Climate change has always picked winners and losers from the animal world. Some, like unbearably cute, mountain-dwelling pikas are already retreating from lower, warmer elevations in places like Yosemite National Park, and heading for cooler heights.
Beyond existing research on how climate change is responsible for certain species, like pikas or polar bears, shifting elevation, latitudes, or just disappearing from some locations, scientists have yet to find the larger patterns that explain how and why climate change impacts some animals and not others. But a new study of North American mammals, many of them from Western mountains has discerned that smaller mammals, like mice and voles, and those with flexible activity schedules, are much less likely to be affected by climate change than larger ones like elk, caribou and even pikas.
This is good news for diminutive critters, but bad news for many of the charismatic species that people adore. However, this new understanding of which American mammals are most impacted by climate change, and why, could help set conservation priorities – and perhaps make climate more tangible to people who want to see their favorite creatures succeed.
The study published in Global Change Biology this month looked at commonalities among 73 species that have been changed by climate, and those that haven’t. Co-author of the study, University of Colorado, Boulder-based mammalogist Christy McCain, has been examining where exactly mammals live in Colorado’s San Juan and Front Range mountains, compared to where they lived in the past, using records as far back as the 1860s. From this comparison, McCain can understand how mountain mammals have responded to climate change, and why. Along the way, she’s noticed that some animals, like tiny mole-like shrews, or voles, weren’t being pushing around the way she expected.
Those observations in Colorado piqued her curiosity about what mammals are doing elsewhere. So McCain and her co-author pulled data from 140 past studies on how North American mammals are relocating, declining, increasing, changing physical appearances and genetics, changing seasonal timing, or just blinking out because it’s getting warmer. Then they looked for common traits and behaviors, like size, hibernation, burrowing and choice of habitat, that are most common among animals with reactions to climate change.
McCain found that only 52 percent of species studied have responded to climate change, 7 percent responded in the opposite way researchers expected and the rest haven’t responded. While she thought that animals living in places that are already warming rapidly would react most dramatically, that wasn’t always the case.
Size of the animal – not the severity of climate change where they live – was the first deciding factor of whether species responded to change. McCain and her co-author think this may be because smaller mammals can find cool pockets, or microclimates, by burrowing or hiding under vegetation. But larger animals, like moose or desert bighorn sheep have to migrate on a larger scale, and that’s often not possible because their habitats are too fragmented. If a herd of desert bighorn sheep tries to escape drought they are probably going to have to have to cross over vast, inhospitable valleys to do it.
Even animals that we consider puny were impacted more than their even smaller counter-parts. For example, the pint-sized American water shrews are the largest of those pointy-nosed animals in the new study, and some of their ranges have shrunk in response to climate change.
Other relatively big, yet small, beasts with shrinking ranges or numbers include certain chipmunks and ground squirrels. These two rodents illustrate a second major factor in whether or not a mammal is impacted – the time of day they are active. Animals that move around in both day and night do better than those that are only active during one or the other, like chipmunks, which are diurnal. Animals with more flexibility in their behavior – whether it’s in small-scale movement, or when they are awake – can more easily escape unfavorable conditions.
Though McCain’s study is a unique contribution to understanding climate impacts, the idea that size-matters isn’t exactly a new phenomenon. “In the fossil record, there’s this evidence that the smaller mammals were able to survive major climate changes whereas the largest organisms were more likely to go extinct,” says McCain. “In retrospect the results shouldn’t have been surprising, but we were in this mind frame that everybody else was.”
Despite evidence that larger mammals are responding more dramatically, McCain found gaps in existing research that mean climate impacts on many species are still a mystery. Many North American mammals haven’t been examined for long-term climate impacts, especially mid-sized carnivores that are hard to capture and study, like foxes, ermine, long-tailed weasels and wolverines. And the relationships between bats and climate change are virtually unknown, says McCain, partly because there’s no baseline data from decades ago for comparison.
Then there’s the problem that not all animals behave the same way everywhere. At one high-elevation location near Crested Butte, Colo., yellow-bellied marmots have started munching on plants earlier in the spring and now go into winter fatter. But that marmot data set that McCain included in her analysis, as thorough and valuable as it is, may not represent all marmots, especially those at lower elevations. Plus, climate impacts on the larger hoary marmots in the Northwest, for example, are virtually unknown.
While this study could be good news for the biodiversity of smaller, less charismatic animals that often slip under the radar, it doesn’t bring much cheer for fans of megafauna. McCain hopes that having a better understanding of which mammals are more likely to respond to climate in the near term will help conservationists prioritize action for the most at-risk species. And perhaps it could even inspire people to more actively combat environmental degradation by putting a big, furry face on our rather uncharismatic climate.
Sarah Jane Keller is a High Country News correspondent. She tweets @sjanekeller.