SMART THINKING

Sometimes targeting an invasive species for eradication can actually make its numbers explode.

Dubbed ‘the Hydra Effect’, sometimes trying to eradicate a species can backfire.

Some invasive species targeted for total eradication bounce back with a vengeance, a new study from the University of California has found. The ‘Hydra Effect’, named after a mythical Greek serpent that grew two new heads for each one that was removed, is the phenomenon of a population increasing in response to an increase in its per-capita mortality rate. The University of California study is the first to demonstrate a dramatic population increase in response to full eradication. The study chronicled the effort – and failure – to eradicate invasive European green crabs from a California estuary. The crab population actually increased 30-fold after about 90 per cent had been removed.

“A failure in science often leads to unexpected directions,” said lead author Edwin Grosholz, a professor and ecologist with the UC Davis Department of Environmental Science and Policy. “It 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.” The European green crab is native to the Atlantic Coast of Europe and northern Africa; however, the species was introduced to North America, Japan, South Africa and Australia. The European green crab is considered among the world’s top 100 invasive species, costing the commercial shellfish industry about $US20 million in annual losses as the crab is a voracious predator that feeds on shellfish. The study showed that in 2013 the population at the Californian estuary had decreased from 125,000 to fewer than 10,000 individuals. But one year later, in 2014, the population exploded to about 300,000 green crab in the lagoon – a 30-fold increase and nearly triple the pre-eradication population size. The scientists did not observe such population explosions of green crab at any of the four other nearby bays they were monitoring, suggesting the increase was the result of eradication efforts and not atmospheric or oceanographic changes.

“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, a co-author and marine biologist with the Smithsonian Environmental Research Center.