As temperatures rise over the next century, three California amphibian species could be pushed to the cusp of extinction because the warming climate will effectively block their migration to more suitable habitats. Interventions by humans who physically relocate the animals may be the only way to help them survive.
Managed relocation, or assisted migration, for climate change is a controversial topic because of the challenges of moving an endangered species and the potential harm it may cause in a new ecosystem.
The Torreya Guardians, a self-organized group of naturalists, botanists, ecologists and others, are the most well-known proponents of assisted migration. Last July, the group planted endangered Torreya taxifolia seedlings in new habitat patches north of their customary domain in Florida, where it is becoming too hot for the conifers to survive. More recently, the British Columbia Ministry of Forests and Range launched an Assisted Migration Adaptation Trial, testing out nine tree populations from the U.S. in that Canadian province, to ensure that the latter nation's timber production stays strong as the climate warms.
One practical issue pertaining to managed relocation is deciding which species need to be moved and when to move them. To study the problem, Regan Early and Dov Sax at Brown University created a detailed simulation of how suitable habitat for 16 well-studied—and mostly common—frogs and salamanders will expand and contract over the next century in response to climate-induced changes in precipitation and temperature. Although some species' current and future ranges overlap, a decade-by-decade analysis shows that corridors to new habitats may appear too late or vanish at the wrong moment. Left to their own devices, the amphibians would then have nowhere to go and wink out.
"That was an unexpected result," Early said at the Ecological Society of America meeting in Albuquerque this week. "Species might have no apparent barriers between their current and future ranges, but climate variability and species traits interact to prevent a range shift."
Early used a moderate climate change scenario of 2 degrees Celsius warming by the end of the century. Then the team assumed that animals could expand their ranges by about 12 kilometers per decade, and also could persist in an unsuitable habitat for 10 years.
Early first described two well-known scenarios for how different California amphibians could be affected by climate change. The black-bellied slender salamander, for instance, would have no problem spreading from its home range around Santa Barbara to the more northern central coast region. But for other species, like the black salamander, a changing climate produces new pockets of habitat to the north, but they don't ever overlap the salamander's current or future range in the San Francisco Bay Area, leaving the animals stranded.
Early's step-by-step simulations revealed a third scenario that is not apparent, simply by comparing current and future habitats. In coastal California newts the current and future ranges appear to overlap, but by the time that future habitat becomes available late in the century, her simulation indicates that the current range will have contracted so much that the animals have no route to get there.
Overall, these findings mean that the three California amphibian species will become critically endangered—defined as inhabiting less than 100 square kilometers—by 2100. Other amphibian species also will become vulnerable or threatened, lacking a way to reach a more suitable habitat. The good news, Early explained, is that managers may only have to move species very short distances to give them access to suitable habitat.
"This is something we've definitely been concerned about," says ecologist Lee Hannah of Conservation International, who has studied climate paths in South Africa and Mexico and is part of the managed relocation working group. "Ten years ago it was good enough to do a century or a half-century snapshot...but now we see we have to look more closely."
"Inevitably," Hannah says, "we are going to have to rescue, captive breed, and move some species."