Ecologists catalogue bird and mammal populations as warming transforms Death Valley.
Death Valley, California
Jim Patton brushes a packrat’s furry white belly with a vibrant green marker as his wife, Carol, croons over the animal. “We’re making you beautiful — punk mice!”
Patton, a retired mammologist, is trapping and releasing desert wildlife as part of an ambitious project to repeat surveys conducted by renowned ecologist Joseph Grinnell from 1908 to 1939. Known as the ‘father of field notes’, Grinnell criss-crossed California in his Ford Model T to catalogue its birds and mammals. His descriptions are so complete that researchers today can compare the density and distribution of animal populations then and now.
Grinnell’s records provide an unparalleled baseline for researchers to explore how urbanization, farming, mining and climate change are reshaping the state’s ecosystems. The Grinnell Resurvey Project, run by the University of California, Berkeley, has sought over the past 14 years to capture current conditions, with an eye to quantifying future ecological shifts. The latest phase of the work, which began last month, is focused on cataloguing small mammals in California’s rapidly changing deserts.
“The only way to get a sense of what is happening under climate change, and what to expect in the future, is the kind of work going on in the Grinnell research project,” says Josh Tewksbury, a sustainability scientist at Future Earth, an environmental-research group in Boulder, Colorado. “It’s hard to see how the water boils when you’re in the pot.”
When Grinnell began his project in the early twentieth century, he was struck by California’s varied geography, from snowy mountains to blazing deserts to rocky coasts. Anticipating the state’s inevitable transformation as Americans moved west, he documented the distribution of species in about 700 locations. His team deposited more than 100,000 specimens in natural-history museums, including the skull from one of California’s last grizzly bears (Ursos arctos californicus), as well as 74,000 pages of field notes and 10,000 images.
“The student of the future will have access to the original record of faunal conditions in California,” Grinnell wrote in 1910, two years after he became the first director of Berkeley’s Museum of Vertebrate Zoology. “This value will not, however, be realized until the lapse of many years, maybe a century.”
In 2003, Grinnell’s academic descendants set out to retrace his survey of Yosemite National Park. Five years later, they reported that 14 of the 28 mammal species monitored in Yosemite had migrated to higher elevations since Grinnell’s time, averaging a gain of 500 metres (C. Moritz et al. Science 322, 261–264; 2008). The animals’ climb occurred during a period when winters in the park warmed by about 3 °C. Because Yosemite has been a protected area since 1864, the researchers concluded that land-use changes were not a major factor in the species’ shifts.
Steve Beissinger, a conservation biologist at Berkeley and the project’s leader, says that recent surveys have yielded less-coherent results. “As we look more broadly across sites in California, we find that responses are much more complicated,” he says. “Some species [are] moving to lower elevations in areas that have become rainier, and in some places we see stasis.”
But a growing number of studies suggest a dim future for desert dwellers in the coming decades, as they face warmer, drier conditions. Temperatures in Death Valley in July were the hottest for any month anywhere in the world in 2017, averaging 41.9 °C.
Many biologists think that desert organisms are living at the limits of survival — and that cooler regions may be out of reach for slow-moving or short-lived species. Preliminary results from the Grinnell Resurvey Project corroborate this idea. Of the 135 bird species surveyed in the Mojave Desert, only the common raven (Corvus corax) has significantly expanded its range since the early twentieth century, Beissinger says. The ranges of 38 other species have contracted.
A changing landscape
Yet on a cool morning in the Lee Flat area of Death Valley, most of the 160 box traps set out by Patton contain small, furry animals. Within 24 hours, he and Carol mark 90 squirrels, mice and rats belonging to nine species — one more than Grinnell listed in the same area in 1917.
Patton rejects the idea that climate change will soon drive many desert mammals to extinction. Like Grinnell, he is awed by the animals’ ability to adapt to extreme conditions. Kangaroo rats (Dipodomys sp.) extract water from seeds, and lose little of it because their kidneys concentrate urine to a crystal-like consistency. The rodents’ oily coats also prevent water loss through sweat.
Still, Patton sees signs of change. He has not yet captured a bushy-tailed woodrat (Neotoma cinerea), prominent in Grinnell’s Death Valley accounts. But Patton hesitates to speculate on the species’ absence, because reliable data on its distribution come only from Grinnell’s time and now. The rat’s numbers might have dwindled before desert warming intensified in the 1970s.
Others on the resurvey project are exploring how hotter, drier conditions might harm birds and mammals, by studying species’ metabolisms and how much water they lose through evaporation. Ecological modellers can combine these findings with the latest population data to better project how the desert ecosystem might fare as the planet warms.
Ideally, scientists would revisit these forecasts in a few decades using fresh data. But fieldwork of this sort is falling out of favour. Staring at the blue mountains on the horizon, Patton says that he doesn’t know who will replace him: very few students today train as naturalists, and museums and national parks are chronically underfunded. “Everyone wants to know how nature is changing and why,” he says. “But there’s almost nobody doing this kind of work.”
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Maxmen, A. The ambitious effort to document California’s changing deserts. Nature 550, 168–169 (2017). https://doi.org/10.1038/550168a