Bats that swoosh from publicly funded roosts are on the radar of Florida's highway planners.
By JEAN HELLER
Published September 14, 2004
[Times photo: Thomas M. Goethe]
A bat leaves a colony underneath the Laurel Street bridge over the Hillsborough River. The Tampa Bay area is a hot spot for bat populations that favor bridges, which transportation and wildlife agencies are studying.
[Photo by Jeff Gore, Florida Fish and Wildlife Conservation Commission]
A colony of southeastern bats roost in the expansion joint of a bridge in Walton County. Concrete bridges hold heat and help bats conserve energy.
When planning a new bridge, engineers always consider the geeky stuff: traffic projections, roadbed materials, geology and watershed.
Starting this fall, transportation officials in Florida also will start considering bats.
Not the baseball variety, but the little night-flying, bug-eating critters that seem, in increasing numbers, to be using Florida bridges as roosts.
"They're welcome," said Jeff Gore, statewide director of wildlife diversity for the Florida Fish and Wildlife Conservation Commission. "They are very helpful in controlling insects, particularly mosquitoes and other pests like moths and beetles. They protect crops and people. They're a key component of pest management."
About a decade ago, wildlife officials realized that bridges around Florida providing habitat for bats were being replaced with structures that were not always bat-friendly.
Eventually, the wildlife commission teamed with the Florida Department of Transportation on a study to determine which species of bats inhabit Florida bridges, which of the state's bridges they favor for roosting, and how those bridges could be maintained and even replaced without destroying them as habitat.
The study, which cost about $92,000, will be completed this fall.
"We're hopeful that bats will be allowed to stay in the bridges because they've lost so much habitat around the state," said Cindy Marks of the Florida Bat Center in the Bay Pines area of St. Petersburg. "The bridges are good for colony species that normally would roost in dead hollow trees. But those trees are disappearing."
State officials have confirmed bat populations in 151 state bridges and estimate the total number of bridges with homesteading bats probably exceeds 430. They are distributed throughout the state with the exception of extreme South Florida and the Keys.
"Tampa-St. Pete is a little hot spot for them," Gore said.
Locally, biologist have found populations on several Hillsborough bridges, including the Laurel Street bridge over the Hillsborough River in Tampa and Dickman Road over Dolphin Cove Creek near Apollo Beach. There also have been reports of bats using a Snell Isle bridge in St. Petersburg.
Biologists have found five varieties using state bridges: the southeastern bat, the big brown bat, the big-eared bat, the evening bat and both Mexican and Brazilian free-tails. Populations range in size from 100 to 5,000 per bridge.
"The research indicates that bats don't harm the bridges at all, and they're no danger to motorists," said Vicki Sharpe, the DOT's transportation ecologist. "The guano doesn't do any damage, though some of the bridges do require extra cleaning."
While guano doesn't harm the bridges, humans have to take some precautions, Gore said.
"People who work around the guano, inspectors or cleaning crews, have to be protected from histoplasmosis, a fungus that grows on the droppings," he said. "Other than that, there are no problems."
Most bat bridges, according to Sharpe, aren't structures like the Sunshine Skyway, the Howard Frankland or drawbridges.
"Typically they're concrete bridges, some over water, but many are overpasses that cross or carry highways that many people don't really think of as bridges," she said.
Condo-shopping bats favor bridges built of concrete because they have more nooks and crannies suitable for the little mammals, including the expansion joints, those horizontal splits in the roadbed that allow the concrete to expand and contract with the temperature.
"They like openings that are less than 2 inches wide but more than a half-inch," Gore said. "They're really small creatures, and a lot of them can pack inside an expansion joint."
An adult Mexican free-tail, for example, may weigh less than a half-ounce, despite a wingspan of 13 to 16 inches.
Bats don't like to have to expend energy by adjusting their body temperatures as the nighttime cools, and concrete bridges hold the heat of the day for hours. So far as anyone can tell, a good view is not a factor when bats choose a home.
Among recommendations in the study is making sure that repair and maintenance work is necessary where bat populations live, and if so, scheduling it for winter months when some bat populations move south.
Some work can be done through the night when the bats are away eating as many as 3,000 insects each. If a population absolutely has to be disrupted, engineers can hang nets from the bridges that let the bats fly out but stop them from returning. The technique also can be used to clear out a population when a bridge has to be dismantled.
"You hang the nets for a few nights and it convinces them to go somewhere else," Gore said.
And do they return when the maintenance work or bridge replacement is done?
"Sometimes they come back, and sometimes they find new roosting areas," he said. "It's not like there's a bat housing crunch."
People have little to fear from bats, Gore said. Normally, they are very passive creatures.
"If you ask local health departments, they will tell you that rabies in bats occurs at high rates," he said. "But that's based on the fact that they count animals that are turned in to them, and most of those are sick or hurt. When you consider that these are individuals from populations of hundreds of thousands, it doesn't seem so common."
In fact, bats make up one in every five mammal species on earth.
All of Florida's bat species are nocturnal, Gore said.
At the moment, at least, Florida's bats don't seem destined for the same notoriety as those that inhabit the Congress Avenue bridge in Austin, Texas.
There, according to Bat Conservation International, a population of up to 1.5-million Mexican free-tailed bats put on a nightly show from March to November as they leave their roosts in great bat clouds to go hunt for the city's plentiful insects.
It has been estimated that as many as 100,000 people visit Austin annually to view the nightly bat flight, generating as much as $8-million in revenue.
