A marsh can be a quiet place in November. No singing red-winged blackbirds nesting in the cattails. No bullfrogs bellowing from the water's edge. No mosquitoes buzzing in your ear. And when the water freezes, even ducks, geese and herons abandon the marsh for open water farther south.

By mid-winter, a marsh seems a dead zone. And on the surface it is. But beneath the ice, fish swim and muskrats go about their business of managing the marsh.

No animals are more important to a marsh than muskrats.

Their appetite determines the relative distribution of open water and aquatic vegetation such as cattails, bulrushes, and water lilies. Muskrats eat these and other aquatic plants.

When the muskrat population is low, aquatic plants spread quickly. Open water disappears.

When muskrat populations rebound, these prolific aquatic rodents reduce the abundance of aquatic plants and create more open water.

More muskrats eat more vegetation. Soon there's not enough food to feed all the muskrats so they starve or disperse, and the 'rat population declines.

While the muskrat population bottoms out, aquatic vegetation rebounds. The increased food supply triggers population growth among the muskrats and their numbers increase.

More muskrats eat more food ... and so it goes.

The consequences of all this extend far beyond muskrats and aquatic plants.

Marshes act as natural sponges that absorb flood waters and filter out pollutants. That's why conservationists preach, "Save wetlands! Don't drain them!"

When a muskrat population explodes, it can destroy a marsh quickly. A denuded marsh becomes a stagnant mud hole with little water holding capacity or filtering ability. Thus, muskrats affect people.

The open water that muskrats create and maintain provides habitat for fish, aquatic invertebrates, wading birds, ducks and many species of songbirds.

The aquatic vegetation that remains provides cover for fish, broods of ducklings and adult ducks left flightless by their annual molt.

Some ducks and geese build their nests on top of muskrat houses. Muskrats affect other wetland wildlife.

They may be just overgrown water rats, but muskrats are a keystone species in marsh ecosystems.