In a warming world, it’s better to be a small mammal than a bird | Science

In the early 1900s, Joseph Grinnell traversed the wilds of California in his Ford Model T truck, meticulously surveying its fauna. Along the Californian coast, he trapped pocket mice and watched condors soar; in the Mojave Desert, his team chronicled American kestrels swooping for insects and caught cactus mice hiding among rocks.

Now, by comparing Grinnell’s data with modern surveys, ecologists have shown that climate change has not been an equal opportunity stressor. As the Mojave warmed by about 2°C over the past century, bird numbers and diversity declined dramatically, but small mammals like little pocket mice are holding their own. The survivors’ secret seems to be a nocturnal lifestyle and an ability to escape the heat by burrowing, the team reports today in Science. 

Until now, researchers have often assumed climate change challenges mammals and birds in similar ways, because both need to maintain their body temperature. But, “There are clearly winners and losers,” says Elise Zipkin, a quantitative ecologist at Michigan State University.

The pineapple heiress who established the Museum of Vertebrate Zoology at the University of California, Berkeley, in 1908 wanted it to do research, and Grinnell, the founding director, took that mandate to heart. Loaded with binoculars, clunky cameras, snap traps, and shotguns, his team drove through mountains and deserts, camping and collecting along the way. When flat tires halted the convoy, Grinnell hired prospectors and mules. Mindful of future researchers, he had his teams take copious notes and photos and map study sites. “He was draconian about it,” says Steven Beissinger, an ecologist at the museum and a co-author of the new study.

“The Grinnell-era field notes are so detailed, I know I put my boots on the same talus slope they did,” adds co-author James Patton, a retired museum ecologist.

The animals Grinnell studied now live in a markedly hotter, drier climate. Resurveys published in 2018 and 2019 in the Proceedings of the National Academy of Sciences showed “the bird community has collapsed to a new, lower number of species found per site,” Beissinger says. On average, every spot surveyed had lost more than 40% of its desert bird species, such as American kestrels or mountain quail. At most sites, even the remaining species were scarcer.

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 2 years measuring heat transfer and light absorption in the fur and feathers of museum specimens of 50 desert bird species and 24 small mammals. He then fed those 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 keep cool, birds must expend energy, for example by dilating blood vessels to evaporate moisture from their legs or mouths. The energetic costs of cooling in birds were more than three times higher than in mammals.

That’s because most small mammals take refuge underground during the hottest parts of the day. Such behaviors even helped mammals such as woodrats, which are not specially adapted for desert life. Only mammals that find themselves in soil too shallow to provide much cooling, such as the cactus mouse, suffered from the heat.

In contrast, many birds, such as the American kestrel and the prairie falcon, are exposed “to the full brunt of global heating,” explains Andrew McKechnie, a physiological ecologist at the University of Pretoria who was not part of the study. “The models establish a convincing biological mechanism to explain why birds and mammals responded differently to climate change,” says Lauren Buckley, an ecologist at the University of Washington, Seattle.

Other studies have shown declines in biodiversity as the climate warms, but this one is “impressive … because they provide the why,” says Robert Cooke, an ecological modeler at the UK Centre for Ecology & Hydrology. “This hints at a worrying scenario that may be replicated for deserts across the globe as temperature rises.” To ecologist Marlène Gamelon of the French national research agency CNRS in Lyon, the results suggest climate change poses as big a threat to desert ecosystems as it does to those in the fast-warming Arctic.

Mammals, too, may be at risk in the future. Thin soils cover just 2% of deserts today, but such areas are expected to grow as deserts get more arid. That’s why “this paper shows the importance of preserving large areas with a diversity of microhabitats,” says Linda Deegan, an ecologist at the Woodwell Climate Research Center.

Modeling studies like this one will also help conservationists make hard 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 those species that are doing just fine.”