'Assisted migration' may save some species from climate change doom

How biology can help species affected by climate change

Most people have never seen a pika. The small mammals with oversized, round ears and stubby legs live at high elevation, on rocky slopes in the mountains of western North America and Asia. But there's a reason we need to be talking about them, and the discussion has to include two of the most polarizing topics in science these days, climate change and evolution.

First some background.



A previous version of this article incorrectly stated Thomas B. Smith's title. He is founding director of the Center for Tropical Research within the Institute of the Environment and Sustainability, not the institute's director. 


With the rate of global warming now faster than at any time in the last 65 million years, we are facing increasing threats to the biodiversity of the planet — something scientists have started to call the next great extinction. Even under moderate scenarios of warming, recent studies suggest we may lose 40% of all African mammals and 100% of coral reefs by the end of the century.

Some species are managing to cope by heading toward the poles. The range of the British Comma butterfly, for example, has moved northward 137 miles in the last 20 years. But the demise of other species is almost inevitable. Take polar bears: Populations have declined 30% in the last 25 years, and many scientists think it is only a matter of time before a scarcity of pack ice dooms them to extinction. Snow leopards and Siberian cranes are also at the limits of their range.

And then there are cases like that of the pika, where it might be possible to mitigate the threat through human intervention.

If warming were happening slowly enough, evolution might come to the rescue without human involvement. Natural selection would favor pikas that could thrive in slightly higher temperatures, and over millenniums, pikas might well evolve to the changing climate. In fact, that is what happened with pikas in the distant past. Today, different species of pika reside at different elevations and latitudes, having evolved in response to different environmental conditions. But now there's no time to let evolution play out naturally.

One possible intervention might be what biologists refer to as “assisted migration.” This would involve physically moving populations of pika northward. An alternative approach would be to leverage evolutionary science by introducing pikas from southern populations into the breeding pool of northern populations, in the hope that the genes they bring would better adapt the northern pikas to warmer climates.

This kind of “prescriptive evolutionary management” is already being implemented in other contexts. Foresters, for example, have transplanted varieties of trees with more heat-tolerant genotypes from lower to higher latitudes.

Some believe humans shouldn't intervene in nature so directly, but we already have. It's human activity that has produced the warming we're seeing and the consequent pressure on so many species, necessitating that we intervene in ways that can help.

Another way to leverage evolutionary biology in the fight to save plants and animals is to try to preserve as much genetic variation within populations as possible. The variation we see in nature — differences in the size of house finch beaks, say, or variations in the coat colors of mice — is the raw material for natural selection. Since we can't always predict which of these variations are most useful or which species or populations will survive the pressures of warming, it is important to protect as much heritable variation as possible, in the hope that some of them will prove adaptive and withstand climate change. This is a bit like maintaining a diversified stock portfolio — the more variation you maintain, the less likely any single market event will bankrupt you.

But how can we accomplish this? My lab's research over the last 20 years suggests that adaptive variation is often concentrated along ecological gradients. Altitude is a good example of such a gradient. Traveling from the base of a mountain to its peak, you travel through different vegetation zones. And just as plants have responded to changing climatic conditions along the gradient, so too have the resident animals.

As one travels up a mountain, one typically sees significant differences within species in characteristics such as body size, coat color and other traits linked to survival. One way to help protect such adaptive variation is to consider it when establishing new protected areas. Unfortunately, this is rarely how protected areas have been designed in the past. We might establish a park or protected area in the lowlands and another in the highlands, but rarely do these tracts extend along the gradients that connect the two ecosystems.

In the 1930s, Aldo Leopold, one of the founders of conservation biology, once wrote, “To keep every cog and wheel is the first precaution of intelligent tinkering.” The quote epitomizes some of the most basic principles we hold dear as conservation biologists.

Since Leopold's time, advances in evolutionary biology have made it clear that we need to maximize the amount of adaptive variation we preserve under climate change. In light of this, we can update Leopold's quote to point out that the preservation of biodiversity requires more than simply keeping the parts but also about preserving those parts that matter the most — the natural heritable variation in populations that allow populations to evolve and adapt to new conditions.

Thomas B. Smith is a professor in the department of ecology and evolutionary biology at UCLA and founding director of the Center for Tropical Research, Institute of the Environment and Sustainability.

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