A cactus mouse hides in the rocks, a strategy that helps many small desert puppies stay cool.
JACK DAYNES
By Elizabeth Pennisi
In the early 1900s, Joseph Grinnell traversed the natural world of California in his Ford Model T truck and carefully examined the fauna. Along the California coast, he trapped pocket mice and watched condors soar; in the Mojave Desert, his team captured insects searching for insects and caught cactus mice hiding among rocks.
By comparing Grinnell’s data with modern surveys, ecologists have shown that climate change was not a source of equal opportunity. As the Mojave has warmed by about 2 ° C over the past century, the numbers and diversity of birds have declined dramatically, but small mammals such as small marsupials hold their own. The secret of the survivors looks like a nocturnal lifestyle and an ability to escape the heat by digging, the team reported today in Science.
Until now, researchers have often assumed that climate change will challenge mammals and birds in similar ways, as both must maintain their body temperature. But, “There are clear winners and losers,” says Elise Zipkin, a quantitative ecologist at Michigan State University.
The Pineapple Heir, who founded the Museum of Vertebrate Zoology at the University of California, Berkeley in 1908, wanted it to do research, and Grinnell, the founding director, took the mandate to heart. With binoculars, awkward cameras, snap traps and shotguns, his team drives through mountains and deserts, camping and gathering along the way. When the convoy stopped with flat tires, Grinnell hired prospectors and mules. Taking into account future researchers, he had his teams take abundant notes and photos and map study sites. “He was draconian about it,” says Steven Beissinger, an ecologist at the museum and co-author of the new study.
“The field notes of the Grinnell era are so detailed, I know I put my boots on the same slope of the talus as they did,” adds co-author James Patton, a retired museum ecologist.
The animals that Grinnell studied now live in a significantly warmer, drier climate. Surveys published in 2018 and 2019 in the Proceedings of the National Academy of Sciences showed “the bird community has collapsed into a new, lower number of species found per site,” says Beissinger. On average, each place surveyed lost more than 40% of its desert bird species, such as American towers or mountain hawks. In most areas, even the remaining species were rare.
But the new study, led by Iowa State University physiological ecologist Eric Riddell, tells a more hopeful story for rats, mice, chipmunks and other small mammals. Since Grinnell’s survey, three species have declined, 27 have remained stable and four have increased in number. “This paper is really big news for small mammals,” says Rebecca Rowe, an ecologist at the University of New Hampshire, Durham.
To find out why birds are so much more vulnerable, Riddell spent two years heat transfer and league absorption in the fur and feathers of museum specimens of 50 desert bird species and 24 small mammals. He then entered the numbers and data on the species’ behavior and habitat into a computer program that modeled how much heat stress an animal would be exposed to, and how well it could cool itself under different temperature conditions. To stay cool, birds need to expend energy, for example by diluting blood vessels to evaporate moisture from their bones or mouth. The energetic cost of cooling in birds was more than three times higher than in mammals.
Where desert roads in California disappeared and trucks got stuck, biologist Joseph Grinnell and his team hired mules to reach their study areas in the early 20th century.
Museum of Vertebrate Zoology / University of California, Berkeley
This is because most small mammals hide underground during the hottest parts of the day. Such behavior has even helped mammals such as wooden gears, which are not specially adapted for desert life. Only mammals that are too shallow in the ground to provide much cooling, such as the cactus mouse, have suffered from the heat.
In contrast, many birds, such as the American kestrel and the coyote, are exposed to ‘the full pressure of global warming’, explains Andrew McKechnie, a physiological ecologist at the University of Pretoria who was not part of the study. “The models establish a compelling biological mechanism to explain why birds and mammals respond differently to climate change,” said Lauren Buckley, an ecologist at the University of Washington, Seattle.
Other studies have shown that biodiversity declines as the climate warms, but it is ‘impressive … because it gives reason’, says Robert Cooke, an ecological model of the UK Center for Ecology & Hydrology. “This indicates a worrying scenario that could be repeated for deserts around the world as temperatures rise.” For ecologist Marlène Gamelon of the French national research agency CNRS in Lyon, the results suggest that climate change is a major threat to desert ecosystems, as are those in the rapidly warming Arctic.
Mammals may also be at risk in the future. Thin soils today cover only 2% of the deserts, but such areas are expected to grow as deserts become drier. That’s why ‘this article shows the importance of conserving large areas with a variety of microhabitats,’ says Linda Deegan, an ecologist at the Woodwell Climate Research Center.
Modeling studies like these will also help conservationists make difficult choices, says Mark Urban, an ecologist at the University of Connecticut, Storrs. “Understanding how species differ in their vulnerability to climate change will help us save money and time by ignoring the species that are doing well.”