By Valerie Hamilton

With guns blasting, helicopters flying and sweaty recruits marching day and night, Marine Corps Base Camp Pendleton might not seem like an ideal wildlife preserve. But on Southern California's densely populated coast, its 125,000 acres of open space have become a kind of Noah's Ark for 16 endangered species, the last place many of them can live.

“Camp Pendleton is 18 miles of almost undeveloped coastline,” says Sherri Sullivan, a wildlife biologist who works for the base. “We have populations of wildlife almost on an island where the surrounding communities don't.”

Take the Pacific pocket mouse, known in military speak as the PPM. PPMs used to live all along California's coast from Los Angeles to Mexico. Rampant development fragmented their habitat, effectively squeezing them out. Now there are so few of them left in the wild that until 1993, biologists thought they were extinct. Two of the last three surviving colonies are on Camp Pendleton. The third is in nearby Dana Point.

One recent evening on the base, about 300 recruits from Golf Company filled outdoor bleachers to listen to a safety briefing before an all-night training mission. After lessons in navigating barbed wire and crawling on the ground with a rifle in hand, the instructor turned to the base's wild inhabitants.

“We have snakes, we have pocket mice, we have tarantulas, we have deer, foxes, coyotes,” he said. “Don't mess with anything out there. Don't pick it up. Don't poke it. Don't pet it. Don't put it in your pocket. It does not want to be your pet.”

“Yes, sir!” answered the recruits.

Warnings notwithstanding, it's unlikely that Marines are tangling much with the PPMs. The Endangered Species Act and the Marine Corps Order require the base to protect the mice and other endangered species, working around their habitats for training and infrastructure.

The mice are known to live on two sites on the base. The first, a high, wild plain strewn with coastal sage scrub, is used by recruits for marches, although they're supposed to stay on the road to avoid PPM burrows. The other is the wide field at the back of the marksmanship range.

“[There] they don't get impacted,” explains Sullivan. “Nobody walks back there. Nobody wants to get hit by a stray bullet.”

The base works to avoid damaging PPM habitat with infrastructure projects, Sullivan says, pointing out a construction fence set back from the road and sandbagged, rather than dug in, as an example.

But the mouse's habits present challenges to coexistence. PPMs dig shallow burrows in soft soil, often at the edge of roads, making everything from construction equipment to a misplaced footstep a potential hazard.

“Any impact is a detrimental impact,” says Sullivan.

With only three known PPM colonies, two at Camp Pendleton and one in nearby Dana Point, inbreeding, a fire or just bad luck could destroy the species forever. Scientists know so little about them that it's hard to guess what the wider impact could be. What they know is, the Pacific pocket mouse is hanging on by a thread.

“Much like the California condors, where there were only a handful left in the wild, chances that this species will blip out are very good,” says Sullivan.

And so, 30 miles away in a concrete bunker at the San Diego Zoo's Institute for Conservation Research, biologists are working on what you might call a strategic pocket mouse reserve.

Biologist Debra Shier lifts Male 8 out of a row of Plexiglas cages. Six-and-a-half grams and about the size of a small human thumb, he nests easily in a baby food jar.

“They call them pocket mice because they can fit in your pocket,” Shier says. “At least that's my favorite explanation.”

Last summer, Shier led a team of researchers on nighttime mouse missions on Camp Pendleton, which helps finance the Institute's work. They captured 22 PPMs, the “founders” they hope will help them increase the pocket mice population by doing what comes naturally. Females should come into estrus in the spring; researchers will use genetic testing to minimize inbreeding.

“Captive breeding is sort of our last ditch effort at recovery,” Shier explains.

The idea is to produce more mice in this safe, controlled environment, and then release them to start new PPM colonies in the wild.

It's risky. Captive-bred mice will need to learn how to survive in nature, and there's so little habitat left that finding a new place to release them is a challenge. But the hope is the Pacific pocket mouse will multiply and prosper to the point they're taken off the endangered species list.

Sherri Sullivan says that would be good news for everyone.

“If the species are delisted, we don't have to have the intense management,” she explains. “We have more freedom to conduct activities on that land space.”

The balance struck by Camp Pendleton and the Pacific pocket mouse may be imperfect, and most agree that in an ideal world, the best thing for the PPM would be to have more habitat of its own. But according to biologist Will Miller of the U.S. Fish and Wildlife Service, without Camp Pendleton, there might not be many PPMs left to protect.

“Probably the survival of the species has been greatly enhanced by Camp Pendleton,” he says. “The fact that they maintain their land as open space and do some limited training on that land.”

With any luck, the first captive-bred babies will be born in the spring. Until then, the only thing standing between the Pacific pocket mouse and extinction may be the U.S. Marines.