Carlsbad's 8 million 'lost' bats likely never existed
Thermal imaging and algorithms challenge famous estimate
By _Susan Milius_
(http://www.sciencenews.org/view/authored/id/70/name/Susan_Milius)
Web edition : Friday, January 23rd, 2009
Eight million is a lot of bats to lose, and now a new study may explain what
happened to the possibly lost bats of Carlsbad Cavern.
Short answer: According to a Boston University team, the famous 8 million
bats never existed in the first place.
From spring to fall, the cave Carlsbad Cavern in New Mexico’s Carlsbad
Caverns National Park still hosts hundreds of thousands of migratory Brazilian
free-tailed bats that thrill visitors by boiling out of the cave at dusk for a
night’s foraging. All the bats roosting in the cave emerge in a dense plume
that streams on and on and on, sometimes for an hour or three.
As with many wildlife spectacles these days, always present is the
disturbing possibility that today’s show is a mere wisp compared to the great
Carlsbad
bat clouds of yore.
In 1937 V.C. Allison published an estimate of the Brazilian free-tailed bat
numbers based on timing an emergence (14 minutes at great density; four
minutes at half that) and eyeballing the speed and size of the stream. About
8.7
million bats roost in the cavern, he reported.
Since then, methods and numbers have varied, but estimates haven’t topped a
million. Consequently, conservationists have raised alarms about perils to
bats. Or maybe Allison’s eyeballs played tricks on him, or the great emergence
flights really have shrunk drastically.
Starting in 2005, bat scientist Thomas Kunz of Boston University and
colleagues brought new technology to Carlsbad Cavern to count and observe the
animals. Parts of the cave where bats roost are closed to visitors to prevent
disturbances to the animals. But to improve the census and studies, the park
allowed Kunz’s team to venture into these portions of the caves.
One of the first field biology groups to use military-derived thermal
imaging, Kunz’s team attracted the U.S. Park Service’s interest by pointing
out
that the researchers didn’t need to shine any lights, even at infrared
wavelengths, on the bats; the cameras detect heat directly.
“Surreal” and “disgusting, yet absolutely amazing” is how Nickolay
Hristov, now at Brown University in Providence, R.I., describes the roosting
sites. “
Imagine standing on a 20- to 30-foot cushion of bat poop covered with a
constantly moving carpet of dermestid beetles and their larvae,” he says.
“As you move around you are being rained on by bat urine,” Hristov says.
Bat excretions don’t have the same odor as human equivalents, he says, but “
the smell of ammonia is so strong that your eyes burn.” A single bat barely
makes any noise that humans can hear but tens of thousands of them together
get “
quite loud,” he says. ”I would grab the camera and go back in a heartbeat.”
To count the bats emerging, the researchers set up cameras around the cavern
mouth to get a clear view of the stream. Magrit Betke of Boston University’s
computer science department developed algorithms for analyzing the camera’s
recordings. Her work basically allowed a computer to pinpoint bats as spots
in a camera frame and then track the spots across enough frames to confirm the
dots were indeed bats. The analysis ends up with a count of each spot in the
vast stream.
In a series of counts in 2005, numbers varied from a low of not quite 70,000
as bats started to arrive from their southern winter caves, to a peak about
10 times higher weeks later as migrating bats on their way elsewhere took
shelter.
Even at the peak, counts came up some 8 million bats short of the old
estimate. So the Boston team used the Brazilian free-tails’ average 0.28-meter
wingspread to model how many bat wing-beat “spheres” would fit through the
cavern in a minute.
A choke point inside the cavern narrows to only 120 square meters, and bats
don’t fly wall-to-wall. At most, 50,000 bats per minute could fit through
that choke point and emerge from the cavern mouth. Thus a single million would
be closer to the number of bats possible that wowed Allison.
For 8.7 million bats to have flown through the choke point in 18 minutes, as
Allison reported, the densest crowd would have had to pass through at
500,000 bats per minute. Their wings and bodies would have had to pass through
each
other to somehow squeeze through the passage.
“The Boston study clearly shows there’s no physical way that could happen,”
says Renée West, supervisory biologist for Carlsbad Caverns National Park. “
That’s a relief.” The park has discounted Allison’s numbers as excessive,
she says, and she’s glad to have the new analysis.
“That doesn’t mean these bats aren’t declining,” Hristov says. “The
declines just haven’t been as bad.”
And for the cavern’s human visitors, hundreds of
