When researchers following in the boots of biologist Joseph Grinnell, who a century ago created a pioneering survey of California wildlife, began sampling birds and small mammals in the Mojave Desert, they expected the harsh conditions would magnify population changes driven by the climate crisis.
“We knew when we were going to the desert that life was on the edge there,” says Steven R. Beissinger, a professor of ecology and conservation biology at UC Berkeley who has been part of what’s called the Grinnell Resurvey Project since it began 17 years ago. The goal is to revisit the same areas Grinnell and his students observed between 1904 and 1940, comparing new data with old. “If there was going to be any place where we would see these physical impacts, it should be there,” he says.
There were, at least among birds. A three-year desert survey, completed in 2018 by the Berkeley team, revealed a stunning community collapse. Thirty-nine of 135 species counted by Grinnell were less common and less widespread. The 61 sites sampled lost, on average, 43 percent of the species that were there a century ago. Only one species, the common raven, increased. The reason? Like many deserts, the Mojave is getting hotter and drier, warming by about 3.6 degrees Fahrenheit and experiencing rainfall declines of 20 percent in some areas over the last century. “That was a shock,” Beissinger says.
So he figured if climate change had altered the desert ecosystem by heating and drying, they’d see the same collapse among mammals when the results of that survey, which took longer to analyze, were complete.
They didn’t. Rather than suffer a similar collapse, they wrote earlier this month in Science, small mammals in the Mojave proved resilient, sheltering underground during the hot day and foraging above ground during the cooler nights. Overall, the researchers sampled 34 small mammal species—kangaroo rats, kangaroo mice, pocket mice, wood rats, cactus mice, ground squirrels, and grasshopper mice, among them—at 90 sites in and around Death Valley National Park, Joshua Tree National Park, and the Mojave National Preserve. What they found were numbers similar to those in the Grinnell survey.
That contrasted to Yosemite National Park, the first of Grinnell’s locations, which the Berkeley team resurveyed from 2003 through 2006. That study discovered small mammals moved to higher elevations or reduced their ranges as temperatures increased. Of the 28 mammal species in the survey, half expanded their range upward by more than 1,600 feet. Low-elevation species like the California vole and California pocket mouse moved to higher elevations while high-elevation species like the bushy-tailed woodrat and Allen’s chipmunk decreased their range.
The results, Beissinger says, indicate that scientists should reconsider the notion that all species respond in the same way to changes in temperature and precipitation patterns. “In the Mojave, we showed that birds are experiencing a very different level of climate change than the mammals,” he notes. “So this ‘one size fits all’ doesn’t really work very well. We need to take a more nuanced approach.”
Understanding those nuances is possible only because Grinnell and his students recorded unusually detailed field notes of the animal and plant life of the early 20th century. They collected 100,000 specimens and recorded 74,000 pages of cursive field notes, with more than 10,000 images, from more than 700 sites across the state. The result is a rare foundation to compare how a century of warming and drying has transformed ecosystem diversity.
For Josh Tewksbury, the interim director of Future Earth, a global research and innovation collaborator, and a researcher who is not part of the resurvey, the new study illustrates the value of digging deeper into the complex influences of a warming and drying climate before taking corrective action, whether it’s for natural or human systems. “It’s fairly rare to have such a complete picture of how big the changes are,” he says. “It’s just not easy to make a really strong, stark comparison that allows for this kind of insight.”
Grinnell began his surveys worried that California’s natural history would be lost as people streamed into the state. He and his students focused on seven sites, including several national parks, which is especially useful today because researchers don’t have to account for land use changes and can focus on the effects of climate change. The meticulous journals, preserved at Berkeley’s Museum of Vertebrate Zoology, document information missing from many museum collections composed solely of specimens.
Those notes, including topographical maps from the time, mean members of the Berkeley resurvey team have been able to recreate his expeditions. “Arrived from Berkeley yesterday afternoon at 2 p.m. and got settled in time to get out traps before night,” Grinnell wrote on March 14, 1914, about his expedition to the Mojave Desert. A hundred and one years later, James Patton, a 78-year-old professor emeritus at UC Berkeley and curator of the museum’s mammals collection, followed the Grinnell team’s survey during weeks-long camping sessions with his wife, Carol.
Patton often carried copies of those journal pages with him. In a few instances, the resurvey teams became amateur detectives, following the clues documented a century ago. In Death Valley, Grinnell’s notes recorded Kelley’s Well as a site. Patton found two spots with the same name, eight miles apart, but was able to determine the correct one by looking at sepia-toned campsite photographs.
Grinnell was a taskmaster who demanded his students write down everything they saw, when they saw it. They traversed the state by narrow-gauge railroad, Model T Ford, buckboard, burro, and on foot, camping for days and sometimes months at a time. He created what’s now known as the Grinnell Method, requiring staff and students to use India ink and what he called permanent paper to keep a field notebook, a field journal, a species account, and a catalog of specimens. They described the desert quail and red-shafted flickers they saw, the traps they set, the plants along their hikes and the grasshopper mice they captured live. They drew cross-sections of landscapes from the hills to the rivers.
“I don’t think there’s anybody today that could hold a candle to what those guys could do, both physically and in terms of deep thinking about what they were seeing,” Patton says. “We’re too keyed to our mechanical, digital devices to actually look at things.”
While Grinnell collected and surveyed specimens by shotgun and deadly snap traps, the Pattons set 200 live traps each night, recording their catch over four or five days. They preserved a few specimens for the museum, spending eight weeks every spring and six weeks every fall from 2015 through 2018 in Death Valley.
The results were unexpected, Beissinger says. “We were very surprised that what we saw was resilience for the small mammals,” he adds. “When we looked at the proportion of sites that a small mammal occupied a century ago, when Grinnell and his colleagues were out [surveying] and the proportion we have now, it was almost a straight line.”
Patton, who has studied small mammals for more than 50 years, says they are buffered by their nocturnal nature, their burrows, and their ability to meet water needs by metabolizing seeds. Birds, meanwhile, forage in the heat of the day and often require open water sources, like springs, pools, and surface waters. “Many of the birds require exogenous water to survive,” he says. “But most of the small mammals out there are manufacturing their own water [through seeds].”
Beissinger puts it in simple terms: Birds have more exposure to heat and are more sensitive to the effects of climate change.
To understand those differences, they brought in Eric Riddell, an assistant professor of ecology, evolution, and organismal biology at Iowa State University. Riddell had been a postdoctoral researcher at Berkeley who built computer models to calculate the cooling needs of 49 desert birds. While Patton and his wife were camping in the desert, Riddell camped out at the museum, spending six months over two years beginning in 2017 taking measurements of bird specimens, determining their rough dimensions, the length and density of their feathers, and even how much sunlight bounces off of them or is likely to pass through their plumage to their skin. From models created using those measurements, he was able to estimate the amount of extra water needed for evaporative cooling by each bird species today compared to 100 years ago. The species that declined from Grinnell’s time were the ones that had the most difficulty keeping cool, notably larger birds, especially those like the violet-green swallow and the white-throated swift that get most of their water from insects.
For small mammals, he returned in 2019 to do the same, cataloging body size and fur density for another six months. The models looked at how their bodies absorbed or reflected heat, including direct sunlight, reflected sunlight, and radiant heat from the ground. A rodent with fluffy fur might transfer that heat slowly while one with short fur, like a ground squirrel, might transfer it quickly.
His program simulating the effects of climate change—increased temperatures and decreased precipitation—consisted of more than 1,000 lines of code. Riddell used UC Berkeley’s supercomputer: 240 linked computers, running for 18 hours to calculate 1.2 billion hourly simulations. Translated, that means the model calculated how much heat each species of mammal gained or lost every hour of every day over the last 100 years in the Mojave Desert.
The key to the different outcomes for birds and mammals proved to be water consumption. Riddell found that birds required almost three times as much water as small mammals to cool themselves. “In the desert, water is very limiting, and there isn’t much of it. And you need that water to cool off,” he says. “In the last century, birds experienced this really massive increase in the amount of water that they needed just to stay cool, just to function, and small mammals haven’t experienced that change.”
“It’s pretty remarkable to think about birds and small mammals living in the exact same environment through climate change with one community having this really intense experience, and another community not experiencing anything at all,” he continues.
Even the few small mammals that have larger water needs, like the desert woodrat, didn’t decline. “That was really surprising,” Riddell says. “What do all these rodents have in common? Well, they all have access to these underground microhabitats, where it’s nice, cool, and stable.”
Without Grinnell’s foundational work, though, those comparisons and that understanding of the effects of climate change would not be possible. While Grinnell could not have imagined a warming Earth, he envisioned his work enduring. “I wish to emphasize what I believe will ultimately prove to be the greatest value of our museum,” Grinnell wrote in 1910, two years after he helped found the Museum of Vertebrate Zoology. “This value will not, however, be realized until the lapse of many years, possibly a century … And this is that the student of the future will have access to the original record of faunal conditions in California and the west.”
Beissinger and Patton are the first generation of those future students, researchers who transformed Grinnell’s work from the shelves where it sat preserved, but largely unexplored, into a tool to examine the effects of climate change. “Somebody will be repeating this again in 30 years,” Beissinger says. “Climate change is probably even going to be more severe.”
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