Muskrats to the Rescue

Several decades ago, cattails with narrow leaves and short, brown flower spikes started growing along the bays of Voyageurs National Park in Minnesota. They looked innocuous enough — in fact, only astute observers could distinguish them from the cattails that had always been there. But soon the invasive, reed-like plants cross-pollinated with the native variety, and before long the offspring had supplanted the parent species. The hybrid didn’t stop there. Stands of cattails became so dense that they crowded out every other wetland plant, including sedges, wild rice and rushes, and left no room for ducks to nest or fish to spawn.

Now, the destructive hybrid has conquered 500 acres along the shores of the park’s lakes and shows no signs of slowing down. Mats of floating cattail as large as four football fields break off and drift for miles before colonizing new bays. They also create navigational hazards invisible on GPS maps, smashing into docks and blocking off boaters inside coves.

“The cattail problem is getting worse and worse,” said Bryce Olson, a biologist at Voyageurs. “People are getting frustrated.”

Enter cattail-loving rodents: muskrats. Biologists are placing their faith in the cattail’s natural foes to help cut through thick stands and contain the future spread of the plant. Not only do the semi-aquatic rodents devour cattails (it’s one of their favorite foods), they also use the plants to build their huts. In a move biologists involved in the project say is unprecedented, they are planning to import 125 muskrats into the park and release them in five locations with the hope that the animals establish stable populations and put their incisors to work.

Dan Svedarsky, a recently retired professor at the University of Minnesota who co-authored a report on cattail management, said muskrats can effectively control cattail, and he pointed to a marsh in Waconia, southwest of Minneapolis, where the rodents cleared out areas around their huts and created pathways between thick cattail stands. “There, the muskrats really made a dent in opening the cattail and probably increased the amount of open water by 50 percent,” he said. As the cattail recedes, seeds that have been dormant for decades sprout again, and native wetland plants return.

Voyageurs is not the only place dealing with hybrid and invasive cattails. Before the 1880s, the narrow-leaf cattail that originated in Europe was found only in the North Atlantic coastal region. By the end of the 19th century it had reached the Great Lakes, and it was spotted in North Dakota in the 1940s, having traveled there along roads, ditches and railroads. Svedarsky said the hybrid has now settled in parts of Utah and Manitoba, where scientists and private companies are collaborating to turn the harvested cattail into pellets of biofuel used for heating and cooking.

In some places, herbicides have been used to control cattail growth, but the designation of Voyageurs’ waters as an outstanding resource by the state of Minnesota disqualifies that option. Fire can destroy cattail comprehensively only if the water is drained and the roots are allowed to dry for extended periods. This is possible in small ponds but not at Voyageurs, where the water levels of the main lakes, which are controlled by dams, have been set by the International Joint Commission — a body that addresses water issues between Canada and the U.S.

To manage the cattail problem, park officials have devised a 10-year plan that includes using a range of mechanical tools from barges known as “swamp devils,” which chop up floating cattail mats, to a small vehicle that scrapes off the plants above the ice in winter. These approaches are no panacea, however. The barges can’t be used in shallow waters where their blades would hit the hard, granite bottom. Power mowers and brush cutters only cut off the plant, allowing it to grow back through underground stems that produce stands of genetically identical cattails. Using shovels and spades to remove the root systems is effective, but it’s also time-consuming and inefficient.

That’s where the muskrats come in. Voyageurs has one of the highest beaver densities in the world, Olson said, but even though muskrats favor the same habitats as their much larger fellow rodents, they are few and far between in the park. Researchers have long suspected that the country’s muskrat population was declining based on anecdotal evidence from fur trappers. In a recent study of harvest data, Adam Ahlers, an assistant professor of wildlife and outdoors management at Kansas State University, found that muskrat populations indeed appear to have decreased since 1970 in most states including Minnesota. Many factors may be contributing to the decline, including habitat loss and degradation as well as increased episodes of drought and flooding resulting from climate change, Ahlers said.

Biologists suspect that the winter drawdowns from the dams between Namakan and Rainy Lakes have played a major role in the relative absence of muskrats in Voyageurs. When the water level is too low, muskrats can’t swim under the ice and must walk on top of it, where they’re more vulnerable to predators. Also, the water acts as a heat source for the muskrat’s hut, so when the water level drops, temperature inside the hut plummets. “They end up freezing to death,” said Steve Windels, Voyageurs’ wildlife biologist.

That may change soon. A recent report funded by the International Joint Commission recommended limited winter drawdowns to increase muskrats’ chances of surviving the winter. This is good news for the new arrivals, which will be recruited from areas around the park. Ahlers, who is collaborating with park biologists on the reintroduction, will help trap the muskrats and equip some of them with radio transmitters to track their whereabouts.

Because importing muskrats to control cattails has never been done before, Olson is uncertain about the outcome. What’s sure is that if the first group of muskrats performs well, another batch will follow. “These areas can support many more,” Olson said. “I don’t think you can have too many muskrats.”