A wildlife bridge that Thoreau would love

Two Massachusetts towns that surround Henry David Thoreau's beloved Walden Woods are considering whether to walk in the American philosopher's conservationist footsteps.

At a projected cost of $2 million to $3 million, a proposed 25-foot-wide by 100-foot-long overpass would span a four-lane stretch of Route 2, allowing wildlife and humans to safely traverse between the two sections of the Walden Pond State Reservation that the state highway bisects. The passage, a rarity in the United States, will be covered with earth and planted with native trees and grasses, providing a scenic footbridge for humans by day and an animal-friendly wooded corridor by night.

Proponents say that the land bridge would keep animals and tourists safe, as well as help preserve the rich history of neighboring Concord and Lincoln, Mass.

"The overpass will allow people and wildlife to utilize both sides of Walden Woods, and experience its beauty in a more safe, pedestrian setting," says Matt Burne, Land Conservation Coordinator for the Walden Woods Project (WWP), a nonprofit research and education center focused on Thoreau's writings and influence on environmentalism. Reconnecting the fragmented woods would "provide visitors with a greater opportunity for interpretation of Walden Woods' historical and ecological significance," he adds.

Hundreds of thousands of people visit the state reservation each year to hike its celebrated forest and swim in its large pond. The reservation's extensive networks of walking trails also presents a safety hazard for any visitors wanting to cross Route 2. "I am unaware of any vehicle collisions with pedestrians in that area, but frankly I am surprised. I have crossed Route 2 and it is not fun at all," says Mr. Burne.

Animals, including black bears, deer, and coyotes, would also benefit from the proposed Walden Passage. While small animals such as skunks and rabbits already traverse culverts that pass under the highway, larger animals and arboreal species, such as native tree frogs and flying squirrels, typically won't use such ground-level enclosures.

To minimize interference with animals, the overpass may have a cross-section "camouflaged" with side and overhead plantings that visitors can pass through during the day. "There will largely be a temporal separation of use," says Burne, "with humans using it during the day and animals using it at night, with little overlap."

"For the most part," he continues, "wildlife in this suburban landscape is increasingly acclimated to human presence, so I don't think that something like the lingering scent of people will dissuade them."

By recreating or protecting links between disparate habitats, wildlife corridors provide animals with access to food and mates that might otherwise be obstructed by roads, housing, or other human developments. This increased interconnectivity helps preserve biodiversity and boosts the sustainability of ecological communities by repairing fragmented "islands" of undeveloped land.

Maintaining habitat connections is particularly pressing in Massachusetts, a state projected to have 37 percent of its forestland transformed by urban growth by 2050, surpassed only by Rhode Island and New Jersey, according to a recent report in the Journal of Forestry.

Some residents are debating the value of the project. While the overpass seeks to minimize the detrimental effects of ecological fragmentation, improving transportation safety is the prime concern for many in the community, says Jack Ahern, head of the University of Massachusetts at Amherst team leading a feasibility study to determine whether the overpass would benefit wildlife populations and humans.

"The people here are first of all worried about keeping the roads safe and have had other roadwork projects on the drawing board for some time that they don't want to lose funding for," says Dr. Ahern, who has received public comment on the proposal throughout the year.

Concerns that funding of the overpass might shelve a popular highway safety project has led the WWP to promise that the proposed land bridge would not compete for federal money.

Well-planned corridors may help improve highway safety. Nationwide, vehicle collisions with animals are responsible for more than 200 human deaths, millions of animal deaths, and billions of dollars in vehicular damage every year, according to the National Cooperative Highway Research Program.

When used in conjunction with roadside fencing, wildlife overpasses and underpasses at Canada's Banff National Park have reduced ungulate road kill (hoofed mammals) by 96 percent. If built, the Walden Passage would take a page from that park's playbook, constructing fences along the highway to funnel wildlife toward the safe-crossing zones.

Supporters of the overpass see it as a step toward combating further development of Walden Woods, which has been chiseled away for decades. Some 30 percent of Walden Woods' 2,680 acres are still unprotected from development. The National Trust for Historic Preservation has twice listed the area as one of America's 11 most endangered historic places.

While the University of Massachusetts team won't reveal its final recommendation on the overpass's feasibility until later this month, its preliminary recommendation in June was favorable. Those closely involved with the project say they are confident that that will be the team's final recommendation later this month as well.

But it is ultimately up to the communi­ties of Concord and Lincoln, says Ahern, to decide if the land bridge's benefits are worth the money.

Linking the current proposal to the area's rich history, Burne says, "Walden Woods is the ecosystem that Henry Thoreau lived in and wrote about. His writings about Walden are arguably the cradle of the American conservation movement."

That leaves some conservationists wondering: If a wildlife overpass can't gain support around Walden Woods, then where could it?