For immediate release: December 9, 2002
Contact: Steven Schultz, (609) 258-5729, email@example.com
Stung by success: Intensive farming may suppress pollinating bees
Study shows native bee species provide valuable services when
allowed to flourish
PRINCETON, N.J. -- Intensive, industrial-scale farming may be damaging
one of the very natural resources that successful crops require: pollinating
bees. A study by Princeton scientists found that native bee populations
decline dramatically as agricultural intensity goes up.
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
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 species and we haven't
developed other avenues, we could really be in great difficulty."
The researchers spent two years examining watermelon farms located
at varying distances from oak woodlands and chaparral habitats that
are native to the Sacramento Valley. They also looked at land that
was farmed conventionally (with pesticides) and organically (without
pesticides). They focused on watermelon because it requires a lot
of pollen and multiple bee visits to produce marketable fruit.
The research required painstaking work. Kremen and Williams first
put fine mesh bags on watermelon flower buds, so that when the flowers
opened they had no pollen. They then removed the bags, put the freshly
opened flowers on the ends of sticks and presented them in front of
bees to tempt them to land. For each of about 20 species of native
bees that frequented the flowers, they determined the median number
of pollen grains deposited in each visit.
Then, in each of their selected locations, the researchers watched
watermelon flowers over long periods and recorded how many of each
kind of bee visited. They found that native bee visits dropped off
dramatically in the farms that were distant from natural habitats
and that used pesticides. "We could then multiply the number
of visits by the number of grains deposited per visit and sum that
up for all the species and figure out how much pollen the watermelon
plants were receiving," said Kremen.
"We found that, where it still flourished, the native bee community
could be sufficient to provide the pollination service for the watermelon,"
Kremen said, adding that the result is likely to apply to a variety
of other species. Farmers began renting bees many years ago to improve
yields and became dependent on them as the size and concentration
of farms increased. Typically, farmers whose lands are located near
natural habitat don’t bother to rent bees, presumably because
they receive sufficient pollination from the natural community, said
One interesting finding, said Kremen, was that the mix of native
bees providing the pollination was very different in the two years
of the study. In one year, a few strong pollinators accounted for
most of it, while in the other, many species contributed.
"That says something about the need for long-term studies and
also argues for the need to maintain diversity," said Kremen.
The research fits into a broader question that Kremen and others
are studying regarding the relation between biodiversity and what
ecologists call "ecosystem services," the economic benefits
that natural systems provide to people but that are not normally accounted
for in the marketplace. Scientists need carefully collected data to
quantify the value of biodiversity, Kremen said.
Kremen is now working on follow-up studies to determine what parts
of the natural landscape are critical for native bees and what parts
of the man-made agricultural landscape also may support native bees.
"Ultimately, we should be able to come up with a plan for restoring
this natural service across the agro-natural landscape," she