Watch out, Rudolph: Climate change endangers reindeer games

Reindeer may or may not be able to fly – that's a matter of belief – but they definitely need room to roam and lichen to eat, and global climate change is threatening that, say scientists. 

"We want to know how reindeer and caribou will cope with climate change," says Glenn Yannic of Université Laval in Quebec, lead author on an article in the current issue of Nature Climate Change. "These animals have already faced two climate changes: We're now in a warming period, but ten to twenty thousand years ago, the Northern Hemisphere was very cold." 

By genetically mapping the migrating population during the dramatic climate change since the last Ice Age, the researchers are able to predict both where the herds will move as the planet warms, and how diverse their herds will be.

The idea that climate change is reducing wildlife habitat is nothing new, but this international team of researchers took it a step further.

"We've tied habitat stability to reservoirs of genetic diversity," says Kris Hundertmark, a wildlife ecologist at the University of Alaska – Fairbanks and a co-author on the paper. "We've always thought that this would be the case, but ... we finally have that link now." 

Genetic diversity strengthens a species, making it better able to withstand changes and challenges. "Genetic diversity is a bit like a Swiss (Army) knife," says Dr. Yannic, a native French speaker. "In a Swiss knife, you have a lot of tools that allow you to cope with different situations in your life. Genetic diversity is the same for a species... The more genetically diverse, the more a population has tools to cope with new or abnormal situations."

The researchers mapped out the genetic variation within reindeer and caribou populations in North America, Europe, and Asia. ("Reindeer" and "caribou" are the European and North American labels, respectively, for members of the same species, Rangifer tarandus.) This included gathering DNA samples from 1,297 animals. The scientists found two subspecies of Rangifer, indistinguishable to the eye but genetically very different.

The "Euro-Beringia" subspecies is found throughout the northern reaches of Europe, Asia, and the western two-thirds of North America, while the other subspecies lives only in northeastern North America, south and east of the Hudson Bay.

"The caribou from the western part of the Hudson Bay are more related to caribou in Europe – to 'reindeer' – than to caribou from the eastern part of the Hudson Bay," says Yannic. 

Survivors of the Ice Age

These giant members of the deer family make up one of only a handful of species left over from the "Beringia megafauna," the group of massive creatures from the last Ice Age that includes mammoths and mastadons, saber-toothed tigers, musk oxen, moose, and steppe bison, the ancestor to the American bison and the wood bison, still found in Canada but extinct in Alaska for about 200 years.

"Actually, the wood bison is a poster child for what we don't want to happen to caribou," says Dr. Hundertmark. The wood bison were driven locally extinct both because of overhunting and because of ecological factors, he says. "The types of habitat they were adapted to – grasslands – there just weren't many of them left in Alaska. The boreal forest had more or less taken over, so the suitable areas of habitat for them were very small. And then, that allowed them to be hunted out very easily, because they had such small populations.... When you have that isolation, things can go badly wrong."

The eastern Canadian caribou could face a similar fate, he says. 

"It looks like those herds in northeastern Canada, on the east side of Hudson Bay, are going to be isolated from those (Euro-Beringia) herds on the west side of Hudson Bay," says Hundertmark. "That whole band of suitable habitat is going to move to the north, and more or less cut them off at the pass, at the south end of Hudson Bay, so those herds in the northeastern part of Canada are going to become more isolated and start losing genetic diversity."

Isolated populations have too few breeding choices, says Hundertmark. "That's what worries us. We want natural selection to be guiding what genetic diversity we have or what forms of the genes are going to be favored in the next generation.... There could be a gene that would be very advantageous, but because the population is small, it may randomly not make it to the next generation, due to the way breeding occurs."

Unlike the Euro-Beringia reindeer, who have the Canadian and Siberian steppe to wander through, the caribou found east of the Hudson Bay prefer very cold woodland habitats, which are becoming scarce as the climate warms. Assuming current carbon emission trends continue, the researchers anticipate that the eastern subspecies will lose up to 90 percent of their habitat in the next 60 years. 

The Euro-Beringia herds are in better shape, but still stand to lose 60 percent of their habitat in the same period, the team says.

The past is the key to the future

To create their model of future populations, the researchers started by looking at the present and the past. They gathered temperature and precipitation data from habitats where reindeer and caribou live today, and then used climate records over the past 21,000 years to map out where those habitats used to be, and where they would therefore expect the giant deer to have lived in the past. Whenever possible, they cross-checked their projections against known locations of reindeer and caribou fossils, and found a strong correlation.

"Once we were confident on how species responded to past climate change, we projected it to the future," Yannic says, using the "Business As Usual" climate projection from the Intergovernmental Panel on Climate Change, which he describes as "not the most pessimistic scenario, but not the best, optimistic scenario."

Once they had maps of reindeer and caribou habitats going 21,000 years into the past and 70 years into the future, they added another component: genetic diversity, that Swiss Army knife of adaptability.

When they tracked genetic diversity as a function of geographic location and time, the results were striking, they say.

"Where the climate remains stable for a long time period, there's higher genetic diversity," says Yannic. "When a habitat shrinks, we observe a decline of genetic diversity."

The conclusion is obvious, says Yannic. "If you want to conserve the maximum of genetic diversity, we have to focus on regions that remain stable," such as the Alaskan and Russian arctic. "Alaska, in particular, has been very stable for a long time over the past and for the future."

Hundertmark, who has studied Alaska's ecology for decades, says that the most stable climate for caribou is found north of Alaska's Brooks Range and along the western coast, comprising between a quarter and a third of Alaska's total area.

"This research is exciting because we finally have that link now, a documented link between habitat stability and modern reservoirs of genetic diversity," says Hundertmark. "It can point the way forward for us to maintain these areas that we think are going to be stable, going into the future. It gives us targets for the future to worry about, for management."