This mouse is as big as a human thumb. Its bitsy body weighs no more than three pennies. But the Pacific Pocket Mouse has a big job. It helps maintain coastal ecosystems.
But for 20 years, the critter seemed to vanish from the region. Habitat loss decimated mouse populations, leading researchers to believe the rodent was extinct. Then in 1993, something surprising happened.
“They were doing some trapping for a different species, put some traps out and found one pocket mouse out there," said Debra Shier, who is the associate director of the recovery ecology team at the San Diego Zoo Wildlife Alliance.
Now, the San Diego Zoo Wildlife Alliance is reviving the Pacific pocket mouse from the brink of extinction through its Pacific Pocket Mouse Conversation Program. The organization released a new population into North San Mateo, a shrubland located inside the Marine Corps Base Camp Pendleton. It is only the third group to be reintroduced to the wild.
“The recovery plan calls for ten populations before we can consider downlisting to threatened. And we’re really just making a huge, huge progress toward that goal,” Shier said.
The Pacific pocket mouse was listed on the endangered species list in 1994. It once lived between Los Angeles and the Mexico border. But upon rediscovery, only three small wild populations persisted.
They were found in Dana Point, Santa Margarita, and South San Mateo, an area in Camp Pendleton. The Pacific pocket mouse lives in a sensitive habitat called coastal sage brush. It’s usually found within 2 and half miles of the beach.
Underneath the sandy earth, the Pocket mouse builds borrows. The shallow caves supply shelter to other species. They also help improve soil health by boosting nutrient cycling and helping water move through the ground.
However, these key ecosystem functions were disrupted after development increased in coastal regions. They faced a host of problems that severely knocked the population back.
“Housing, roads, night lighting is a problem. Noise is a problem. But conversion of habitat really, habitat loss has been the biggest impact to them,” Shier said.
This fragmented the populations, leading to inbreeding and a lack of genetic diversity in offspring. Shier observed wild populations who inherited genes that made it difficult to survive.
“You end up with things like morphological problems. Kinked tails. They might not be able to breed very well. Those things actually manifest. And so we’re seeing lowered fitness. Lowered reproductive success in the populations with really low genetic diversity,” she said.
To help the rodent create healthy populations, the San Diego Zoo Alliance created the Pacific Pocket Mouse Conservation Program. Researchers peered into the genetic codes of the mice to better understand how to breed offspring best suited for survival.
The team then bred individuals from different populations. They used previous genetic analysis to ensure the animals were not too genetically similar. Mixing up the populations builds a stronger genetic tool kit to deal with harsh or unusual environments.
“Let’s say we’re talking about really heavy rains or a really long drought year or things like that, there are some individuals in normal populations that could survive that because of genetic diversity,” Shier said.
To breed the animals, Shier and her team placed a male and a female in an enclosure that simulates a natural shrubland. The cage is in a room with red lighting. That is because the animals are nocturnal and are sensitive to light.
Shier and her team joined the mice on their late night schedule. In the wild, it is difficult to observe matinging behaviors because the animals are so small, live underground, and only come out at night. The conservation program exposed information about the mouses’ behavior that was previously unknown.
“Females are just as likely to try and kill that male as they are to try and mate with that male. And there are behaviors that we know now that we have learned over time that indicate that it's going well or it's not going well,” Shier said.
The mice bred in captivity will also be trained to have wild behaviors. The researchers teach the pocket mouse how to fear predators like snakes.
For example, they’ll “fly” taxidermy owls over the mouse cage. Once the owl soars overhead, the mouse is sprayed with water.
“We wouldn’t scare them unless we absolutely had to,” Shier said. “But if they don’t understand that an aerial predator can fly over the top of them once they’re reintroduced and take them, then they’re going to die the moment we release them.”
The Alliance has tucked more than 500 mice into their tunnels. The species is one of the key speed dispersers in the ecosystem. Returning their populations to self-sustaining sizes will help ensure that native plants remain in the area.
However, the program’s hard work may be jeopardized by El Nino this year. Heavy rains could threaten the Pacific pocket mouse, which is adapted to live in dry environments. Shier is concerned the predicted unusual weather could hammer the already vulnerable species.
“The species doesn’t do well with really wet soil, especially through the overwinter period. And that's what we’re expecting to see this winter,” Shier said.
Still, she is hopeful that the Pacific pocket mouse can recover even with El Nino related setbacks. The new population in North San Mateo is a significant milestone. It brings the overall number of Pacific pocket mouse populations (new and wild) to six.
“We’re well on our way to getting them to the point where they’re self-sustaining,” She said.
The team will test offspring in spring to evaluate if the program has successfully helped wild populations become more genetically diverse.
If successful, Shier hopes to expand the program to Dana Point, where the population is the most threatened.
