Scientific Method / Science & Exploration

Can mammals outrun climate change?

For about 10 percent of species, the answer could be "no."

Despite its reputation, the sloth apparently disperses to new habitats with relative ease.

One of the ways in which climate change impacts Earth’s ecosystems is through the movement of climate zones. In order to stay in the climate zones they’ve adapted to, species will have to shift poleward (or, in the case of mountainous regions, upward). This trend is already apparent, occurring at an average rate of about 6 kilometers poleward per decade, or 6 meters higher in elevation.

Obviously, some species are more capable of this than others. Trees, contrary to the depictions in The Lord of the Rings, are not terribly good at getting from place to place. They rely on the scattering of seeds and growth of new individuals to advance into a new area. Migratory species, on the other hand, already undertake tremendous journeys every year. These disparities could lead to some significant shuffling of ecosystems as fast-moving species begin catching up to and interacting with slow-moving species that they've never seen before.

The Red Queen in Through the Looking Glass tells Alice that “it takes all the running you can do, to keep in the same place.” The plight of species in shifting climate zones is similar. Previous research on the subject had focused on the climate zones, using the simplifying assumption that mammals would be able to move along with them. In a paper published last week, researchers from the University of Washington examined mammals in the Western Hemisphere to find out how well they'll keep up.

To find out, the researchers took model projections of climate change for the A2 emissions scenario, a slightly higher than middle-of-the-road version of future greenhouse gas concentrations. They calculated average climate zones for two time periods—1961-1990 and 2071-2100—to find local rates of change for the 21st century.

Next, they pulled together data for 493 species of mammal in the Western Hemisphere. The speed at which a species can move as climate changes is controlled by their dispersal behavior—how individuals strike out from their home range or group in search of greener pastures. In many species, this is at the culmination of adolescence. Adults sometimes move as well, usually dictated by breeding conditions. Ultimately, the maximum velocity a species can shift its range depends on how far individuals can travel when they disperse and how frequently they do so.

They found that, on average, just over 9 percent of the mammal species in any given location would be unable to keep pace with their climate zones. In some places, that number is nearly 40 percent.

A heat map of species that will have trouble migrating to track a changing climate. (Blue = none, yellow = 40 percent)
U of Washington

Generally, the problem appeared to be driven by especially rapid climate shifts. That’s the case in parts of the Amazon, the Yucatan Peninsula, the Appalachians, and the south-central United States. In other places, like the American West and Midwest, as well as western Mexico, the limited dispersal ability of mammals is to blame. Primates stand out as one of the hardest hit groups, along with shrews and moles. Carnivores—everything from raccoons to wolves—and deer have the best chances, along with (ironically) sloths.

A critical additional factor is human land use. Not all land is suitable habitat for other mammals, so they may be unable to settle in or move through some areas. To roughly estimate this effect, the researchers forced movement paths for a random subset of mammals to go around things like large cities. That increased the average number of species that couldn’t keep up from around 9 percent to about 11.5 percent. The true impact of human land use is very difficult to determine, though.

The researchers also looked at how the size of species’ ranges changed over time. They found that roughly 87 percent of mammals would see their ranges shrink (though a few could actually expand), for an average projected range reduction of 37 percent by the end of the century. For primates, that number climbs to 75 percent.

Tropical species look to suffer greater impacts, even though climate change is more rapid at the poles. The researchers suggest this could be because tropical species are adapted to very specific conditions, making them more sensitive to changes in climate.

The authors end with some pragmatic implications of their results. “Our study highlights a number of species that may be candidates for assisted colonization and for which further research into the costs and benefits of these actions may be practical. Our results also highlight areas in which landscapes could be made more permeable to allow species to better track climate change. Conservation planners in these regions can focus on how land-use practices can facilitate climate-change–related dispersal and range expansions into regions of newly suitable climate."

But, even with planning, unpredictable things are likely to happen when species that have never seen each other before begin to interact, so this model can only tell us so much.

PNAS, 2012. DOI: 10.1073/pnas.1116791109  (About DOIs).

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