Unlike most Army installations, U.S. Army Yuma Proving Ground's primary purpose is not to train troops, but to conduct natural environment testing on virtually every piece of equipment used by Soldiers.

Also atypical is the presence of abandoned mines in numerous locations across the proving ground's more than 1,200 square miles of range space.

Prior to Yuma Proving Ground's existence, Southwest Arizona was home to all manner of mining--gold, silver, copper, lead, mercury--and there are more than a dozen old shafts and mining sites scattered about the installation. Some prospecting digs penetrate the earth only a few feet, while other more elaborate works snake hundreds of feet, with multiple horizontal turns that plunge into deep, sometimes flooded vertical shafts.

Disused for at least a century and frequently excavated out of soft and unstable rock, the mines are isolated and cool, perfect roosting for the 11 species of bats known to call YPG home. They are also incredibly dangerous to anyone who trespasses within the proving ground's boundaries--stories abound throughout the American West of fatal or near-fatal mishaps involving abandoned mines. From treacherous drops to insufficient oxygen, old mines are no place for casual desert wanderers to tread.

"They're hazardous to people, but really good habitat for wildlife," said Daniel Steward, YPG wildlife biologist. "Our goal is to keep people safe and keep the habitat available for wildlife."

As such, personnel from YPG and the Arizona Game and Fish Department recently outfitted several of the more dangerous mine openings with barriers that will prevent unauthorized people from accessing them while allowing bats to come and go. They worked quickly to finish their work as far before the bats' spring mating season as possible to minimize impact on the creatures.

"The worst thing you can do to a bat is walk into its home during the maternity period and disturb it," said Dr. Joel Diamond, a research ecologist for Arizona Game and Fish. "They'll vacate the roost, and that is what has happened in most of our major care systems in Arizona. YPG is a functional refuge since there is no recreational activity here."

All mines are different, and the team had to use different methods to close off each. In one mine shaft that had been drilled through solid rock, they installed a heavy steel gate, with resin covering the bolts that hold it securely in place. In a more complex mine elsewhere on the range with an irregular opening in soft rock that had partially caved in, the team draped a massive single strand of wire-cable mesh that will continue to block human access as the mound shifts and erodes over time.

With this done, the team soon emplaced electronic data loggers that will record bat calls from sunset to sunrise over the next six months, allowing the officials to track what species of bats are using the mine and in what magnitude.

"We want to determine as soon as possible whether bats are still using it," explained Steward. "Any time you put an alteration on a mine, it will affect the way bats use it."

Fortunately, when Steward placed the data logger in one of the mines there was ample evidence that it was still hosting a large bat population: there were scattered areas of inches-deep guano, full of discarded pieces of insect wings.

Healthy bat populations are enormously beneficial to the community as a whole: A pregnant female bat can eat more than double her body weight in insects every day.

"Yuma is a big agricultural producer," said Diamond. "In the absence of these bat populations, you would need a lot more pesticides on the crops."

As average temperatures increase, so do insect populations, making bats even more necessary than ever. Officials have observed a long-term trend of subtropical bat species migrating north.

"Over the past 50 or 60 years, we've seen animals that didn't winter in this latitude do so now," said Diamond. "They can now forage all year long. Years ago there were enough cold snaps to kill off the insect population every year, but no more."

Though home to anywhere from 60 to 120 unique military tests per week, YPG actively strives to minimize the footprint of its activities, particularly around desert washes.

"We take great pains to avoid washes whenever we're doing any kind of construction or testing," said Steward. "By doing that we protect the forage habitat of a plethora of species."

In addition to bats, YPG is home to a wide variety of wildlife. It is home to one of the healthiest populations of bighorn sheep in the state, and was specifically chosen as a place for the once-critically endangered Sonoran Pronghorn to recover from the brink of extinction.

"YPG provides a net benefit to a variety of wildlife species," said Steward. "While we're using our land intensively, we're also intensively managing our natural resources."