Mine program aims to be bat friendly
TUMCO – During its heyday in the late 1800s and early 1900s, the town of Tumco supported a population of several thousand individuals who toiled primarily in the region’s mining industry.
Today, the historic site and surrounding Cargo Muchacho Mountains are home to thousands of bats who typically reside year-round in the subterranean passages left behind by decades of mining activity.
The pockmarked mountainous region is also the focus of the Bureau of Land Management’s Abandoned Mine Lands program, which mitigates hazardous mine features to help ensure safe recreational opportunities on public lands.
As part of the AML program, BLM officials are also tasked with determin- ing whether a particular mine serves as habitat for bats and, if so, what actions may be taken to preserve the mine’s bat colonies.
Locally, the BLM El Centro Field Office has had considerable success identifying mine features that serve as bat habitat and installing bat-friendly gates that prevent recreational visitors from accidently falling or deliberately venturing inside, said BLM wildlife biologist Camden Bruner.
“Down here we’ve never seen the need to remove a gate because the bats stopped using (the mine), which is a pretty good sign for us,” Bruner said.
As of January 2017, the BLM’s Abandoned Mine Lands program had identified more than 52,000 abandoned sites in 12 Western states. Of those, about 80 percent required further investigation and/or remediation, the agency’s website reported.
The number of estimated abandoned mines here in Imperial County is about 1,200, Bruner said.
Although Native Americans are reported to have visited the region’s quarry sites for thousands of years, modern day mining activity in the region reportedly dates back to the 1780s, when Spanish soldiers, settlers and laborers mined placer gold in the southeastern Chocolate Mountains.
“Imperial County has a rich historical mining heritage, being the site of the first gold production in California,” according to the 1981 book “Desert Fever: An Overview of Mining History of the California Desert Conservation Area,” which was researched and written at the request of the BLM.
Mining activity in the Cargo Muchacho Mountains appears to have ramped up considerably following the completion in 1877 of the Southern Pacific Railroad between Yuma and the coast, resulting in further degradation of the bats’ natural habitat.
“A lot of the natural rock work has been altered by humans so bats are now using the abandoned mines to compensate for the habitat they lost,” Bruner said.
Bat monitoring efforts
Bats account for about 20 percent of the total mammal species on Earth. Locally, California leaf-nosed and mouse-eared bats are the two species that primarily occupy the mine features within the Cargo Muchacho Mountains.
On any given night, a bat can consume between several hundred to more than 1,000 insects that are typically considered a nuisance to both humans and agricultural crops, Bruner said.
“Some of them can almost eat their entire body weight in insects a night,” Bruner said. “We really benefit from having them around.”
Contrary to popular opinion, bats statistically pose no health risk to humans, and the United States is void of the bloodsucking or aggressive species found elsewhere across the globe.
“There’s no real hazard to people as long as bats are given their space,” Bruner said.
BLM considers the California leaf-nosed bat a sensitive species, and while not endangered still remains a species of concern that is subjected to monitoring and conservation efforts.
Monitoring efforts include annual surveys that help determine the size of the colonies found within some of the region’s larger mine features.
Such surveys require BLM staff to position themselves outside of the mines on moonless nights with night vision binoculars and handheld clicker counters to determine the number of bats that exit to forage.
Often, enough staff can be dispersed to monitor about eight mines in a night if enough night vision binoculars can be obtained from other BLM field offices, Bruner said.
Infrared lighting also is placed along the perimeter of the gated mine features to help get as accurate a count as possible.
Recent monitoring efforts have focused on the Jackson Gulch area of the Cargo Muchacho Mountains, where such survey efforts have not previously taken place and where mine closure projects are scheduled to get underway by year’s end, Bruner said.
“That’s one of the primary things we look for when we close a (mine) feature,” Bruner said. “We want to make sure it’s not only closed but that the bats still are using it.”
Designing a bat-friendly gate
After years of trial and error, the BLM settled on a gate for occupied abandoned mines that allow bats to conveniently enter and exit while limiting opportunities for opportunistic predators.
The horizontal crosspieces on these gates prevent predators such as snakes from perching on the slats and striking at the bats as they enter and exit, Bruner said. The spacing between the crosspieces also prevent individuals from crawling inside.
The gates can be fairly elaborate, and costly, as in the case of the cupola-style gate installed at the King Mine last year that came with a $25,000 price tag, Bruner said.
“It’s a big investment for a gate this big,” Bruner said. “This is kind of top of the line at the moment.”
The Cargo Muchacho Mountains are also littered with mining features that aren’t occupied by bats but yet have historical significance. The BLM is also tasked with preserving those features, and in some instances, it will enclose them as well.
“These mines are really old and a lot of them have quite a bit of historical value to them,” Bruner said. “When we install gates and close features we try not to disturb any of the historical landscape.”