It is dusk as we cross the Congress Avenue bridge at the edge of downtown Austin. A passenger in the back seat looks east to the horizon's fading rim of lavender and says, ``Look at that smoke.'' But the plumes of swirling blackness that trail across the sky are not smoke at all: They are bats. It happens every dusk, from March to October: First a bat or two, then a few more, come flitting out from under the concrete bridge where they have spent their day, sheltered in deep narrow channels that are a fluke of engineering.
Within minutes the trickle becomes a torrent, as more than three-quarters of a million bats, the largest urban bat population in the world, come whooshing out to spread into three or four lacy columns across the darkening sky.
Their ``emergence,'' as scientists call it, will last for up to a half-hour; then from dusk to dawn these bats will fan out across the land, eating up to 10,000 pounds of insects within a 50-mile radius of Austin.
Such spectacles of nature were once much more common, as large colonies of bats, some topping tens of millions, populated caves and crevices, treetops and rain forests, around the globe.
But the sight of large numbers of feeding bats is much rarer today, as the species, like so many others, has been decimated by the steady encroachment of man on its environment. In addition to man's ignorant neglect, moreover, bats have had the double misfortune of being labeled pests, or disease carriers, or worse.
But nothing could be further from the truth, says Merlin Tuttle, a world authority on bats and founder of Bat Conservation International, an Austin-based organization that labors to educate the world about bats' many irreplaceable roles.
``Bats form a pretty crucial link in nature's system of checks and balances,'' says Dr. Tuttle, whose organization has 6,500 members - double the number of six months ago - in 50 countries. ``Forty percent of the tree species on Guam depend on bats for pollination and seed dispersal,'' offers Tuttle as one example.
``One common species in the Southwest eats scorpions - that ought to make a lot of people happy,'' the scientist adds. ``But it lives primarily on grasshoppers and crickets, and I'm not sure a lot of cattlemen realize how much their cattle compete with those insects for grass.''
The examples continue - of desert plants whose flowers are only pollinated nocturnally by bats, of hundred-million-dollar tropical fruit crops that would be lost without the winged mammals.
Quite simply, Tuttle says, bats are the most important natural controllers of night-flying insects, from agricultural pests to mosquitos.
Rain forests probably could not exist without large numbers of bats, he adds.
One study done in West Africa showed that bats, through their role as seed dispersers, were responsible for 98 percent of a destroyed forest's initial regrowth. A typical fruit-and-nectar bat will eat three times it's weight a night. With an incredibly rapid digestive period - about 15 minutes - the bats are soon defecating in flight. And since they prefer to fly through forest openings, ``they end up dropping seeds in the very parts of the forest that need regrowth,'' Tuttle says.
When tropical fruit growers in Kenya were found killing bats because they eat fruit, Tuttle researched the problem and discovered that the bats were actually eating only the very ripest fruit that often had been left behind by pickers. Able to prove that bats played a vital role in keeping down fruit flies and fungi, Tuttle got many fruit growers to stop bat eradication projects.
Yet despite the evidence, the battle to protect bats remains difficult.
``It's a `glamor species' problem,'' Tuttle says. He notes that the same people who are willing to volunteer or send money to save pandas or spotted leopards or even whales ``wouldn't give a bat the time of day, and might even consider bat eradication an accomplishment.''
Tuttle says that he headed one bat preservation committee for several years but he never got a major project funded by big environmental organizations, even though a number of bat species became extinct during that time.
``People are far more likely to support efforts for furry, big-eyed, winsome mammals,'' says the gray-blond mammalogist, ``but eye size and cuteness are no measure of ecological value.''
Besides, according to Tuttle, who has been studying the species for more than 25 years, bats really are cute.
In his Austin office, Tuttle keeps the world's largest and most complete collection of bat pictures: 60,000 slides and photographs, many of them showing furry, big-eyed, winsome creatures that, often resembling chihuahuas or mice, present bats as Tuttle knows them.
His picture-taking began 10 years ago, when the National Geographic Society asked him to write a book chapter on bats. When Tuttle went to Washington to view mock-ups of the chapter, he was horrified to see that all the photos to go with it were of snarling, teeth-baring bats.
``They told me that was all they had, and I was amazed,'' says Tuttle. ``I'd never realized before how just one of those pictures, which were about all you saw at the time, could destroy everything I had to say about bats.''
Tuttle notes that, contrary to myth, bats are not vicious and do not suck blood (other than the vampire bats found in parts of Latin America, which do cause some problems for cattle growers).
Nor are bats common carriers of rabies, Tuttle adds - although he cautions against picking up sick or slow-moving bats, which like any animals could be infected.
Many of the world's recent success stories in bat conservation bear Tuttle's imprint.
In Samoa, where a lucrative market exists for bat-as-delicacy, Tuttle persuaded bat hunters to limit their kills in order to maintain the species. In Wisconsin, he helped pass the first state law against bat poisoning, and helped put two bat species on the federal list of endangered species as of next month.
Right now he's busy pressing for preservation of one central Texas cave where more than 20 million bats - more bats than there are Texans - summer each year.
Down at the Congress Avenue bridge in Austin, the bats have become a nightly summer attraction. But Tuttle says his happiness over the public's acceptance and growing understanding of the bat population here is tempered by knowledge of their continuing fragility.
The bats winter in caves in Mexico, many of which have already been firebombed or dynamited by misguided or fearful neighbors.
``If these bats have the misfortune of going back to the wrong cave in Mexico,'' he says, ``we might never see them again.''
But even people who continue to harbor an uneasiness about bats should realize how such a loss would effect them, Tuttle says.
Noting that the 20 million bats in that Texas hill-country cave eat up to a half-million pounds of insects a night, Tuttle adds, ``For people who like to sit outside or take an evening stroll, those bats certainly have an impact on their quality of life.''
As the last of the bats head off into the night sky, the small clumps of people who have assembled to witness this natural wonder seem to second Tuttle's words.
No lesser authorities than three teen-age boys, often thought to define the word ``blas'e,'' stand in awe beneath the waning ebony plume. One of them whispers, ``Wow. Now that's entertainment.''
(More information about bats may be obtained from Bat Conservation International, P.O. Box 162603, Austin, Texas 78716-2603.)