Invasive Species Thought Eliminated From California Shores Return in Force

In attempting to wipe out an empire of destructive crabs, scientists discovered that sometimes the empire strikes back.

A mass of European green crabs captured at Seadrift Lagoon in California. (Credit: Edwin Grosholz, UC Davis)

(CN) — At Stinson Beach in Northern California, with its impressive rock formations and nearby coastal redwoods, the invasive and aggressive European green crab threatens both the local wildlife and commercial fishing industry.

In 2009, researchers began their task of eradicating the crabs, one of the top 100 invasive species in the world, from Stinson Beach’s Seadrift Lagoon. At first blush, it seemed their efforts were successful. In four years, the species’ population declined from 125,000 to 10,000.

When scientists went back in 2014, they were befuddled by what they discovered. The population jumped to about 300,000, more than doubling in size. Other nearby lagoons which did not undergo eradication measures did not see the same growth in population. 

As a result of the attempt to wipe out the creatures, the species came back with a vengeance.

“A failure in science often leads to unexpected directions,” said lead author Edwin Grosholz, professor and ecologist with the UC Davis Department of Environmental Science and Policy. “We slapped our foreheads at the time, but with thought and understanding, it’s told us a lot about what we shouldn’t be doing and provided a way forward for us. The world should get less focused on total eradication and work toward functional eradication.” 

Grosholz and the team of researchers from University of California, Davis detailed their findings in a study published Monday in the journal Proceedings of the National Academy of Sciences.

In their investigation, researchers quickly discovered what led to the population boom. Many crustacean adults, including crabs, often eat their young. When the scientists began eradicating the adults, the younger crabs were able to thrive and quickly grew in number.

The study authors called this process the “hydra effect,” named after the nine-headed monster from Greek mythology that could regrow heads that were cut off. 

“Don’t try to get them all, or it could come back to bite you,” Grosholz said.

The researchers said their efforts will be used to refine eradication measures that can both reduce the population numbers of invasive species and do so in a way that ensures they won’t return.

The scientists recommend a “Goldilocks level” method, whereby measures are used to reduce the population of invasive species just enough to ensure the local ecosystem is protected.

“Instead of a one-size-fits-all approach, this study highlights the need to evaluate possible unintended consequences in selecting management strategies and tailoring these to the particular context and expected outcome,” said Greg Ruiz, co-author and marine biologist with the Smithsonian Environmental Research Center.

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