Study Also Informs Eradication Efforts, Offers Toolkit for Future Research Worldwide
Scientists with the Wildlife Genetics Research Unit at the California Department of Fish and Wildlife (CDFW) have completed a genomic study of nutria, an invasive South American rodent, linking their 2017 discovery in Merced County to a population in central Oregon. This was the first genomic study of the semi-aquatic pest species that is harmful to wetland habitats and agriculture and has challenged control efforts of land and wildlife managers worldwide for decades.
To identify the origins of nutria in California, CDFW research scientists compared DNA of invasive nutria populations around the world. This led to building a dataset showing the genetic similarity among nutria on a global scale that will serve as a toolkit for future research. The genetic data also give insight into how nutria populations spread across a landscape — information that will inform eradication efforts and development of management strategies.
“CDFW conducts sophisticated genetics research to improve management outcomes for California’s wildlife populations,” said CDFW Wildlife Genetics Research Lead Mike Buchalski. “The results of this study additionally will benefit future national and global genetic research on nutria.”
“Genetics allowed us to narrow down the most likely source of California’s nutria population and provide land and wildlife managers with information about how these animals are spreading across the landscape,” said CDFW Research Scientist Kristen Ahrens. “We have also created a global reference dataset that others can use to compare populations and trace invasion pathways.”
Nutria were introduced to countries all over the world to be raised for their pelts at the height of the fur trade in the late 19th and early 20th centuries, including in California. When that trade collapsed nutria were set free from fur farms, leading to a limited number of feral populations on the landscape, which seemingly failed to thrive. Localized nutria eradication efforts followed, and they were ultimately declared eradicated from California by the late 1970s.
There was no sign of nutria presence in the state until 2017 when a pregnant female was found in Merced County. Unlikely that nutria could have gone undetected since eradication in the 1970s, particularly given the rapid rate of reproduction and geographic spread observed following their discovery, CDFW went to work to assess genetic variation in past and present nutria populations in California to find the origin of this recent discovery.
To compare today’s nutria with those present before eradication, historical genetic data were essential. CDFW’s Ahrens located and sampled nutria skins and skulls housed in museums and universities across California, and one unexpected source in a taxidermy nutria found at the Butte County Weights and Measures Department office. Recovering DNA from decades-old specimens allowed direct comparison of past and present maternal lineages, providing critical insight into the origins of the current invasion.
CDFW’s Nutria Eradication Program team provided tissue samples for DNA sequencing of California’s current nutria population.
For broader comparison, Ahrens obtained tissue samples from federal and state wildlife management agencies for other nutria populations in the U.S. — Louisiana, Texas, Maryland, Virginia, Oregon and Washington. Ahrens was also able to tap into publicly archived sequenced DNA from invasive nutria in South Korea, Japan and Brazil.
Comparisons showed that nutria in California today are most genetically similar to populations in central Oregon. Historical California nutria represented multiple maternal lineages, based on analysis of the museum bone and taxidermy specimens. In contrast, today’s population traces back to a single lineage, also detected in historical samples, a pattern most consistent with a recent reintroduction rather than survival of the earlier, more diverse population.
Ultimately, the genetic research suggests California’s current nutria invasion was the result of intentional reintroduction, with the original source animals likely being transported from the central Oregon population, which is prohibited by Fish and Game Code section 2118 and Title 14, California Code of Regulations section 671. Eradication efforts are now costing the state about $5 million per year.
“This study supports our long-held belief that the current invasion is the result of reintroduction rather than explosive growth of a remnant, undetected population,” said Nutria Eradication Program Manager Valerie Cook. “While we can only speculate on the ‘reasoning’ behind the reintroduction of this incredibly destructive invasive species, these findings highlight the critical importance of biosecurity inspections aimed at preventing the importation and transportation of invasive plants and animals into and within California.”
CDFW’s Nutria Eradication Program has taken 7,841 nutria in California since 2017. Eradication efforts are underway across hundreds of thousands of acres of wetland and riparian habitat in California’s Sacramento-San Joaquin Delta and Central Valley, spanning from San Pablo Bay to Kings County. Visual observation surveys, motion-detection cameras and scat-detection dogs help CDFW’s eradication team detect nutria presence for subsequent trapping efforts.
Findings of the nutria genomic study will be another valuable tool for the CDFW Nutria Eradication Program. This study demonstrates how genetics can shed light on population structure and patterns of gene flow to better understand how nutria disperse across a landscape.
The nutria genomic research took place over the past five years and was funded by the California Department of Water Resources. Research was done in partnership with the U.S. Department of Agriculture’s Animal and Plant Health Inspection Service.
The recently published paper on the study, Population Genomic Insights Into Recent Nutria Invasion Dynamics, is available online.
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Media Contacts
Michael Buchalski, CDFW Wildlife Health Laboratory, (916) 508-1385
Krysten Kellum, CDFW Communications, (916) 825-7120