When the muskrat population is low, aquatic plants flourish and take over the marsh. Open water disappears. When muskrat populations rebound and climb, they reduce the abundance of aquatic plants and create more open water.
More muskrats eat more food. Soon there's not enough food to feed all the muskrats so the population declines. Mink compound the situation by eating the abundant muskrats. When the muskrat population bottoms out, aquatic vegetation rebounds. The increased food supply triggers a population rebound among the muskrats. Then the mink population follows. More muskrats eat more food ... and more mink eat more muskrats. ... And so it goes.
The consequences of all this extend far beyond mink, muskrats and aquatic plants. Marshes act as natural sponges that prevent flooding and filter pollutants from surface water to make it safe for human use. (Those are just a few reasons why conservationists promote wetland conservation.)
When a muskrat population explodes, it can denude a marsh quickly. A denuded marsh becomes a stagnant mud hole with little filtering and water-holding ability. Consequently the quality of surface water suffers. Muskrats affect people.
The open water muskrats create and maintain attracts shorebirds, wading birds, ducks, and many species of songbirds. The aquatic vegetation that remains provides cover for fish, broods of ducklings and adult ducks rendered flightless by their annual molt. Some ducks and geese build their nests on top of muskrat houses. Muskrats affect other wildlife.
If only there was a way to manipulate muskrat populations so a marsh could be stabilized to consist of about half open water and half vegetation. Such a tool exists. It's called predation. And mink are a key predator that regulates muskrat populations. Absent predators, trapping is another tool wildlife agencies use to manipulate muskrat and mink populations, and thus manage marshes.