I stood on a ridge top today looking out over an ocean of trees - the largest area of nearly unbroken hardwood forest in the central Midwest. To get there I rode in a jeep driven by Dan Shaver, director of the Nature Conservancy's Brown County Hills Project. We climbed steadily up from the valley cupping the little town of Nashville, Ind., along Greasy Creek Road to Bear Wallow Road, and up to Freeman Ridge. Stepping out of the jeep, we stood in the soft winds passing over the ridge top. The forest below and all around us is the most successful songbird habitat in Indiana, where populations of the little Cerulean warbler (nearly an endangered species) remain stable, and may even be growing. The Nature Conservancy calls it "The Big Woods."
The Big Woods has many owners. Much of it is already protected as nature preserves, state parks, and national forest. Some areas, though, are privately owned. We were visiting the Bear Wallow Natural Area, where private land mingles with three forest preserves, separating two of them. The Nature Conservancy is working with private owners, encouraging them to keep their land wildlife-friendly. That way, the private land can continue to serve as a passageway for wild animals to travel safely between government-protected lands.
Such "wildlife corridors" are critical links. They are natural "hallways" between large "rooms" of protected natural spaces. The corridors can be miles wide or as narrow as a road culvert. About 20 years ago, in response to the rapid disappearance of wild areas, biologists began designing and researching wildlife corridors on different scales for lots of different animals. Today, corridors have been established and more are being proposed.
Wildlife corridors add to the space available to animals and help keep the spaces from being broken up by roads, housing, unsustainable logging, or farming. The corridors help wildlife evade predators and adjust to climate changes. They help animals find food, water, and mates. They increase the range and "living room" for wildlife. This increases the health and genetic diversity of animal populations and reduces the risk of the animals becoming extinct.
If, on the other hand, habitat continues to be broken up, wild areas become smaller. This is called "fragmentation." If the areas get too small, they may no longer support enough animals to maintain a healthy population. Animals in a habitat that's too small may die out.
In the case of tigers, researchers estimate that at least 50 breeding females are needed to sustain the population. Tiger habitat in Asia is now so broken up that many patches contain 30 or fewer tigers.
Biologists often refer to fragmented habitats as "islands." Real islands in the ocean are perfect examples of habitat isolation - animals often can't get on or off on their own. But, unlike true islands, habitat "islands" on land can be linked by natural bridges. These bridges provide more overall wild space, so that larger populations of animals can live together and mix. They also may provide escape routes from such catastrophes as drought, flood, disease, or fire.
In southern California, for example, Coal Canyon is a critical link between two areas of wildlife habitat totaling 512,000 acres. To get from one area to the other, animals must pass through Coal Canyon - and across a highway. This particular "corridor" used to be on private land. The last 32 acres of the corridor became part of a preserve in 2001.
A farm hedgerow also offers a simple kind of bridge between habitats. On our farm, small animals like chipmunks, squirrels, and deer mice scurry along brushy fence lines between woodlots The brush hides them from hawks and other predators. Streambanks offer similar cover for the rodents. But even songbirds - although they can fly - are reluctant to cross cleared land.
Some ambitious proposed wildlife corridors would connect national parks to other big habitats for animals that migrate between summer and winter ranges. Other animals require hundreds of acres in which to roam. In North America, such animals include bears, wolves, lynx, elk, cougars, and panthers. In Asia, tigers and elephants need lots of space. Corridors would let them roam from one place to another safely.
A few years ago, the state of Florida purchased swampland from a lumber company to maintain a green link between the Osceola National Forest and the Okeefenokee National Wildlife Refuge in Georgia. This wetland corridor contains critical habitat for black bears, sand hill cranes, bobcats, and woodpeckers. The corridor greatly expanded the range for these species.
Out West, the Y2Y (Yellowstone to Yukon) initiative is under way. Conservation groups in the United States and Canada are working to protect links between wildlife habitats along the entire Rocky Mountain range.
"El Paseo Pantera" (The Panther's Path) is the name of another proposed corridor in Central America. The idea is to weave a series of protected reserves into a green chain from Guatemala to Panama - one that a panther would feel at home walking along.
But many critical wildlife corridors are much smaller than these. Just five square miles of wild habitat connects a wildlife sanctuary in India to a reserve in Nepal. It lets tigers, elephants, and rhinos move between the two countries.
Roads often break up animal habitats and make it difficult for some species to breed. A friend of mine who lives in southern Germany often drives along a forested road past signs reading "Froschwanderung! Vorsicht!" (Frog crossing! Watch out!) Every March, parts of the road are covered with frogs trying to reach a small pond. Adult frogs live in the damp forest. But the females instinctively return to where they hatched to lay their eggs. The frogs are especially plentiful - and endangered - at night. Volunteers (kids, too) collect the frogs in buckets and carry them across the road to safety.
Here's how a simple corridor has helped the frogs: When a new road was built over a nearby ridge a few years ago, environmentally concerned citizens got a resolution passed to create "frog underpasses." These culverts serve as frog tunnels. They work perfectly: That stretch of road does not require the services of volunteers with pails and flashlights.
Highway engineering with wildlife in mind also includes culverts for salamanders and overpasses for bear and elk. In southern California's Coal Canyon corridor, animals - including cougars - use an underpass and two culverts to get safely across the busy Riverside Freeway. (A 1995 study found that one cougar used the culvert corridor 22 times!)
Montana plans to add more than 70 wildlife crossings to its roads in the next few years. Some will be big enough for grizzly bears to cross.
Are corridors the answer? Yes and no. Large areas of wild habitat are still needed for healthy wildlife populations. Where this isn't possible, many scientists say there is good reason to maintain corridors between habitat "islands." Field research has shown that even small patches of habitat that are interconnected have more species and larger populations of animals than those that are not.
Adults may spearhead big wildlife corridor projects, but kids have made important contributions to many of them, too. Two years ago, schoolchildren raised more than $50,000 for the World Wildlife Fund in a 'Pennies for the Planet' drive. Some of this money went to create wildlife corridors in the forests of South America.
Grade-schoolers in Hayden, Ill., are helping to figure out the best places to put culverts and warning signs along US Highway 40. They are doing this by keeping track of where animals are most likely to try to cross the road.
While habitat corridors are designed to keep animals away from human disturbances, humans often seek contact with wild creatures. This kind of contact can benefit both human and animal. For example, anyone with a backyard bird feeder can help migrating birds. Now is the time to begin feeding, as many migratory birds are on their way south.
Birds fly over many barriers that other wildlife cannot cross, but they still need places to rest along the way. Biologists have found that migrating birds use more energy seeking food and shelter along their routes than they do in flight. Homeowners who live along bird-migration 'corridors' can help by offering seeds (or nectar water, for hummingbirds) as well as a yard with fall-blooming and springtime flowers. Birds also appreciate patches of brush and undergrowth in which to rest. Backyards with unmown areas along the edges and native plantings of differing heights can also provide 'steppingstones' for wildlife moving along the ground from place to place.
If you're more ambitious, you may be able to spark interest in preserving or designing a habitat corridor between parks in your area. Look at a local map: Can you see potential links between green spaces? If a river or stream connects them, protecting the waterway and its banks would be a good first step. A local nature organization may have a plan under way.
Chipmunks and other rodents use unofficial wildlife corridors - such as the grassy habitat provided by farm fences and the banks of streams - to dash from one stand of woods to another. But larger wild animals need much more space to live, migrate, and avoid the impact of human development.
Elk and wolves (below) are just two of the many species that would benefit from the Goliath of all wildlife corridor projects: the Yellowstone-to-Yukon Conservation Initiative (Y2Y). The idea for Y2Y was thought up in 1993 by a group of scientists and conservationists near Calgary, in Canada's Alberta province.
Canada's Rocky Mountain Trench is part of Y2Y's protection plan and provides crucial winter habitat for elk and a passageway for wolves. Up to 12 miles wide, the trench's long line of valleys stretches between the Columbia Mountains and the Rocky Mountains.
• See: www.y2y.net for more info.