Can Wildlife Corridors Heal Fragmented Landscapes?

Conservationists have long called for creating ecological corridors that would enable large mammals and other wildlife to roam more freely across an increasingly developed planet. But now scientists are taking a closer look at just how well these corridors are working and what role they might play in a warming world.

The rugged Cabinet Mountains of northwestern Montana are an island of wild country with a population of fewer than 30 grizzly bears, their existence tenuous because they are cut off from others of their kind by distance, roads, and other development. Biologists are concerned about the small number of females, since they reproduce only every three to four years. So in recent years, the U.S. Fish and Wildlife Service has occasionally caught a sow near Glacier National Park, trucked it to the Cabinets, and sent it running off into the woods to increase the number of females.

But the Fish and Wildlife Service is pinning its hopes for the long-term survival of this population on a different strategy: the protection of an ecological corridor that would connect the marooned Cabinet grizzly bear population with a larger, more intact ecosystem, 50 miles to the south. That ecosystem is the Selway-Bitterroot Wilderness, some 600 square miles of rugged bear habitat now devoid of bears because they were wiped out to protect sheep.

The effectiveness of corridors remains an open question, especially as climate shifts in unpredictable ways.

The Cabinet bears could make it to the Selway-Bitterroot Wilderness under their own steam, but standing in their way is the formidable obstacle of Interstate 90 — six lanes of concrete, with more than 8,000 vehicles a day zooming past at 75 miles per hour. Some tunnels exist under the highway to allow wildlife to bypass the road, but in the last few years only one grizzly bear has apparently made it to the other side, and he was shot by a black bear hunter. Biologists aren’t sure grizzlies will even make the trip but they are currently studying options for preserving land for a corridor; in August, a non-profit group bought a key, 71-acre parcel of land to expand the grizzly bear corridor near the Cabinet Mountains. “We can’t make them move,” said Chris Servheen, recovery coordinator for the U.S. Fish and Wildlife Service. “We can only provide the opportunity.”

Connecting the Cabinet Mountain grizzlies to the Selway-Bitteroot wilderness — part of the larger “Yellowstone to Yukon” corridor project — demonstrates the challenges involved in efforts to link up isolated populations of wildlife by establishing ecological corridors. With the planet increasingly carved up by human development, biologists and conservationists have for decades realized the importance of establishing ecological corridors that will enable remaining populations of animals — particularly large mammals — to have the room they need to thrive. Now, numerous studies are underway and the effectiveness of corridors remains an open question, especially as the climate, and natural systems, shift in unpredictable ways.

“We’ve studied the small ones, a couple of hundred yards [wide],” and they work, said Paul Beier, a conservation biologist at Northern Arizona University in Flagstaff and an expert on wildlife corridors. “We think the bigger ones will work too, but we don’t really know that.”

Still, the creation of corridors is moving ahead. In Germany biologists are planning to protect or create thousands of miles of corridors to connect national parks and conserve a range of species, especially the imperiled European wildcat. In India, conservationists have raised money to resettle several villages in the Tirunelli-Kudrakote corridor, a critical 2,200-acre swath that connects elephant habitat between two preserves that are home to roughly 6,300 elephants, the largest population of Asian elephants in the world. This summer, residents of a fourth village in the corridor agreed to abandon their land for new homes elsewhere.

In the Amazon, conservationists and international organizations are working to create corridors for animal and plant migrations upslope as the climate continues to change. “Extinction estimates for the Amazon Basin are terrifyingly high,” said Miles Silman, a Wake Forest biologist who is gathering baseline data on Andes ecosystems as the region warms. New fragmentation is unceasing; Peru and Brazil completed a massive construction project this year, the Interoceanic Highway, which slices through the protected tropical wilderness of both countries.

In Central America and South America, conservation groups such as Panthera are attempting to create a web of jaguar corridors in many of the 18 countries where the great cats live. The corridors will include parks and wilderness, but also agricultural areas and other human-dominated landscapes through which jaguars can pass without fear of being hunted by local residents.

Areas connected by corridors had 20 percent more plant species than those without, a study showed.

In western North America, conservationists are hoping that bears and other large animals in the northern Rocky Mountain region will eventually be linked by the Yellowstone to Yukon corridor, which would connect major parks and wilderness areas and allow the flow of species across hundreds of miles of the wildest landscape in North America. “These are large blocks of public land separated by mountain valleys with private land,” said Servheen. “We want to reconnect all the blocks of public land.”

The textbook example of the perils of isolating populations of large mammals is the Florida panther, whose numbers dwindled to just two dozen individuals due to habitat fragmentation and resulting genetic impoverishment; the big cats were dying, in part, because of a heart defect related to inbreeding. But by introducing eight mountain lions from Texas — the same species, even though they have different names — and by building highway overpasses and tunnels that have reduced mortality from cars and trucks, the Florida panther has been pulled back from the brink of extinction. Roughly 100 to 160 exist today.

A critical element of conservation is the need to keep large mammals on the landscape, especially predators. And essential to protecting the large mammals is the preservation of their migration routes, whether they’re moving for food and water, for breeding, to make seasonal changes, or, more recently, to follow preferred habitat as a changing climate causes shifts in plant communities.

Some ecologists question, however, whether corridors are the panacea that conservationists make them out to be. Dan Simberloff, an ecologist at the University of Tennessee at Knoxville, said that some corridors will work, and some won’t — it’s site specific because habitats are so different. But he thinks they are a compromise that avoids the real problem, and diverts critical funds. “A general concern I’ve had with the corridor bandwagon is that it perpetuates the notion that we can somehow have conservation on the cheap by providing a technological solution to the problem of habitat destruction and fragmentation,” he said. “It’s seductive, but unlikely to work in many cases. Unfortunately to conserve biodiversity we have to conserve habitat.”

A study published in late September in Ecology Letters suggested that global warming could occur so rapidly that some creatures, including certain amphibians, might not be able to adapt, even with the aid of ecological corridors. “Our work shows that it’s not just how fast you disperse, but also your ability to tolerate unfavorable climate for decadal periods that will limit the ability of many species to shift their ranges,” said Dov Sax, assistant professor of biology in the Department of Ecology and Evolutionary Biology at Brown University. “Ultimately this work suggests that habitat corridors will be ineffective for many species and that we may instead need to consider using managed relocation more frequently than has been previously considered.”

‘We don’t know if species will use the corridors we think they will,’ an ecologist notes.

Few studies exist on the conservation effect of corridors on large mammal populations, but there is some good data on small- to medium-sized species. The longest-running study of corridors has gone on for 18 years at the Savannah River Site near Aiken, South Carolina, a 310-square-mile federal nuclear reprocessing facility that is also a National Environmental Research Park. Nick Haddad, a professor of biology at North Carolina State University has investigated the impact of a restored corridor 150 meters long and 25 meters wide between fragments of native mixed longleaf pine and savannah. Haddad and his colleagues have done painstaking work, capturing butterflies and small mammals, marking them, and then recapturing them to see which creatures made the trip across the corridor. They have also dusted plant seeds with fluorescent powder, and then found those seeds again in bird waste on the other side of the corridor.

The verdict? “Corridors work as a superhighway for plants and animals and they use them a lot,” Haddad said. Of the 20 species studied, 18 moved more frequently with a corridor, some even ten times as much as species with no corridor. Areas connected by corridors also had 20 percent more plant species than those without according to a 2009 study in the Proceedings of the National Academy of Sciences.

Scientists have shown that highway overpasses for wildlife on Arizona Highway 260 and in Banff National Park are well used and have reduced the number of large animals killed by traffic by more than 90 percent. They are widely considered to be a success, and similar structures are being built around the West. The question, though, is whether less road kill has an appreciable effect on a species’ long-term viability. A 2003 study along the 16-lane Santa Monica Freeway, used by 150,000 vehicles each day, found that bobcats and coyotes used the existing underpasses. But they also crowded the animals’ home ranges together and newcomers were fiercely challenged and did not stay long enough to breed.

Other studies say there is little or no effect from corridors. A 2002 study found, for example, that corridors did not offset the impacts of logging-caused fragmentation in the boreal forest in north-central Alberta, Canada, on most bird species.

Thomas Hoctor, a professor of conservation biology at the University of Florida, and a colleague, Reed Noss, drew up a connectivity plan for their state in the 1990s called Ecological Greenways, which proposed purchasing corridors from the coast to inland habitat. In the last few years, that project has become integral to the state’s plans for adapting to climate change. Sea levels are projected to rise by as much as three to six feet in the next hundred years, and much critical habitat will likely be inundated, forcing species to migrate away from the coast. Over the past decade, the state bought hundreds of thousands of acres for corridors, but funding for the program was eliminated this year under the administration of conservative Republican governor, Rick Scott. Still, said Hoctor, the land that has already been purchased could help some species make the journey inland across the state’s densely developed landscape.

Oswald Schmitz, an ecologist at the Yale University School of Forestry & Environmental Studies, is investigating climate change and the movement of species across large landscapes. He thinks the jury is still out on the efficacy of corridors. “The hypothesis is there, but there hasn’t been a lot of empirical work done,” he said. “We don’t know if species will use the corridors we think they will.” What is important about corridors, he said, is that “they create a dialogue and awareness that these are things we need to pay attention to.”

To some, the notion of preserving and creating corridors seems obvious, especially as a warming world will put more pressure on species to move. “Dozens are being created, and it will become hundreds quickly,” said Beier.

But other scientists are less sanguine. University of Minnesota ecologist Craig Packer, writing in Science in 2010 about the Florida Panther, said, “Once the entire planet reaches the same state of economic development and urbanization as the United States, wildlife managers all over the world can look forward to carting rare species from one park to another until the end of time.”