Seeking Safe Passage
Scientists are discovering the benefits of protecting corridors that connect isolated wildlife habitats
Copyright Jessica Snyder Sachs
(first published in National Wildlife)
LINE AFTER SKINNY LINE of pine trees flicker past the truck window like so many rows of corn, as U.S. Forest Service biologist David Dorman patrols the sandy roads of northern Florida’s remote Pinhook Swamp. These fast-growing slash pines, like the spiky saw palmetto weeds matted between them, are native species, notes the scientist. But their regimented and crowded condition is anything but natural—the result of 150 years of logging, draining and machine planting.
A far different landscape emerges as Dorman begins to talk about the long-term future, when restoration of the Pinhook’s natural water flow and seasonal fires brings back a “pre-European” ecosystem of blackwater cypress swamp dotted with flatwoods of longleaf pine and burn-resistant grasses. “A hundred years from now, you’re going to be able to drop a minnow at the top of the Okefenokee Swamp [60 miles north] and see it swim all the way to me and keep on going,” says Dorman.
Nature's Natural Highways
Some of the most passionate advocates of Macon, Georgia’s new Ocmulgee Heritage Greenway are bird-watchers who come to the river each spring and fall. “So many Neotropical migrants pass through here,” says Rose Payne, a resident of nearby Musella. “Scarlet tanagers, black-throated blue warblers, rose-breasted grosbeaks, even turkey vultures.” Indeed, waterways such as the Platte River have always served as natural highways for migrating birds, their wetlands providing vital rest and refueling stops for journeys of hundreds to tens of thousands of miles. The Mississippi Flyway alone conducts millions of birds of more than 300 species to and from their summer and winter homes each year. Beneath their surface, North America’s rivers likewise provide corridors for hundreds of fish species on their own seasonal migrations.
Unfortunately for wildlife, civilization also feels this draw to the water’s edge. As a result, more than two centuries of dumping, development and dam building have altered many North American rivers in ways that hinder and block wildlife movement.
Though serious problems persist, the good news is that conservation groups and government agencies have begun to work together to restore safe passages along some of the nation’s rivers and their surrounding habitats. Four years ago, the U.S. Fish and Wildlife Service initiated the National Fish Passage Program to help local communities and agencies restore natural flows and fish migrations by removing or bypassing man-made barriers such as dams and poorly placed culverts. That same year, the National Wildlife Federation released Higher Ground, a report encouraging responsible floodplain management by steering new development away from flood-prone areas and restoring these vital wetlands through voluntary property buyouts. The report resonated strongly in the wake of the multibillion-dollar damage of the Midwest spring floods of 1997.
“What we’ve seen in the last few years has been revolutionary, a sea change of thinking,” says the report’s author, NWF Resource Specialist David Conrad. “Across the country we’re seeing communities identifying ways of rolling back development away from rivers and restoring the natural functions of floodplains.”
After the 1997 floods in Grand Forks, North Dakota, and East Grand Forks, Minnesota, for example, city officials created a 2,000-acre open space area between the towns along the Red and Red Lake Rivers. The Greater Grand Forks Greenway now protects residents from future floods, restores ecological stability to the river corridor, and connects wilderness areas on both sides of this twin-city metropolitan area.—Jessica Snyder Sachs
Such future prospects don’t come cheap. In a celebrated buyout last year, nearly 60,000 acres of this 170,000-acre swamp passed from private to public ownership at a cost of $60 million. The rangers of nearby Osceola National Forest will now manage the land with the primary intent of restoring native habitat, switching from a 15- to 20-year cycle of clear-cutting to a 100- to 120-year cycle of selective harvest. Government land managers estimate they need another $116 million to buy the remaining prime timberland from private interests.
The patch-by-patch buyout of the Pinhook Swamp means far more to conservationists, however, than restoration of an isolated habitat. The swamp forms a perfect puzzle piece connecting the even larger wetlands of the Okefenokee National Wildlife Refuge in southern Georgia with the pine flatwoods and cypress swamps of the Osceola National Forest in northern Florida.
“The acquisition of the Pinhook will give us the largest protected wildlife corridor east of the Mississippi River,” says Andrew Schock, director of the National Wildlife Federation’s Southeastern Natural Resource Center. “We’re especially pleased about the land purchases last year, but there is still a lot of work to be done.”
Conservationists have championed the Pinhook as a vital travel route for Osceola’s population of Florida black bear, a subspecies that is considered threatened in the Sunshine State. The national forest being too small to sustain them, the bears often wander north through the Pinhook and into the Okefenokee, searching for food and mates while avoiding the towns and highways that hem them in on all other sides.
For his part, Dorman is more excited about connecting populations of less charismatic species, such as the threatened flatwoods salamander and the countless insects, worms and mollusks awaiting discovery in the Pinhook’s little-studied ecosystem. “A pine plantation doesn’t pose much of an obstacle to a bear that can travel 30 miles in a single night,” he explains. “But drain even 100 yards of swamp and you’ve thrown up a roadblock between populations of amphibians and invertebrates on either side.”
Less than 150 miles away, another biologist shares a similar vision as he stands on a sandbar of Georgia’s Ocmulgee River. “We have an interesting bottleneck here, in that the wildlife corridor between Georgia’s coastal plain and its piedmont runs straight through downtown Macon,” says Brian Rood, chair of Environmental Science at nearby Mercer University. Hoping to reestablish this corridor, Rood has thrown his scientific support behind a local plan to establish a 35-mile-long riverbank greenway that would run through the city and connect national wildlife refuges on either side. The project would also create a recreation area for Macon’s residents.
At first blush, efforts to save a vast, remote swampland and those to establish an urban recreational greenway would seem to have little in common. Yet both are being championed as vital wildlife “corridors”—among the most widely debated topics in wildlife conservation efforts today. Where once environmental activists concentrated on protecting isolated parcels of prime habitat, a new drive is for “connectivity.” Indeed, there is something intuitively appealing about the concept of reconnecting a modern landscape fragmented by urban sprawl and endless highway.
Few places illustrate this fragmentation as well as America’s most populous state, California, where last year interest in wildlife corridors drew more than 160 scientists, conservationists and land managers for a one-day mapping marathon. The result: a 100-page atlas and report identifying more than 300 “linkages” needing immediate protection across the state. The findings, published last summer, won the endorsement of California’s director of natural resources, who vowed to make the corridors her agency’s “top priority.” Already, California authorities have spent more than $20 million to acquire the first 2 of 232 identified corridors—one a ranch in the mountains behind San Diego, the other a canyon in Los Angeles.
But not everyone is cheering such efforts. “Serious conservation isn’t as easy as skinny linkages between insufficient natural areas,” argues University of Tennessee biologist Daniel Simberloff. “Clearly, something about the concept of wildlife corridors has ignited public interest, and that’s a good thing. But we don’t want people thinking they’re preserving nature when they’re really not.”
Critics such as Simberloff are particularly concerned about what they see as a siphoning off of conservation dollars for strips and parcels of high-priced urban and suburban land—money that might be better spent on acquiring intact blocks of healthy habitat in more remote locations. The science behind wildlife corridors and how plants and animals use them is still relatively new. The concept springs from the “island biogeography” work of two noted American scientists, Edward O. Wilson and Robert McArthur, in the late 1960s. Studying actual islands cut off from continents at the end of the last Ice Age, they observed that habitat isolation inevitably results in a loss of plant and animal species—with the greatest losses on the smallest and most isolated islands.
Twenty years later, ecologists began applying the concept to landlocked “islands,” arguing that the same kind of extinction patterns occur when hemmed-in wildlife populations can’t migrate or replenish themselves during times of disturbance such as drought or fire. Of particular concern: populations so small they suffer inbreeding without an infusion of migrants from surrounding habitat.
Driving home such concerns, a recent study of wildlife reserves in the eastern United States and Canada confirmed that conservation areas smaller than 1,000 square miles invariably lose species, mammals in particular. Worse, only 14 of the 2,355 refuges in the researchers’ study area met the 1,000-square-mile minimum threshold. The only hope of stemming the losses, they concluded in Conservation Biology, was to combine smaller reserves into assemblages with migration corridors.
“The idea that corridors are the solution to such problems is intuitively appealing and certainly intriguing,” comments University of Colorado ecologist Sharon Collinge. “But we need to be cautious about making sweeping generalizations about their importance.” To test how organisms use corridors, Collinge mowed a large field of mixed-grass prairie to create a collage of native grassland and bare patches. She then compared the success and failure of insect populations living in habitat patches with or without connecting corridors of vegetation. Collinge found that the corridors made little difference to the survival of insects in either very small or large patches of habitat. “Where patches were too small, the populations couldn’t persist regardless of corridors,” she explains.
“Where they were sufficiently large, the insects succeeded regardless of corridors.” Where Collinge saw an advantage was among populations in medium-size patches. Corridors might prove most important, she explains, when they connect habitats just large enough for a population to hang on, but small enough that individuals feel the pressure to migrate.
Collinge goes further to suggest that corridors might prove more important in situations where plants and animals face more daunting obstacles than mown prairie. “If the matrix around our habitat patches had been, say, a paved parking lot, we might have seen a much more dramatic effect,” she notes.
Clearly, the importance of corridors also varies depending on the plant and animal species involved, says ecologist Nick Haddad of North Carolina State University. Like Collinge, he began researching corridors using insects. Eight years ago, he created 27 four-acre clearings in a dense South Carolina pine plantation, connecting some of the new meadows with clear-cut corridors between 70 and 420 yards long. A year later, Haddad began monitoring the abundance of four native butterflies (cloudless sulphurs, common buckeyes, variegated fritillaries and spicebush swallowtails) in the clearings. Within a year and a half of the time the scientist created the corridors, populations of all but one species had become significantly larger in the connected versus isolated meadows.
Since then, Haddad and his colleagues have scaled up their experiments to look at a variety of small mammals as well as plants that depend on birds to disperse them. “Of the 15 species we’ve studied so far,” reports Haddad, “all but two move more frequently between connected than unconnected habitats.” The two exceptions thrive in a variety of habitats. “The message,” he adds, “seems to be that corridors prove especially important for habitat specialists—species restricted to a certain type of habitat.”
“Clearly, we are getting beyond the arguably simplis- tic question of whether corridors are good or bad,” says ecologist Paul Beier of Northern Arizona University. “The question may only make sense when talking about a particular species in a particular set of circumstances.” Beier’s own research has helped elevate this discussion. In 1998, he coauthored a landmark review of 32 previous corridor studies. The results cautioned against a blanket endorsement of corridors as a universal boon to wildlife. At the same time, the preponderance of studies supported corridors as one important tool in the conservationist’s arsenal. More importantly, perhaps, the review found no evidence that corridors have ever caused harm to a resident population, despite the hypothetical possibility that they could introduce predators and other threats.
Moreover, Beier’s overview helped expand the perception of corridors beyond that of mere highways for quick travel. Their greater importance, he explains, may lie in their use by organisms—from wildflowers to frogs—that would move across only over the course of many generations. In other words, an ideal corridor must provide the resources needed for an organism’s survival, not just its conveyance. “Plants don’t often move as individuals,” says Beier. “Some spread no farther than the adult plant’s seed shadow. Consequently, a corridor that allows plant migration would, in my mind, meet the highest standard.”
Florida’s Pinhook Swamp clearly falls into this category of corridor as habitat in its own right. However, there’s no avoiding the reality that such large tracts of habitat have become rare, especially in the populous eastern United States and the rapidly developing Southwest. The situation is somewhat better in the Northwest, where corridor advocates still have a chance of preserving large swaths of connectivity between even larger ecosystems.
Last year, the Interagency Grizzly Bear Committee—a group made up of staff from the U.S. Fish and Wildlife Service, Forest Service, Bureau of Land Management and state wildlife agencies—released a report that represents the most detailed long-term study ever undertaken of areas linking large blocks of public lands in the Northern Rockies. Although it deals primarily with grizzlies, the report essentially addresses the needs of other species that use the same corridors, such as lynx and wolverines. “We’re trying to find a consensus of where linkage zones are located in the Rockies so we can take steps to manage those areas effectively,” says Bill Ruediger, a member of the committee who also heads a new Forest Service program designed to find solutions to highway construction and other habitat-fragmentation in national forests.
Road construction became an issue not long ago in northwestern Montana, when the state Department of Transportation proposed widening a section of Highway 93 that passes through Salish-Kootenai tribal lands. Tribal members insisted that the plan include a variety of wildlife-crossing structures, from culverts for salamanders to overpasses for bears and elk. “The project is an excellent example of what highway engineers must consider and implement if we are going to avoid further fragmenting wildlife habitats into smaller, isolated patches,” says Tom France, director of NWF’s Northern Rockies Project Office in Montana.
In more heavily populated parts of the country, conservationists argue that it is not enough for developers to offer narrow set-asides of open space to mitigate their habitat destruction. “A coyote might use a dirt canyon to get through a suburban subdivision, but a bobcat probably won’t. And something as small as a six-inch step can prove insurmountable to a salamander,” says M.A. Sanjayan, a lead scientist for The Nature Conservancy who coordinated the California wildlife-corridor inventory with the California Wilderness Coalition. Recognizing such limitations, Sanjayan insisted that the corridor project place top priority on identifying large areas of healthy habitat where animal movement had been documented with scientific methods.
At the same time, Sanjayan argues, there’s no escaping the need to create what he calls “corridors of the last resort”—highway underpasses, riverbank buffers and other vegetation strips that might safely channel some animals through urban areas.
Meanwhile, proponents and critics of wildlife corridors can agree on the need to move quickly to preserve those intact tracts of wilderness still large enough to afford not only movement but long-term survival of resident wildlife. “If we don’t secure these lands now, they’re not going to be here 50 or 100 years from now,” warns David Dorman. “Now’s our chance.”
“If we don’t secure these lands now, they’re not going to be here 50 or 100 years from now.”
New York City journalist Jessica Snyder Sachs wrote about Georgia’s Chattahoochee River in the June/July 2001 issue.