The future of bees may depend on understanding their past.
Bees are in trouble, any entomologist will tell you. Honeybee colonies in the United States have suffered devastating losses
in recent years. But colony collapse disorder, as it’s called, affects
only the species kept in beehives — the European honeybee, Apis
mellifera. There are almost 20,000 species of wild bees, and they aren’t
faring well, either.
Nearly
a third of bumblebee species in the United States are declining. In the
Netherlands, more than half of the country’s 357 species of wild bees
are endangered. Many species of plants, including crops, depend on wild
bees to spread their pollen. When they lose their pollinators, they may
suffer, too.
“It’s
essential to know what is causing those declines,” said Jeroen Scheper,
a graduate student at Alterra, a research institution at Wageningen
University in the Netherlands.
But
it is not enough to consider the many challenges — from pesticides to
parasites — that wild bees face right now. “We need to go back in time,”
said Mr. Scheper.
Mr.
Scheper and other scientists have tried to solve this puzzle by taking
advantage of the patient — some might say obsessive — work of
naturalists over the past 140 years. Through much of North America and
Europe, these unsung heroes carefully tallied sightings of bees year
after year. They caught bees, stuck them on pins, and stored their
desiccated little bodies by the thousands in museum cabinets.
Those impaled bees have been resting in their darkened drawers, waiting for scientists to pay them a visit. And now they have.
Recently, Ignasi Bartomeus,
then a post-doctoral researcher at Rutgers University, and his
colleagues tapped this vast supply to reconstruct the history of bees in
the Northeast. They searched the bee collections at the American Museum
of Natural History, the New York State Museum, and a number of
university collections.
All
told, they examined more than 40,000 wild bees. They whittled their
survey down to just 30,000 specimens for which there was clear
information about when and where they had been caught.
Studying the 438 species in their database, they found that the diversity of wild bumblebee species in the region declined by 30 percent between 1872 to 2011. (The diversity of the bees overall declined by a more moderate 15 percent.)
As
scientists gain a better understanding of the history of bees, they are
also starting to gather clues about what has been driving the changes
they are documenting. In their new study, published this week in
Proceedings of the National Academy of Sciences, Mr. Scheper and his
colleagues analyzed detailed records about bees in the Netherlands to determine how their populations changed during the twentieth century.
Then
the scientists looked for what the declining species had in common.
They examined a number of possible factors — how common bee species were
at the beginning of the century, for example, and how far they
typically flew to find food, and how big they grew.
The
scientists were even able to study what bees ate all those decades ago.
Mr. Scheper and his colleagues visited seven Dutch museums, where they
inspected the bee collections. When they peered closely at the insects,
they could see pollen grains stuck to the legs of some them.
Placing the pollen grains under a microscope, the scientists identified the plants that the bees had visited.
As it turned out, the fate of the bees often was tied to that of the plants they depended on.
The
growing intensity of farming in the Netherlands since the 1950s hit
many wild plant species hard. “There were a lot more flowers in the
landscape before,” said Mr. Scheper.
Dutch farmers cleared more land, used more toxic herbicides, and blanketed their farms with fertilizers.
Some
wild plants were able to survive these challenges, but others became
scarce. Mr. Scheper and his colleagues found that the bees that
preferred declining plants also declined.
This
link held true even for bees that collect pollen from dozens of plant
species. The results suggest that without the preferred kind of pollen,
the bee larvae suffered.
Mr.
Scheper and his colleagues also found that big bees were at greater
risk than small ones. He suspects that is because big bees need to eat
more. If the plants they depend on get harder to find, they are more
likely to suffer than smaller bees.
“The
results are compelling and make a lot of sense,” said Dr. Bartomeus,
who was not involved in the Dutch study. “If your food source is
declining, your populations will suffer.”
Laura
A. Burkle, an ecologist at Montana State University who also was not
involved in the new study, cautioned that food might not be the only
explanation for the results. The changes in landscape that stripped away
pollen might also have ruined bee nesting sites.
“We don’t have a solid understanding of which of these main resources is most limiting to bees,” said Dr. Burkle.
Mr. Scheper said that policies for restoring bees will have to take their preferred plants into account.
“If
you want to slow down or reverse the decline of a species, you can’t
suffice with general measures,” he said. “Bee species that need red
clover are not helped with dandelions. I know that policy makers prefer a
simple and quick answer — ‘Just do this and you’ll get this.’ — but
it’s not that simple.”
