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Movers Of The Earth

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Beavers do major ecosytem engineering - from clear cutting forests to filtering water

We live on a landscape shaped by big rodents. The handiwork of beavers turns streams into lakes and dry land into lush meadows. Their dams help filter water, and their homemade ponds provide habitat for countless species. Beavers may even reduce the effects of acid rain.

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A hundred years ago these industrious engineers were driven to the edge of extinction by fur traders. And our own industrious cities and farms covered much of the fertile land they left behind. But today the busy beaver is back again-with nearly 10 million across the United States.

Slow-moving and weighing an average of 50 pounds, beavers turned to water for security at least 30,000 years ago, because they lacked such traditional defenses as speedy flight, armor, or quills. Any predator venturing into water after a beaver must claw through a bulky lodge of sticks.

Until the 1600s an estimated 60 million to 400 million of these rodents populated the North American landscape. Typically, their dams could be found every 500 feet along a stream, forming networks of ponds, wetlands, and lush meadows, from the Arctic to the Mexican desert. In the late 1600s, European fur traders arrived in this wonderland. Driven by a rapacious market for beaver furs, trappers quickly depleted each new region they visited until even the far reaches of the Great Basin Desert in Nevada were trapped out in the 1830s. Because trappers preceded settlers in much of North america, especially in the West, the beaver was already a faded ghost by the time "civilization" arrived, and the fertile landscape the settlers found was simply assumed to be part of nature's generous bounty. That fertile landscape, however, started with the leap and gurgle of a stream. Then a beaver arrived - over land or by water - waddling along languidly with small-eyed myopia.

At first glance they seem innocuous, but beavers are the ultimate modifiers of such water energy. Usually they select small, wooded valleys, where a single dam will flood a large area, giving the beavers protected access to the riverside aspens, willows, and cottonwoods they love to eat. With stubborn diligence, long teeth, and dexterous hands, they build a thick dam of sticks, stones, and mud. Beaver dams look like a jumble of mismatched sticks, but they are sturdy enough at times to support a rider on horseback, and they withstand all but the worst spring floods.

By slowing a river's flow, beaver dams cause water to rise and spread accross the land. Much of the water travels through the root zone of plants. Several hundred acres may be subirrigated before the water - clear and filtered - reemerges downstream as springs running from the stream bank. Raised water tables nourish vegetation. And in the arid landscapes of western deserts, beaver ponds provide rare oases of water throughout the summer. Free-flowing streams rapidly carry away natural debris as it erodes from the land. But beaver ponds, because of their holding action, are important storehouses of sticks, leaves, and other collected material. Over time, a variety of soil microbes process and convert this material into nutrients that are used by other species. Some bacteria, for instance, convert an inorganic form of nitrogen to an organic form that can be used by plants. The sulfates in acid rain are converted by bacteria into sulfides, which don't lead to the problems that acid rain causes.

As a beaver pond ages, the sediment matures into a blackish, oily residue that smells like rotten eggs. Purple-colored sulfur bacteria, reddish iron pigments, and bright green bacteria form in bands, streaking the sediment with color. This rich stew later provides nutrients for the meadow and forest plants that move in after the pond has filled in. We see little of this underwater life, instead we witness the impact of beavers on trees and vegetation. Beavers return to the same home to sleep at night rather than wandering freely, the way deer and other familiar herbivores do. This behavior, combined wih the fact that nearly all of a beaver's browsing takes place within a very narrow area - rarely more than 100 yards from water - essentially clear-cuts favored tree species, such as aspen and cottonwood. Their pruning, however, promotes bushy resprouting of the trees. Flood control by humans has led to a decline in cottonwoods, so in some areas, these trees depend on beavers for their regeneration.

In the patches of forest flooded by beaver ponds, trees die. But these stands of dead trees are used by birds, especially cavity nesters such as chickadees and swallows. The ponds are also frequented by nesting ducks, whose broods are protected by standing dead trees, which disrupt the flight path of hunting raptors.

Beavers tend to abandon a site after several decades, however, because the pond fills up with sediment and the local flood supply runs out. They often relocate upstream or downstream, leaving their pond to slowly transform into a wet meadow. Beavers sometimes reclaim an old site once the trees and shrubs have grown back. Scientists have postulated that old beaver ponds produced most of the fertile farmland in the valleys of North America.

The return of beavers is both good and bad news, for while they benefit willows, water tables, and wildlife, at times they do not see water needs in the same way as humans, and both can be particularly stubborn about their points of view. When beavers cut trees, they block culverts, flood properties, and pollute wells, and many people would be perfectly happy if beavers never returned. But what's at stake when beavers are extirpated from an eara? The nearly total extermination of this ecosystem engineer across the entire landscape at the turn of the century showed what can happen without beaver dams. Steams picked up speed and cut deeper channels, lowering the water table. By the mid 1900s lush meadows had become a thing of the past in some regions. Lew Pence, who worked for Idaho's Soil Conservation Service for 34 years, recalls streams that no longer functioned as ecosystems but merely as drainage ditches. "We tried all kinds of structures to slow the water down and raise the water table again," he says, "but nothing held, and all the techniques were prohibitively expensive."

Then in 1984, the agency brought in some beavers. It's been in the "beaver business" ever since. Pence says the streams he's worked on "start to smell like grass again after the beavers come back." Ranchers along the Front Range of Colorado have had a similar experience, finding that hay meadows along beaver streams don't need irrigating.

Savvy land and water resource managers are increasingly turning twork pretty cheap, and beavers for stream-restoration work. As one Wyoming rancher quipped, "They work pretty cheap, and I've never seen a lazy one yet."


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