Desert mammals survive climate change underground. For birds, it’s a different story

SAN DIEGO —  In the struggle to survive the ever hotter deserts of California, there are winners and losers.

Among the losers are desert birds, whose populations have collapsed amid the heat stress of climate change. The winners, it turns out, are small burrowing mammals, including the cactus mouse, kangaroo rat and white-tailed antelope squirrel, which take refuge from the sun underground.

Researchers, including scientists from the San Diego Natural History Museum, published those results in the journal Science this month, noting that the stable mammal populations formed a hopeful contrast to the dire condition of birds.

“Why was the mammal community relatively stable compared to the bird community?” asked study co-author Lori Hargrove, an ecologist at the San Diego Natural History Museum. “Birds had a higher evaporative heat loss. Birds were more exposed to the effects of warming, so they had higher energy costs to maintain their body temperature, whereas mammals were able to buffer their body temperature by using burrows during the day.”

Temperatures have risen about 4 degrees Fahrenheit on average across the area studied, she said, but the heat affects birds and small mammals differently. As part of the study, scientists modeled each species’ body temperature and cooling needs under different desert conditions. To calculate that, they measured the conductivity of the animals’ fur or feathers, and used information on their heat reducing behaviors, such as evaporative cooling or denning underground.

The models showed that in the fight against climate change, there is not a level playing field for furred and feathered desert dwellers. Cooling costs — or the resources needed to maintain stable body temperature — were about 3.3 times higher for birds than they were for small mammals, the study reported. Rising temperatures resulting from climate change increased cooling costs by 58% for birds, but only 17% for mammals, it said.

“Mammals have shown this remarkable stability,” said study senior author Steven Beissinger, a professor of environmental science, policy and management at UC Berkeley and researcher at its Museum of Vertebrate Zoology. “It’s really quite interesting that, in the same region, with the same level of climate change, these two very similar taxa have responded very differently to the changes taking place.”

The study relied on data that scientists gleaned during years spent retracing historic surveys of California’s deserts and mountains, and measuring ecological changes over the last century. The Grinnell Resurvey Project, a joint effort of UC Berkeley, the Natural History Museum and other institutions, has painstakingly followed the footsteps of Joseph Grinnell, the first director of the Museum of Vertebrate Zoology at UC Berkeley, as he and his team walked sweeping transects across the state.

Starting in 2008, scientists trekked through the San Jacinto Mountains and other Southern California wildlands to examine the exact sites that Grinnell studied a century earlier. Current researchers relied on Grinnell’s extensive field notes, hand-drawn sketches and maps to chronicle the status of plants and animals that his team cataloged in the early 1900s. Whenever possible, Hargrove took photos in the precise locations that the original team visited.

“Exactly 100 years later, we retraced his steps in the San Jacinto Mountains to see how the ecosystem has changed,” Hargrove said. “Then we extended his work to Joshua Tree National Park and Mojave National Preserve. We couldn’t do that without the work of the early 20th century zoologists who surveyed the fauna of California with [an eye to] what they thought would change because of humans and development.”

Grinnell was prescient about the need to document natural conditions before humans altered them, but he couldn’t have anticipated the extent of the transformation to California’s landscape.

“They wanted to document the fauna before it changed,” Hargrove said. “Now we also have climate change.”

The granular, detailed observations of habitats and wildlife has allowed researchers to observe broad trends, such as the well-being of mammals versus birds noted in the recent survey. But it also helps them spot the outliers: individual species faring better or worse than others.

For instance, while most species of rodents were stable, chipmunks that were common in Joshua Tree National Park in the 1940s had nearly disappeared when the team surveyed the park, said Phil Unitt, curator of birds and mammals for the Natural History Museum. That made sense as these tree dwellers are active in daytime and scramble around branches like birds, rather than burrowing underground in the heat of the day, he said.

By contrast, the California Towhee, a chaparral bird, had spread more widely across the park, even as others — such as the mountain quail, titmouse, black-chinned sparrow and gray vireo — have declined.

“So why that species alone, when all these other chaparral birds had done so poorly?” Unitt asked. “It’s always interesting when we find counter examples as well. Why is some species bucking the trend? To me, that’s the most interesting thing in science, when you observe things that seem contradictory, because then you know there’s some richer story going on behind.”

While most parts of the region got hotter and drier, a few pockets of Mojave National Preserve have curiously become wetter, Hargrove said.

“In those cases, we actually saw increases in Joshua trees, and grass and shrubs and cover, and local increases in bird and mammal communities,” she said.

Insights into the status of different species can help scientists and land managers take steps to protect them, such as increasing fire prevention in fragile high desert areas, managing water use to preserve natural springs, and enhancing water sources in those areas, Hargrove and Unitt said. The study also demonstrates that climate change is already reshaping the California desert.

“Obviously, climate change is not something that’s off in the future, but something that we’re dealing with and the components of our natural environment are dealing with right now,” Unitt said.

Brennan writes for the San Diego Union-Tribune.