Volume 11, Issue 2, Summer 1993

Bats and Mines: Finding Solutions

When renewed mining threatened an important bat winter roost in California, the mining company decided to help . . .

By Brown, Patricia E., Berry, Robert D., Brown, Cathi

When renewed mining threatened an important bat winter roost in California, the mining company decided to help . . .

by Patricia E. Brown, Robert D. Berry, and Cathi Brown

Abandoned mines are important roost sites for many North American bat species, but perhaps none are more dependent on mines than California leaf-nosed bats (Macrotus californicus). These bats of the desert Southwest are candidates for federal threatened or endangered status and are found year-round in abandoned mines and caves at elevations below 2,500 feet. Unable to lower their body temperatures and enter torpor, leaf-nosed bats conserve energy during the winter by congregating in groups of hundreds in especially warm roosts where temperatures are greater than 80 degrees Fahrenheit. In California, less than 20 such winter roost sites are known--all of them in mines.

Sixteen years ago we began to study the ecology and behavior of these bats at the American Boy and Golden Queen mines in southeastern California's Cargo Muchacho Mountains. As a result of this and a 25-year banding study of California leaf-nosed bats, we found that most live their entire lives--up to 15 years--in a single mine, rarely traveling more than a few miles from the original site where we first banded them. The population in the American Boy Mine was fairly stable, averaging 150 bats each winter.

This stability was threatened in February 1989 when renewed mining in the area began under the direction of the American Girl Mining Joint Venture (AGMJV). Initially, the new open pit mine and underground operations did not affect the American Boy Mine, and AGMJV allowed us to continue our research. We found, however, that by December 1989 the bat population in the American Boy had declined to 50. A year later only 11 bats remained.

Until this time, the mine itself had not been altered, but vegetation in the nearby dry washes had been removed to create an open pit, mill site, and cyanide leach pads and ponds to extract the gold. Evidence suggested that desert wash vegetation was where these bats foraged, gleaning large moths and katydids. Since the bats' actual roost had not been disturbed, and the cyanide ponds were covered, the population decline likely resulted from loss of foraging habitat.

Because the California leaf-nosed bat is only a Category 2 candidate* for threatened or endangered status, the Bureau of Land Management, which administers the land the mine is on, did not require any special mitigation when mining was renewed. Recognizing, however, that the species could become listed during the life of the project, American Girl Mining Joint Venture supported our 1992 radiotelemetry study of the bats' habitat requirements in an adjacent drainage slated for future open pit mining development.

The radio-tracking study soon revealed that in summer the bats foraged exclusively among desert wash vegetation within three miles of their roost. In the winter, they fed within a mile of their warm mine roosts and stayed on the surface only for brief periods. This suggests that foraging areas adjacent to the roost are very important for winter survival. The study also showed that bats would move to other nearby roosts when disturbed.

Because AGMJV sponsored this basic research, we are now better able to predict when California leaf-nosed bat populations will be affected by mining operations and how to mitigate for them. With initial baseline counts and monitoring of the bats after operations begin, we will be able to determine the effectiveness of mitigation measures. American Girl Mining Joint Venture has also installed approved bat gates and fences that will protect both bats and people at those mines not directly affected by current operations. It is hoped that California leaf-nosed bats excluded from the mines currently being worked will eventually move to the safety of the gated mines. After operations cease at the active sites, gates will be installed to provide new secure bat habitat, and desert wash vegetation will be re-established in disturbed areas.

The long-term prospect for survival of the California leaf-nosed bat in the Cargo Muchacho Mountains is now much improved due to the foresight and careful planning of American Girl Mining Joint Venture. We hope that this project can serve as a model for how mining companies can collaborate with bat biologists and conservation planners to ensure the safety of resident bats while allowing mining or other commercial projects to proceed.

Over the past 100 or more years, the mining industry has created new homes for bats. They now have the opportunity to play an important role in conservation efforts by protecting this increasingly critical habitat. The result could prevent some of our cave- and mine-roosting bats from joining the ranks of threatened and endangered species.

Patricia E. Brown, Ph.D., a Research Associate in the Biology Department at UCLA, has been involved in bat research and conservation in California for 25 years. Robert D. Berry, Ph.D. in engineering, has assisted in radio-tracking studies on California bats since 1980. Cathi Brown, B.S. in agriculture, has participated in bat research projects since she was four.

(Footnote 1)
* A U.S. Fish and Wildlife Service Category 2 candidate does not have as high priority as those in Category 1 because no conclusive data are yet available to support an endangered or threatened listing, even though their inclusion as a candidate indicates such a listing is probably appropriate.

Above: California leaf-nosed bats are dependent on mines year-round. Unable to enter torpor like temperate-zone bats, they rely exclusively on deep, geothermally warmed mines in which to conserve energy during winter.

The American Girl Mining Joint Venture not only supported the authors' research on California leaf-nosed bats but also installed gates to protect the bats from human intrusion. AGMJV personnel from left to right are Terry Rogers, Monta Zengerle, and Ed Kirwin. Robert Berry is inside the mine tunnel.

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