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Looking for love as the climate heats up

Pushed north by a warming climate, southern species mate with northern cousins, muddying gene pools and conservation efforts alike.


By: Sharon Oosthoek, The Daily Climate

A northern flying squirrel, courtesy Animal Spot
BOBCAYGEON, Ontario February 14, 2013 – In the hush of a snowy Ontario winter woods Jeff Bowman's radio wave tracker beeps insistently, pointing him toward a nearby tree cavity.

Inside, a group of tagged flying squirrels huddle in the comfort of each other's body heat. In a few weeks, the rodents – which glide from tree to tree using flaps of skin between their front and rear legs – will begin to mate.

Some of their babies will emerge looking a bit like a southern flying squirrel, a bit like a northern flying squirrel, and a lot like the product of climate change, says Bowman, a population ecologist with the Ontario Ministry of Natural Resources.

Several wildlife biologists, like Bowman, suspect many species are reacting to a changing climate the way the flying squirrels are: As warmer temperatures change habitats, southern species are pushing north to hybridize with their northerly cousins.

Diluted genetics

The interbreeding has several consequences, none well understood: It could increase genetic diversity, helping species weather rapid ecosystem changes. It also could dilute the genetics of at-risk animals such as polar bears – perhaps even diluting them beyond recognition. And the changes threaten to wreak havoc with conservation efforts.

Not all evolutionary biologists buy the theory that climate change is a big driver in hybridization. Myriad factors influence cross-breeding, notably development, habitat loss and the introduction of invasive species.

Isolating a changing climate's influence is tricky, but Bowman and wildlife geneticist Paul Wilson of Trent University believe they are starting to see its fingerprints.

The pair has been trapping and testing the DNA of flying squirrel hybrids since discovering them in 2003.

Southern squirrels are smaller and have pure white belly fur, while the larger northerners have two-toned gray-white bellies. When they cross-breed, their babies are southern-sized with mottled grey-white belly fur. Looking back at Bowman's habitat studies, the two scientists found that 1995 marked the start of a series of unusually warm winters that saw the southern flying squirrel creep north 240 kilometers.

First documented impact

Wilson's DNA analysis shows roughly four percent of the squirrels within that band are now hybrids.

He and Bowman believe it's the first documented example of cross-breeding following the expansion of a species' range due to modern climate change.

"There are others," said Bowman. In fact, he and Wilson are now studying two different species of mice in Ontario for similar signs.

Cross-breeding wildlife is not new. But human-induced changes such as global warming, development and the introduction of non-native creatures are bringing together previously separated species. While there are no baseline studies to show there are more hybrids than nature intended, anecdotal evidence is mounting.

"Opportunities for hybridization are increasing," said Wilson. "It's largely because you have more southern species moving up into the range of northern species."

All of which raises vexing questions about wildlife conservation.

"Management-wise, it certainly causes a problem," said Charlotte Lindqvist, an evolutionary biologist at the State University of New York, Buffalo. "What should we do with these hybrids? Should we shoot them? Should we preserve them? We need to know more about their impact."

Polar bear/grizzly cross

While flying squirrels are not at-risk in Ontario, the best-known example of hybridization is the result of mating between two at-risk species. In 2006, an American hunter made headlines when he shot a polar bear/grizzly cross in Canada's Northwest Territories.

British Columbia wildlife geneticist David Paetkau, who confirmed the "pizzly's" parentage, thinks warming temperatures may have caused the grizzly father to spend less time hibernating and more time wandering. And once he found himself so far from home, he probably mated with a polar bear because he could find no females of his own kind.

It's not the first time the bears have strayed over the species line. A study published last year in the journal Proceedings of the National Academy of Sciences shows grizzlies and polar bears swapped DNA in ancient times. Lindqvist, the study's lead author, suspects cross-breeding was more frequent during warmer periods as polar bear populations crashed in tandem with grizzlies moving north.

Of course North America is home to other hybrids, most of which fly under the public's radar, and not all of which are linked to climate change.

Climate as a driver?

In the Pacific Northwest, crossing between spotted and barred owls threaten the tiny population of spotted owls whose old growth forest habitat has been substantially squeezed by logging. Across the western United States, pure cutthroat trout populations have declined as they breed with various introduced species of trout. And in central and eastern North America, the red wolf/coyote cross is a long-standing example of hybridization resulting from human development.

But University of California, Berkeley evolutionary biologist Jim Patton cautions that hybridization is fairly common, driven by myriad influences. Overall, he noted, there's no evidence they're on the rise or that climate change is a big driver.

Patton's work in southern Arizona along the Mexican border highlights an area where 11 percent of pocket gophers are hybrids, and whose hybridization is a natural occurrence.

The percentage of hybrid gophers has remained steady for 40 years, he added, with no effect on the genetics of parent species outside the area.

"While I don't think there is any question that climate change has had a role in hybridization over the past many thousands of years, the question is how rapidly is it occurring today – and what are the consequences in terms of species identities?" said Patton.

"Is this going to be a problem for the long-term existence of parental species? Are they going to merge into one big hybrid population? We're only going to find out in hindsight," he added.

Adaptive advantages

The question has experts taking sides now. Some say hybrids are Mother Nature's answer to the changes humans have wrought. That is, the greater an animal's genetic diversity, the more chance there is for it to adapt to rapidly changing environments.

Wilson, the Canadian wildlife geneticist, is studying the hybrid squirrel genome for adaptive advantages, such as the northerner's ability to withstand cold and the southerner's ability to fight off diseases from warmer climes.

"One could look at these hybrids as a creative reshuffling of genetic material for a changing landscape," Wilson said. "Climate change isn't going to go away … maybe these hybrids are emerging as the most adapted flying squirrel for the changing landscape and climate."

Hinder or help?

But it is hard to know in advance if a hybrid's novel mix of genes will harm or help. One example of a genetic gamble that didn't work out so well: Pizzlies bred in a German zoo excelled at hunting seals but didn't have the strong swimming abilities of their polar forebears, noted Brendan Kelly, an Arctic specialist with the National Science Foundation.

Three years ago, Kelly was part of a group of American wildlife experts who sounded an alarm over Arctic hybrids. Disappearing sea ice – which once separated potential cross-breeding partners – could mean more hybrids in the future, and governments must manage hybrids before rare species are lost, they warned.

"We are projecting forward and asking what happens when we very rapidly remove barriers to gene flow," Kelly said in an interview. "This has evolutionary implications and we should be paying attention."

Sharon Oosthoek is a freelance reporter based in Toronto who covers the environment. DailyClimate.org, published daily, is an independent news service that covers climate change. www.DailyClimate.org.


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