Beavers: The Engineers of the Forest- page 2 | Science | Smithsonian
Beavers have long been recognized as the engineers of the forest, constantly reshaping their surroundings. (Peter Busher)

Beavers: The Engineers of the Forest

Back from the brink of extinction, the beavers of Massachusetts are a crucial component of a healthy ecosystem

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And beaver ponds provide habitat for many types of animals. That’s evident as Busher and I hop across the flooded culvert. A great blue heron flaps up from nearby marsh grass, and we hear the deep, hollow thrum of a pileated woodpecker boring into a tree. When we climb from the bank onto a cement-hard beaver lodge, newts scramble for cover in the shallows. Broken eggshells are scattered underfoot–the remains of a snapping turtle nest on top of the lodge, raided by an unknown predator. Browsing moose have left massive tracks in the mud. “It’s much more diverse here than it would be if the land was drained,” says Busher. “But you do lose some trees.” Standing dead trees, killed by rising waters, are a feature of beaver ponds, along with gnawed-off stumps. But those dead trees provide homes for wood ducks and other cavity-nesting birds. And plenty of healthy hardwoods remain around the ponds, including some large maples growing right at the water’s edge.

It’s not always this tranquil when beavers move into human neighborhoods–an increasingly common occurrence across much of the United States. Here in the Northeast, forests have regrown over the past century as farming has declined, providing more beaver habitat, and hunting and trapping are limited in many areas. In Massachusetts, which banned most types of traps in 1996, beaver dams regularly flood roads, backyards and septic systems.

Under state law, beavers can be trapped when their activities threaten human health or safety. “Flooding sewer systems or wells, or damaging the structural stability of buildings, are the kind of impacts that justify trapping,” says Hajduk. But her agency advises that when beavers are simply an inconvenience, “tolerance is the best solution.” Fencing off ornamental trees and shrubs and installing specially designed pipes in dams to regulate water flow can reduce property damage.

Busher thinks many of the problems beavers are now causing for homeowners may be temporary. When beavers reappeared on the Prescott Peninsula in 1952, the number of colonies grew very slowly at first. Then the population spiked from 16 groups in 1968 to 46 in 1975 and remained high for nearly a decade. By 1992, however, it had plunged to 10 colonies, and since then it’s never risen higher than 23 colonies. “At the high point they were using every marginal spot, but that wasn’t sustainable,” says Busher. Once beavers ate all of the available plants in marginal areas, some left the peninsula, failed to breed, or died. Now the population is more stable. He’s documented a similar pattern of rapid growth followed by decline in California's Sierra Nevada. If that model holds true in the suburbs that beavers are now infiltrating, flooding problems may ease over the long term as beaver populations stabilize.

When beavers abandon a site, their ponds and marshes start to fill up with silt and eventually become “beaver meadows,” which provide habitat for many species of grassland birds. Farther down the peninsula we see abandoned dams covered with vegetation and ponds where water levels have dropped. Some of the lodges have holes in them, and they have no food caches—piles of freshly cut branches, some with leaves still on—that typically stretch out in front of an active lodge.

At the last pond, runoff from heavy rains cascades over a beaver dam that’s at least five feet high and 50 feet long. The beavers that built this dam moved upstream after watershed managers broke down a smaller dam that threatened to flood an access road – one of the few times Busher has seen beavers give up on a location so quickly. “It’s not always obvious why they choose one spot over another, or move on from what looks to us like a perfectly nice pond. They’ve got their own aesthetics,” he says.

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