Nutria: The Invasive Rodent Ravaging Wetlands

The nutria, a large, semi-aquatic rodent native to South America, has become a significant ecological problem in the United States, particularly in coastal areas like Louisiana and the Chesapeake Bay. Introduced for fur farming in the early 20th century, these prolific herbivores have since established themselves as invasive species, causing extensive damage to wetlands, agricultural lands, and infrastructure. Their voracious appetites and destructive burrowing habits threaten biodiversity, increase erosion, and pose economic challenges.

A History of Introduction and Spread

Nutria, also known as coypu, were first brought to North America in the early 1900s for fur production. The initial attempts at nutria farming in the late 1800s proved unsuccessful due to high mortality rates and low reproductive success in captivity. However, in the 1930s, nutria were introduced to Louisiana, where they were farmed for their fur. The industry faltered, and many animals escaped or were intentionally released into the wild when fur farmers realized the profits would not be worth the effort. A hurricane in the 1940s further exacerbated the problem, allowing nutria to spread rapidly into coastal marshes, inland swamps, and other wetland areas. From Louisiana, they expanded across the Southern United States, wreaking havoc on marshlands.

The nutria's spread was also aided by deliberate transplantation to the Mississippi River from Port Arthur, Texas, in 1941. A subsequent hurricane that year facilitated their dispersal. The collapse of the nutria fur market in the 1940s led to mass releases, further accelerating their establishment in new regions. Today, nutria are found in at least 20 states, with significant populations along the Gulf Coast, the Pacific Northwest, and the Atlantic coast. They have also been reported in Canada, specifically in the lower Fraser River valley and on Salt Spring Island.

Ecological Impacts: A Wetland Destroyer

The primary concern with nutria is the extensive damage they inflict on wetland ecosystems. These rodents are opportunistic herbivores, feeding primarily on aquatic and semi-aquatic vegetation. They consume all vegetation from foraging areas, severely reducing the biodiversity of native plant species. A significant amount of the vegetation they consume is essential for maintaining the structural integrity of wetland soil.

Nutria herbivory "severely reduces overall wetland biomass and can lead to the conversion of wetland to open water." Unlike disturbances such as fire and tropical storms, which occur sporadically, nutria feed year-round, resulting in constant damage to marshes. Their feeding habits are particularly destructive in the winter when they dig up root networks and rhizomes in search of food due to the scarcity of above-ground vegetation. This creates "eat-outs," areas where most of the biomass has been removed, disrupting habitats for other animals and humans dependent on wetlands and marshes.

The reduction of vegetation caused by nutria contributes to erosion and the eventual loss of wetlands, increasing the vulnerability of nearby environments to erosion and flooding. Their burrowing behavior further exacerbates erosion, weakening the bank stability of streams, lakes, and other water bodies. In Louisiana, nutria have been observed feeding intensively on bald cypress seedlings, the state tree, resulting in mortality. The destruction of marshlands also increases the vulnerability of adjacent upland sites to erosion and flooding during storms.

In the Chesapeake Bay, nutria have destroyed thousands of acres of wetlands through their damaging feeding habits. Scientific studies have shown that damage to marsh and shallow water habitat has resulted in the decline of oysters, crabs, fish, and waterfowl. Nutria movement through the marshes causes sediments to become suspended in the water, increasing turbidity and reducing light penetration. Consequently, submerged aquatic vegetation, which plays a vital role in oxygen production and serves as a refuge for marine organisms, is negatively impacted.

Economic Consequences: Damage to Agriculture and Infrastructure

Beyond their ecological impact, nutria also cause significant economic damage. Their burrowing can undermine and breach water-retention levees in flooded fields used to produce rice and crawfish. Nutria burrows can also damage flood-control levees that protect low-lying areas, weaken the foundations of reservoir dams, buildings, and roadbeds, and erode the banks of streams, lakes, and ditches. In Jefferson Parish, Louisiana, nutria damage to the canal system has been estimated at over $8 million since 1990.

Nutria also damage economically significant crops, including corn, milo, beets, alfalfa, wheat, barley, oats, peanuts, and other vegetables. In the United States, sugarcane and rice are the primary crops damaged by nutria. Grazing on rice plants can significantly reduce yields, with severe localized loss. In some areas, such as Italy, damage by nutria to crops has been calculated in the millions of dollars. Alterations in water supply to agricultural fields, alongside heavy grazing by the animal, have led to significant declines in crop yields, notably sugarcane and rice plants.

Control and Eradication Efforts: A Multi-Faceted Approach

Controlling nutria populations is a complex challenge that requires a multi-faceted approach. Various methods have been employed, including trapping, shooting, habitat modification, and, in some cases, chemical control.

Trapping is currently the most effective method of removal from infested areas. Different trapping techniques are used, such as the deployment of cage traps, foot-hold traps, rotating-jawed traps, colony traps, and the use of snares. Traps are baited using sweet potatoes, carrots, and other vegetables and fruit. Shooting can also be effective, particularly when nutria are concentrated in specific areas. Up to 80% of the nutria in an area may be removed by shooting with a shotgun or small caliber rifle.

Habitat modification involves eliminating brush, trees, thickets, and weeds from fence lines and removing row crops adjacent to ditches, drainages, waterways, and other wetlands to discourage nutria. Improving drainage to destroy travel lanes and managing vegetation to eliminate food and cover can also be effective.

The use of zinc phosphide as a chemical control method has proven effective in eliminating nutria on a large scale. The poison is typically placed on bait-rafts and on carrots and sweet potatoes near burrows and dens; however, this method may lead to the unintended death of non-target species.

In the Chesapeake Bay region, the Chesapeake Bay Nutria Eradication Project began in 2002 with the goal of eradicating nutria from the Delmarva Peninsula. Biologists utilized a variety of techniques to locate nutria and monitor the success of trapping and removal, including releasing sterilized nutria outfitted with tracking devices to locate additional nutria, boat, shoreline, wetland, and platform surveys, hair snares, and detector dogs. As of 2018, all the known nutria populations have been removed from over a quarter million acres of the Delmarva Peninsula.

Louisiana has also implemented a Coastwide Nutria Control Program, which pays licensed trappers a bounty for each nutria tail collected. The goal of the program is to remove up to 400,000 nutria each trapping season from coastal Louisiana to reduce nutria-induced marsh damage.

The Future of Nutria Management

The ongoing battle against nutria requires sustained effort and innovation. While eradication has been achieved in some areas, such as the Delmarva Peninsula, maintaining control in other regions, like Louisiana, remains a significant challenge.

Research into new control methods, such as contraceptives, may offer more humane and effective approaches. Public awareness campaigns are also crucial to educate people about the impacts of nutria and encourage participation in control efforts.

The nutria serves as a stark reminder of the potential consequences of introducing non-native species into new environments. Their story underscores the importance of proactive measures to prevent future invasions and the need for continued vigilance in managing existing invasive populations. By implementing comprehensive control strategies and fostering collaboration among stakeholders, it is possible to mitigate the damage caused by nutria and protect valuable wetland ecosystems.