Pete Schmidt, the wildlife biologist at Tualatin River National Wildlife Refuge outside Portland, stands atop a dirtfilled levee as he looks toward the bank, now bare of the thick vegetation that once lined the waters edge.
This site, among others on the refuge, shows the damage that nutria can cause. The population of the nonnative, semiaquatic rodent, which resembles a beaver with a round, ratlike tail and large orange teeth, is growing rapidly in the Pacific Northwest.
The refuge is focusing on shortand longterm approaches to nutria management: trapping and shooting and, more important, educating refuge visitors and the general public on the dangers of the rodent, which can spread an array of diseases and parasites, and the reasons not to feed or house them.
They drill into the levees that were standing on here, and they build their dens underneath the levee, Schmidt says. This area collapsed all the way back about halfway through into the levee. Shortterm complications of this type of damage include loss of vegetation from erosion and damage to water control structures. Its also a safety hazard for refuge vehicles.
It could cause a drowning if a truck rolled over into the ditch, Schmidt says. Longterm damage includes the displacement of native muskrats, which depend on the same vegetation for food. If nutria managed to burrow all the way through the levee, they could severely harm the habitat and food grown for the tens of thousands of migratory waterfowl that return to Tualatin Refuge each winter.
The refuge was established in 1992 to mimic the natural cycle of the Tualatin River Basin floodplain. Every summer, areas are selectively drained; in winter, they are flooded.
We draw down the wetlands during the summer to promote the growth of annual plants, which feed waterfowl in the winter time. If we had a levee breach and it flooded the wetland at the wrong time, it could kill all the plants that were trying to grow in the summer, Schmidt explains.
The refuge hasnt experienced that catastrophe yet. But a breeding pair of nutria can multiply to 16,000 individuals in just three years.
Oregon is one of 15 states known to have stable or increasing nutria populations. In the early 2000s, Blackwater National Wildlife Refuge in Maryland embarked on a multiyear trapping program in which more than 5,000 nutria were eradicated from the refuge. The rodent, native to South America, was introduced to the United States as early as 1899 for fur farming. When the demand for their fur bottomed out, many nutria were released into the wild. In Oregon, that happened in the late 1930s.
Today, climate change could seriously increase the population of nutria, whose range is limited by cold winter temperatures. Mortality rates, which can climb to 90 percent during unusually cold winters, keep populations in check.
If the climate begins to warm and we don’t have cold winter temperatures that stop the spread of nutria, they could extend eastward up the Columbia Basin to other areas, says Schmidt.
Schmidt says that the refuge is seeking public input into its Comprehensive Conservation Plan process for dealing with nonnative species, including nutria.
Tess McBride is a communications intern in the Pacific Region office in Portland.