In farms studied in and around the Sacramento Valley in California, concentrated farming appeared to reduce bee populations by eliminating natural habitats and poisoning them with pesticides, the researchers reported.

U.S. farmers may not have noticed this effect because historically they have achieved their harvests with the help of imported bees rented from beekeepers. These rented bees, however, are in decline because of disease and heavy pesticide use.

The study, to be published this week in an online edition of the Proceedings of the National Academy of Sciences, found that native bees are capable of doing a lot more pollinating than previously thought. But it would take careful land use to take advantage of that capacity, the researchers concluded, because current high-density, pesticide-dependent agriculture cannot support native bees.

"This is a valuable service that we may actually be destroying through our own land management practices," said Princeton ecologist Claire Kremen, who co-wrote the study with Neal Williams, a postdoctoral researcher, and Robbin Thorp of the University of California-Davis.

Suppressing the many species of native bees and relying on just a few species of imported ones may be unnecessarily risky, said Kremen. Farmers who use managed bee populations -- that is, most commercial farmers -- depend on fewer than 11 species out of the 20,000 to 30,000 bee species worldwide. Other researchers have estimated that $5 billion to $14 billion worth of U.S. crops are pollinated by a single species of bee, the European honey bee Apis mellifera.

"Right now we are really very dependent on that species," said Kremen. "If something happened to that
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Princeton University
9-Dec-2002