The Case of the Ravenous Rodent Invasion

The misplaced

Nutria (or copyu as it is known in some countries) is a type of rodent.
It is native to temperate zone coastal lakes and wetlands of Argentina, Bolivia, Brazil, Chile, Paraguay and Uruguay.

Appearance

Nutria adults are about 50 to 60 cm long (excluding tail) and have thick fur all over their bodies (once again excluding the tail) with white bristles about the muzzle. They are often confused with beavers but 2 anatomical features stand out to distinguish them.

1. bright orange incisor teeth protruding from the front of the upper and lower jaws
2. the tail is long and cylindrical and  thinly covered in bristly hairs

They are excellent swimmers, and seldom move far from a body of water. Adaptions for their semi-aquatic life style include partially webbed back feet and valves that can seal off water entry to their nostrils and mouths while under water. Female nutria even have their teats located so they can suckle their offspring while floating.

Reproduction and Longevity

Interactions and ecology

Nutria are creatures of wetlands and swamps and are well adapted to fitting into these ecosystems.

Nutria are herbivorous and will eat almost all types of terrestrial and aquatic vegetation  available to them. They are voracious feeders and eat 25% of their body weight daily.

Alligators prey upon adult nutria. Turtles, gar fish, snakes, birds of prey and other carnivorous swamp animals may eat juvenile nutria. It is thought the mortality rate of young nutria is high.

In the areas in which nutria are native,  numbers are kept in check by weather patterns of droughts and floods. This means that the population fluctuates in size, but never gets completely out of control in terms of what the ecosystem can support.

The displaced

In terms of displacement by direct competition, nutria are outcompeting muskrats and to some extent beavers, as they have similar dietary needs. In the wider context, nutria disrupt the entire invaded ecosystem by removal of vegetation and extensive digging and tunneling which alters the landscape.

The consequences

Where is the invasion?

Damage done

As can bee seen from the map, nutria have spread to different parts of the world. The USA has the best available documentation of their consequences of nutria invasion, especially in states Louisiana and Washington e.g.
In Louisiana

At high densities and under certain adverse environmental conditions, foraging nutria can significantly impact natural plant communities. In Louisiana, nutria often feed on seedling baldcypress and can cause the complete failure of planted or naturally-regenerated stands. Overutilization of emergent marsh plants can damage stands of desirable vegetation used by other wildlife species and aggravate coastal erosion problems by destroying vegetation that holds marsh soils together. Nutria are fond of grassy arrowhead (Sagittaria platyphylla) tubers and may destroy stands propagated as food for waterfowl in artificial impoundments.¹

In Washington

Nutria damage is related to burrowing and feeding. Nutria construct burrows in the banks of rivers, sloughs, and ponds, sometimes causing considerable erosion. Burrows can weaken roadbeds, stream banks, dams, and dikes, which may collapse when the soil is saturated by rain or high water. Rain action can wash out and enlarge collapsed burrows and compounds the damage.
Nutria numbers may increase to the point where an area is denuded of aquatic vegetation. After foraging on entire plants, including the roots, they leave the area pitted with digging sites and deep swimming canals. This feeding behavior can destroy existing root mats that bind and secure a wetland together, and the area can be quickly eroded by wind and wave action. In parts of southern Washington, nutria may be are out-competing muskrats for food and places to live.²

Note that in all places where nutria have invaded, they have brought with them a number of parasites and diseases which may infect other organisms.

Nutria control measures

In areas where nutria have achieved a high density and have done serious damage , measures taken to control them include

• trapping and killing
• trapping for human consumption
• fencing out
• harassment
• repellents
• encouraging hunters to collect pelts via incentives
• slope management

Trading Places: Did you know that the US imported nutria from South America to replace beaver pelts?
The United States exported beavers to South America’s tip, Tierra del Fuego, where they have become a major ecologic and economic nuisance.

Dig Deeper

• Nutria Population Dynamics – A Timeline
• Hurricanes, Floods, Levees, and Nutria: Vegetation Responses to Interacting Disturbance and Fertility Regimes with Implications for Coastal Wetland Restoration