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Making bridges for animals

Animals need room to roam; one way to provide it is to link up natural spaces using 'wildlife corridors.'

By Sue Wunder / October 5, 2004

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."

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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.