A World of Wildlife in Abandoned Mines

ABOVE: TIMOTHY ARMSTRONG ET AL.

The American West is famous for the scale and expanse of its scenery. But look more closely and it’s easy to see the marks of American westward expansion. Thousands of abandoned mines remain scattered across these vast landscapes, relics of the land and gold rushes of the 19th century. Although park and land managers think of them mostly as hazards (and many are), a new study shows that they may be important habitats for many animal species.  

Research on wildlife and abandoned mines has, until recently, focused mostly on those most famous cave dwellers: bats. But a recent study in the Journal of Wildlife Management by biologist Tim Armstrong and colleagues at Adams State University in Colorado reveals that the winged mammals are only part of the story.

A few years ago, Armstrong got a tip from a friend about the remains of several large carnivores found in a mine in the local Sangre de Cristo mountains. That got him curious about whether there were any other reports of large animals going into or inhabiting mines. “I looked in the literature, and I couldn’t find that anyone had looked at this before,” says Armstrong. He reached out to the Forest Service, the Great Sand Dunes National Park and Preserve, and the Bureau of Land Management. “And no one seemed to know anything.”

Armstrong and his students got a grant to set up camera traps at the entrances to several mines in the Sangre de Cristo range to find out which species of wildlife were using them. This involved long days in the field and up to 23-kilometer hikes to mine entrances. Aside from a few anecdotal reports of wildlife at these sites, there was no guarantee that anything would show up before they went back to check the memory cards months later. 

Armstrong says that the researchers hoped to see a few species of carnivores and other wildlife using the mines, but they never expected the diversity they eventually found.

Armstrong remembers the excitement of sitting down to look at that first memory card. “The first mine that we monitored, we got the images back and there was a bobcat on it, there was a bear on it. . . . When we saw those, it was like, ‘We are really truly onto something here.’”

The team monitored 50 mine entrances over a period of three years. As the researchers started to analyze the data, it became clear that mines, especially smaller horizontal ones, provide habitats for a surprisingly diverse community of animals. In all, they identified 48 vertebrate species, including 11 of the 14 species of carnivores known to inhabit the region. 

In one case, the time stamp on one of the images informed the researchers that they had almost come face-to-face with a top predator. “We know we missed a mountain lion by a day,” says Armstrong. “We got to the mine, we could see tracks, and when we pulled the card and checked, it had been there just the day before.”

One species in particular became the star of the project. Ringtails (Bassariscus astutus) are especially elusive members of the raccoon family found mostly in the arid Southwest. But here several of them were, captured on camera outside of mines in the mountains of Colorado.

“The ringtail was, by far, the most exciting and unexpected,” says study coauthor Tyler Cerny. “I honestly didn’t know that we had them anywhere even close to the Sangres.” With that first ringtail photo, the project not only expanded the researchers’ own list of species that were using the mines, but also widened the known geographical range of that particular species.

Like the settlers who originally cut these mines into the landscape, at least some species of visiting wildlife seem to be coming to access the minerals inside. Bighorn sheep (Ovis canadensis) were seen repeatedly entering and leaving the mines and twisting their heads to bite on the rock around the entrances, likely to ingest salt and other minerals. To the team’s surprise, this even happened at sites where bighorn predators like mountain lions had recently visited, say Armstrong and Cerny. The bighorns almost certainly smelled the predators, suggesting that the benefits of this visit outweighed the potential risks.

However, the researchers say that the main motivation for wildlife to enter the mines probably has to do with thermoregulation. Like caves, mines vary much less in temperature than the world outside, in terms of both daily swings and seasonal fluctuations.

One challenge for managers of land with abandoned mines is how to balance wildlife habitat and human safety. But when the researchers compared wildlife’s use of different sorts of mines, they found that the most dangerous type, vertical shafts like the one shown above, are much less likely to be visited by wildlife than horizontal ones. This puts a clear priority on closing vertical shafts, says Armstrong, who doesn’t mince words about them: “Every one of those, in my opinion, should be closed, because they are a hazard to everything—wildlife, hunters, hikers, you name it.”

This opinion is echoed by Gretchen Baker, a cave specialist and ecologist at Great Basin National Park in Nevada who was not involved in the work. “You’re not going to have wildlife just popping in and out [of vertical shafts] because they would fall down the shaft also. Those would be top priority for dealing with. And then, after that, it’s going to have to be on a case-by-case basis.”

Baker sees the Colorado team’s study as an important step for understanding wildlife in and around mines, with immediate applications for the way these areas are managed. “I’m really delighted that they did this study,” she says. “When they get to their findings, they say we should look at these mine entrances for a year before we put a gate on them. I think that’s a really good idea.” 

Coauthor Cerny says he sees the study as adding a new dimension to the decision-making process about mines and their closures, hopefully leading to a more considered approach. “I think if people understood that there needs to be a balance and that not everything is always about humans and not everything is always about wildlife, then people and wildlife would get along a lot better.”