The nutria, or coypu (\(Myocastor coypus\)), is a large, semi-aquatic rodent native to South America, but it has become an invasive species across North America and other parts of the world. Nutria are recognized as a threat due to their voracious herbivory and extensive burrowing habits in non-native wetlands. This designation as an invasive pest stems from the environmental and economic damage they inflict. The reasons for this status are rooted in the history of their introduction, their unique biological traits, and their significant impact.
How Nutria Became Established
The presence of nutria outside of their native range in South America is a direct result of the global fur trade. The species was first introduced to the United States in 1889 for fur farming, but established populations emerged after a wave of introductions in the 1930s. When the market for nutria fur collapsed in the 1940s, thousands of animals either escaped or were deliberately released into the wild by ranchers who could no longer afford to maintain them.
Once in the wild, the nutria’s biology allowed for rapid population growth and dispersal. Females are highly reproductive, reaching sexual maturity as early as four to six months of age and capable of producing up to three litters annually. With an average of four to five young per litter, this high turnover rate enables populations to recover quickly. Furthermore, in their introduced habitats, nutria lack the natural predators and limiting factors that control their numbers in South America, allowing them to spread effectively.
Ecological Destruction Caused by Nutria
The nutria’s primary ecological threat comes from its intense grazing, which results in the complete removal of marsh vegetation. This feeding behavior is known as “eat-outs,” where the animals consume up to 25% of their body weight daily, but destroy up to ten times that amount of plant material through waste. Their preference for the roots, rhizomes, and tubers of marsh plants is particularly damaging because these below-ground structures hold the wetland soil together.
The destruction of this root mat causes the soil to destabilize, leading to severe coastal erosion and the permanent loss of wetlands. This process can convert healthy vegetated marshes into open water environments, which is an irreversible ecosystem change. Nutria have been documented as a contributing factor to the loss of tens of thousands of acres of marshland in the Gulf Coast and Chesapeake Bay regions. This habitat loss also disrupts the food web, impacting native wildlife species that rely on the wetlands.
Threat to Human Infrastructure and Safety
Beyond the natural environment, nutria activity poses a costly threat to human infrastructure through extensive burrowing. Nutria excavate complex tunnel systems into banks, which can extend up to 150 feet in length and six meters deep. This activity severely weakens the structural integrity of flood control features, such as earthen levees and dikes, putting low-lying areas at risk of failure and flooding.
The burrowing also undermines the foundations of water management structures like irrigation canals, roadbeds, and bridge supports, leading to costly repairs and safety hazards. The collapse of these weakened structures can occur when the ground becomes saturated or when weight is applied. Furthermore, nutria can carry and transmit various pathogens, including tuberculosis and septicemia, which can be shared with humans, pets, and livestock. Parasites such as blood flukes and nematodes that cause an irritating rash known as “nutria itch” can also contaminate drinking water supplies and swimming areas via the animal’s feces and urine.
Managing the Invasive Population
Controlling the invasive nutria population requires intensive management techniques. Direct methods of population reduction center on trapping and culling programs, utilizing tools like leghold traps, body-gripping traps (such as the Conibear traps), and snares. Some states have implemented bounty systems to incentivize the removal of nutria, which provides a continuous incentive for trappers to manage the population.
Toxicants like zinc phosphide can be used in baiting programs, though this requires careful application to avoid affecting non-target species. Habitat modification and exclusion are also employed, involving specialized fencing, the installation of bulkheads to prevent burrowing, and manipulating water levels to stress the animals. Eradication is challenging due to the nutria’s high reproductive rate and wide distribution, meaning that management efforts must be sustained to keep the population density low enough to mitigate their damage.