May 16, 2025Could giant piles of sticks near the San Joaquin River hold the key to solving the ongoing black rat problem in California’s orchards?
That’s not a leap biologists are quite ready to make, but current efforts to restore land near the San Joaquin River near Modesto could help the near-extinct riparian woodrat population rebound. The woodrat is a direct competitor to the problematic black rat that’s been chewing through irrigation lines and electrical wires and damaging trees in orchards across the San Joaquin Valley.
Biologists from the U.S. Fish and Wildlife Service, Stanislaus State’s Endangered Species Recovery Program and River Partners recently conducted camera trap studies at the San Joaquin River National Wildlife Refuge and Caswell State Memorial Park near Modesto with the hope of capturing photos of the elusive woodrat.
“They have lush, fluffy fur, large ears and a thick blunted tail,” said Jaime Rudd, Director of the Endangered Species Recovery Program. “They’re the chinchilla of rats.”
Riparian woodrats went undetected on the refuge for nearly a decade, leaving biologists to wonder if the species was still in existence. But in late 2024, traps set out to humanely capture endangered riparian brush rabbits captured two riparian woodrats.
“I’m so amazed that we found them again after such a long absence,” said Rudd. “It speaks volumes to the efforts to restore the refuge and the resiliency of these rare, little animals,” said Rudd.
According to River Partners Restoration Science Ecologist Haley Mirts, this marks the first time riparian woodrats have been found making a home in restored habitat in the state. “As someone who has spent most of their professional life helping bring back California species on the brink, it’s incredibly uplifting every time we find struggling wildlife thriving in the areas River Partners revitalizes.”
Over the past two decades, the U.S. Fish and Wildlife Service and River Partners have transformed thousands of acres at the refuge and nearby properties into critical native habitat, as 95% of the Central Valley’s riparian habitat has been lost. Most of the land at the refuge was once a dairy and tomato farm.
“Only about 5 percent of California’s riparian habitat remains intact,” said Fumika Takahashi, a wildlife biologist at the San Joaquin River National Wildlife Refuge. “These restoration efforts are aimed at providing good habitat for endangered species like the riparian woodrat and riparian brush rabbit, as well as migratory birds like least Bell’s vireo.”
Recent projects removed huge thickets of invasive blackberry and an overabundance of coyote bush. Areas were re-planted with native grasses, rose and quail bushes, and oak and willow trees. The work created large brush piles on the refuge, which is the exact location where the woodrats were captured.
Riparian woodrats transform those piles into elaborate homes. The homes contain rooms for rearing young, sleeping and storing food. These tidy creatures even include a designated outdoor latrine. Some homes can measure 6 feet in diameter and be used for 20 years by the same woodrat family.
However, biologists aren’t sure that riparian woodrats are solely using these traditional home structures on the refuge.
“These traditional homes take years for woodrats to build, and with the number of floods this area has seen over the past several years, we don’t think they have time to rebuild every year” said Summer Howland, a biologist with the U.S. Fish and Wildlife Service.
Rudd and Howland both hypothesized that riparian woodrats may have adapted to the annual flooding by moving into the tree canopy and making homes in tree hollows.
“It could explain why we’re not seeing them in the images captured by cameras positioned on the ground,” said Howland. “They could be living in the trees, and only coming down occasionally for food or needs they can’t get in the tree canopy.”
Out of the 31 camera traps installed on the refuge for a month, only one captured photos of the riparian woodrat.
“We were hoping that we would see a lot more on the cameras, but we’re not discouraged. We think they’re out here, and we are working to figure out the best method to detect them,” said Rudd.
“Our continued use of high-tech tools like camera traps are invaluable in truly understanding what wildlife are up to on restored Central Valley landscapes and how those areas benefit species recovery,” said Mirts. “The technology not only gives us a snapshot of what’s happening on the ground at any given moment but enables us to see how wildlife diversity changes with the age of our restoration and informs future restoration to maximize wildlife benefits.”
As a next step, biologists are hoping to humanely capture riparian woodrats and equip them with small radio collars that will track their movements.
“We want to figure out where they go and how they move. This data will help inform the habitat restoration efforts on the refuge to support the woodrat,” said Howland.
Simultaneously, the refuge and River Partners are about to start restoring a piece of property across the river from Caswell Memorial State Park.
“We still have a lot of unanswered questions on what habitat features woodrats are using. This restoration project will meet multiple refuge goals, including improving habitat for riparian brush rabbits and helping us learn more about riparian woodrat habitat use,” said Takahashi.
With more habitat options for woodrats, biologists hope the population will expand. This could create natural competition and reduce food, shelter and other resources for black rats. Currently, black rats seem to have an advantage in terms of sheer number and their brutal consumption of young woodrats. Unlike the non-native black rats, riparian woodrats don’t stray far from their riverside homes. Instead, they provide important ecosystem balance by spreading seeds and providing fertilizer to the soil. More research is still needed to examine the relationship between the woodrat and black rat.
Like many rare species, biologists don’t know all the benefits the riparian woodrat brings to the floodplain ecosystem, but their general feeling is that they’re important.
“There are examples of where a species returned to an ecosystem and triggered multiple benefits. The riparian woodrat could be the next example,” said Rudd. “Thankfully, there are a lot of groups involved in this research, and that gives me hope for the future of the species.”
