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When an invasive species is good for conservation...

Procambarus clarkii, the highly invasive (and delicious) red swamp crawfish. Image credit, Mike Murphy.Conservation scientists and practitioners generally hold the assumption that all invasive species are bad for ecosystems and merit eradication. So, what do you do when a really bad invasive species happens to be good for the conservation of threatened birds?

This is the question that Zulima Tablada and fellow researchers tackle in the journal Conservation Biology. Their study focuses on the interesting case of the red swamp crawfish native to North America, which has invaded freshwater systems of Europe.

As a dominant ecosystem engineer, the red swamp crawfish grazes on macrophytes, often "shifting marshlands from macrophyte-dominated, clear-water systems to phytoplankton-driven, turbid systems." The species has emerged as the most common crawfish in the world, leading to the decline of native amphibians and invertebrates.

However the researchers found that in the biologically important Guadalquivir marshes of Spain, the crawfish has become an important food source for a number of birds possibly contributing to the rebound of numerous threatened species.

The scientists looked at the diets and population trends of predators and herbivores in the area. They found that the red swamp crawfish made up a substantial part of the diet of most of the predators. Furthermore, the abundance of predatory birds in the area was increasing compared to local herbivorous birds and avian predators from other parts of Europe.

"Guadalquivir marshes are inhabited by at least 10 predator species that are considered threatened in Europe and constitute the primary protected area in Europe for many of these species," the authors note.

Their research is groundbreaking because it represents possibly the "first study to show the positive effects of an otherwise detrimental invader on a community of predator species."

The research also raises a very challenging question: How do we manage for an invasive species when it has both very bad and very good ecological impacts?

The study authors point out that while the crawfish are benefiting predatory birds, they may also be simplifying the food web by condensing the trophic levels. Furthermore, anecdotal evidence suggests that crawfish invasion in Spain may have impacts on other regions by leading some European birds to overwinter in the Guadalquivir marshes instead of Africa.

Given the mix of both good and bad impacts and the fact that the crawfish would be nearly impossible to eradicate regardless, the authors recommend a middle-of-the-road approach that would focus on moderating the numbers of the invasive species rather than trying to eliminate it.

Since the crawfish is most abundant in the rice fields of the Guadalquivir marshes, the authors suggest that restoring some of these managed areas back to natural wetlands might be a good start. 

--Reviewed by Rob Goldstein

TABLADO, Z., TELLA, J., SÁNCHEZ-ZAPATA, J., & HIRALDO, F. (2010). The Paradox of the Long-Term Positive Effects of a North American Crayfish on a European Community of Predators Conservation Biology DOI: 10.1111/j.1523-1739.2010.01483.x

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