“In Northeastern Pennsylvania, where I grew up, muskrats have basically disappeared from many areas, and I haven’t seen a comeback,” Hardisky said. “Farm ponds are the only areas in the northeast where there are still some muskrats. In the rivers and streams it’s way down.”
Reasons for the decline are still being tossed around by biologists, but disease, at least in Pennsylvania, has almost been completely ruled out because populations usually rebound a few years after an outbreak occurs.
Hardisky said impacts of heavy metals or pesticide contamination also don’t seem to be factors either. Predation is a partial factor, he added, as muskrats are a popular prey species for great-horned owls, red-tailed hawks and mink.
Still, increased predation might be pointing biologists to the main reason why muskrat populations are plummeting. “The cause of high predation goes back to habitat,” Hardisky said. “It goes back to streambank conditions. It all has the symptoms of a bigger problem – environmental changes. One of those changes is cleaner water.”
In the past, when runoff from farms and residential areas was discharged directly into waterways, the increased nutrient load caused vegetation along streams and rivers to flourish. In turn, muskrats benefited from a lush food source and thick cover.
Also, before flood control measures resulted in the banks of streams and rivers being covered in riprap, those shorelines were generally dirt and muskrats were able to easily burrow and make dens. Today, the riprap, which is large stones, prevent muskrats from burrowing and prohibits vegetation from growing on banks.
If less runoff and cleaner water is really the cause of muskrat decline, it's hard to imagine what the solution would be. I doubt anyone will turn back our present water quality standards for the sake of building muskrat habitat. It will be interesting to see if this theory is really confirmed, and what a solution might look like.