THE MEXICAN free-tailed bat ranks among North America's most ecologically and economically important animals, yet this species appears to be in alarming decline. In the early 1960s, the colony in Eagle Creek Cave, Arizona, consisted of 25 to 50 million free-tails; six years later, it had declined to less than 30,000. The famous Carlsbad Caverns, New Mexico, colony, estimated to contain 8,700,000 in 1936, had fallen as low as 218,000 by 1973. Other major declines are reported, though most free-tail caves have not been censused in many years.
Pesticides were a contributing factor in the dramatic decline at Eagle Creek Cave. These bats fed over nearby agricultural areas where many dead and dying bats were observed, and it is documented that sick free-tails in Arizona contained dangerous levels of DDT and its metabolites. Chemical pesticides have also been implicated at Carlsbad. Studies have shown that these bats feed heavily on agricultural pests and thus are exposed to toxic chemicals. Stored in fatty tissues, toxins are released when the bats metabolize fat during migration, or as they learn to fly.
Despite the contribution of pesticides to the decline of these bats, many researchers have concluded that human disturbance and vandalism of key roosting sites are by far the most serious causes of decline. Eagle Creek Cave is easily accessible, and the cave entrance is littered with gun shell casings. The Carlsbad population also likely suffered when a shaft was bored through the hole of the main roosting area to accommodate guano mining in the early 1900s.
But what happens to the bats once they migrate south for the winter is of even greater concern. In a recent BCI-sponsored study, Arnulfo Moreno visited the 10 largest over-wintering caves known in Mexico and found that half had declined 95 to 100 percent [BATS, Summer 1991]. There were no bats at all in one that had been home to many millions. Human disturbance and intentional burning, which may have killed millions of bats at a time, were clearly the cause.
Such enormous mortality has been caused by building fires in the entrances to free-tailed bat overwintering caves. Fueled by old tires and gasoline, these blazes not only can trap millions of bats inside, but also leave a toxic coating which may prevent recolonization. Fires are often set by phosphate miners who want to work year-round, but fear working in caves with bats present. Other caves are dynamited shut in misguided attempts to control vampire bats (Desmodus rotundus), though such approaches always cause far more harm than good [BATS, Spring 1991].
Ensuring the survival of large populations of free-tails throughout their range is a high conservation priority for BCI. This summer, Ambassador Robert Pringle, Director of the Office of Terrestrial Ecology and Conservation at the U.S. State Department, hosted a special meeting in Washington, D.C. at which Merlin Tuttle and freetailed bat expert Gary McCracken explained the plight, needs, and values of free-tailed bats to 15 representatives from appropriate federal agencies. This will be followed by a special State Department initiative and meeting in Mexico City on October 24 to consider collaborative efforts between our two countries on behalf of these important animals.
Just because free-tailed bat colonies still number in the millions doesn't mean we shouldn't be concerned. The examples of the passenger pigeon and the American bison should be reminder enough that even vast numbers of animals can disappear. The time to be concerned is now-before they become endangered.