Invasive Species: Nutria and their Impacts

Nutria are a large bodied semi-aquatic rodent that are an invasive species in North America. Originally brought to the United States as part of the fur trade they escaped or were released when demand diminished. They are highly reproductive animals, with females in a near constant state of either pregnancy, lactation, or both. Nutria can inhabit a range of wetlands and marshes, which they decimate through over grazing and burrowing. They were thought to be well controlled in the California central valley until recently, when they were found in both Stanislaus and Tuolumne Counties. Most of the information about invasive nutria has been compiled in the southern region of the country, but still holds valuable insights for California. Increases in population size threaten our economy through crop loss and our very way of life through damage to our fragile water delivery and storage system. Nutria have been observed to take advantage of polluted dairy ponds as habitat, of which the central valley has many. The eradication of nutria should be an immediate priority if we are to win the war against them, if we even can. In this review, I will analyze the findings that conclude that Nutria are a danger to valuable natural and agricultural resources in the California Central Valley.
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Nutria (Myocastor coypus) are a large rodent native to South America, particularly 23º south of the equator (Borgnia, 2000). Nutria are generally smaller than a beaver (Castor canadensis), but larger than our native muskrat (Ondatra zibethicus). Like the muskrat, they are semi-aquatic, known to inhabit riverbanks and marsh land (Brown, 1975). They are herbivorous grazers and can decimate biomass in their ranges due to their voracious appetites (Shaffer, 2015). These appetites cause them to be a threat to crops grown in these areas (Carter, 2002) They also burrow into river banks, levees and other water control measures. This causes economic loss when fields flood or livestock raised in pools escape (Dixon, 1979). Brown documented that they were out-competing native wildlife for food sources (1975). The presence of more invasive species would be devastating to biological diversity in the California Central Valley (Close, 1998).
Eradication of nutria is nearly impossible once there is an established population (Ehrlich, 1962). I have found only one case of total eradication. Ireland managed to totally eradicate a small population of escaped fur farm nutria, but this was done before there was a well-established population. (Where there’s a Will there’s a Way, 1999) The only proposed method for eradication is a combination of hunting and trapping until absolutely no nutria remain (Pepper, 2005). A more common method of attacking a nutria invasion is controlling their spread. This is done by careful monitoring of the populations, culling and constructing barriers to dispersal (Martino, 2014). Challenges to full eradication include high reproductive rates, low habitat specificity requirements and the large network of rivers, canals and other water system components that afford the nutria travel pathways (Hong, 2014). Nutria often go undetected by humans until it is too late. In the 1950s we were convinced that nutria could help us control aquatic weeds, and many were released into southern wetlands as the fur trade dwindled during world war two (Dixon, 1979). The California Central Valley was once very marshy, to farm the area many flood control levees and irrigation canals were put in place. Some areas have remained in their more natural state (Close, 1998). Our alterations may have made it easier for the nutria to invade further into our communities and lives.
In this review article I will address the habitat selection, their effects on agriculture and marshland habitat and how they might be eradicated or at the least the damage from their activities can be mitigated. Scientists have determined nutria are a highly reproductively active species. Scientists have also studied how they impact the habitats which they invade in comparison to their native habitats in South America.
Nutria are semi-aquatic and prefer marshy areas where food is plentiful and good burrowing sites are present (Hong, 2014). They inhabit both stagnant water and areas which experience slow flow (Nolfo-Clements, 2012). They disperse along rivers and tributaries, exploiting the natural ponding and native plants along the banks (Hong, 2014). They prefer areas which have a natural sheltering area at the edge of the water and sand dunes are a plus (Hong, 2014). A riparian buffering zone between the river and farmland or human presence is also strongly correlated with nutria presence (Hong, 2014). The strongest factor in determining whether a habitat is suitable for nutria is the total number of days which average below -4ºC. Unfortunately, Sacramento, which is in the California Central valley, has record lows of around 3ºC (Sacramento, 2005) which is not cold enough to dissuade nutria from becoming residents (Hong, 2014) In the California Central Valley we have the Sacramento river, the Stanislaus river, the San Joaquin river as well as the Tuolumne river. Each of these rivers have tributaries which could play host to an invasive population of nutria. Large portions of the rivers are lined with a small strip of riparian border and have plentiful vegetation growth along the banks. There are also wildlife refuges in the California Central Valley that would make excellent habitat for invasive nutria. The same habitat required by our native, and sometimes endangered, species is also usable by nutria. Because nutria have been observed by Klima (2012) to use heavily polluted dairy runoff ponds, water quality appears to be insignificant in choosing a habitat. Much of the California Central Valley has sandy loam soil which is easy to burrow into and holds its form well when moist, as would be the case near a river (McElhiney, 1992). Nutria are a flexible species in their habitat selection, but the California Central Valley certainly isn’t a difficult place for them to invade and exploit.
Nutria feed constantly and voraciously, decimating both above and below ground biomass with their appetites (Shaffer, 2015). They were so good at grazing down plant life that some states intentionally released nutria to control aquatic weeds (Dixon, 1979). This ended poorly, with the establishment of feral populations which did not eat the target species of plant, instead eating desirable native plants (Dixon, 1979). Burrowing behavior damages levees, canals and eats away at natural river banks (Nolfo-Clements, 2012). This makes the nutria a danger to irrigation systems and flood control measures. In the California Central Valley there is intentional flooding of some areas to grow rice in the same way that areas in Japan are flooded to farm shrimp (Kawamura, 2018). If nutria were to invade rice growing regions of the California Central Valley they could eat young rice crops and drain the fields of water by damaging levees. This presents an economic challenge to farmers in the region. They are also proven to eat agricultural crops adjacent to rivers (Hong, 2014), in the California Central valley these could include alfalfa, tomatoes, soybeans, melons of various types, and potatoes. In the San Juaquin delta there are large levees constructed to prevent homes and businesses from flooding. If nutria were to burrow into these levees, weakening them, there could be catastrophic results.
Eradication of nutria is difficult due to their high fertility and large litter size (Brown, 1975). It does not take long for a population to become established. Unless a population is very isolated and systematically eradicated the chances of complete removal are small (McFalls, 2010). Efforts to eradicate nutria must start with identifying where the animals have infiltrated, and any paths or feeding locations that they frequent (Pepper, 2005). One method of doing this is to place platforms on land where nutria is suspected as a pest, (Pepper, 2005), these platforms should be equipped with hair snares, which allow you to be sure nutria have visited. Once it is known that a population of nutria is present eradication must be swift and thorough. One major obstacle to eradication is a lack of funding (Carter, 2002), this leads to a lack of hunters and a lack of equipment. Another obstacle is overly optimistic timelines, where an open-ended model would be better suited to the task (Brown, 1975). A combination of hunting and trapping is the most effective (Carter, 2002). These methods must be continued until no nutria remain because even one pregnant female is enough to continue the population. Eradication has been deemed highly unlikely for most, if not all, populations in the USA (Pepper, 2005).
Our future in the California Central Valley most likely involves nutria as our invasive neighbors. Because complete eradication relies on isolated populations and continuous removal it is unlikely that we will ever completely remove this pest from our lives. What we can do is control them, preventing their spread through constant vigilance. We should aim to kill any that we see, particularly if they are found in ecologically or economically sensitive areas, i.e. wildlife refuges or flood control levees. If we can successfully isolate the current populations, we may be able to institute heavy trapping and hunting regimes to control the population size. We may also be able to decrease genetic diversity enough to utilize a biological control. Another possibility is the use of birth control injections as has been done with other wild animals which were overpopulating their ranges. One such use of birth control is on the island of Assateague, made famous in a children’s chapter book for their marsh ponies. This would involve humane live trapping the nutria, tagging them, and administering an injection once per year to prevent conception. Female hormonal birth control has the benefit of not changing the social dynamics of a group, while stymying population growth. This method would also allow for further study of the nutria here in California, as animals could be weighed, measured and aged at the same time as they received their injection. It is not a perfect method, as not all female nutria are guaranteed to be trapped. However, if repeated several times per year for 2-3 years and yearly thereafter it could significantly diminish our population of invasive nutria.

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