may be on the decline worldwide due to habitat loss, pesticide use, and changing agricultural practices.

While there's some debate about the extent of pollinator loss, "we have good evidence that pollinators are being driven to endangerment and extinction," said Scott Hoffman Black, executive director for the Portland, Oregon-based Xerces Society for Invertebrate Conservation.

Most pollinators pick up pollen by accident while trolling flowers for nectar. Bees, however, collect pollen for their young.

When a bee lays its eggs, it also provides a packet of pollen and nectar—like an energy gel for a long bike ride—for its offspring.

"That's what [a bee] lives on for the whole larval life cycle," Hoffman Black said.

Farmers mainly use the non-native European honeybee to pollinate their crops. But a parasitic mite, Varroa destructor, has started to wreak havoc on honeybee colonies.

The more bee species there are, the more likely pollination can happen even if conventional pollinators aren't available.

It's possible, for instance, that native U.S. bee species, such as those found in California cities, may be able to pick up the slack when honeybee populations falter. In a 2002 study in the journal Proceedings of the National Academy of Sciences, researchers found that on organic farms near natural habitat, native bees could provide full pollination services.

"These native bees can be a very good insurance policy when honeybees are scarce," Hoffman Black said.

The Buzz in Berkeley

When it comes to attracting bees, size isn't everything.

Frankie, the Berkeley entomologist, found more than 40 different bee species in a single 15-by-80-foot (5-by-24-meter) space. The key is having a wide range of bee-friendly plants that flower at the same time.

Frankie's studies have even spotted bees that come to a garden for one flower but end up frequenting nearby flowers that aren't normally on their pollen-shopping list—a phenomenon called the "mall effect."

For identification, Frankie sends most of his finds to bee experts Robbin Thorp, at the University of California, Davis, and Terry Griswold, at the U.S. Department of Agriculture's Logan, Utah, bee laboratory.

The large-scale effects of urban gardens on agricultural-pollinator populations isn't yet known. What is fairly certain, though, is that gardeners can draw more bee species if they choose the right mix of flowers.

"It's that little cliché that if you plant it, they will come," Frankie said.

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