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Brown University Galápagos study identifies keystone predator in a complex food web

Despite popular metaphors and cartoons depicting straightforward “food chains,” ecologists such as Brown University Professor Jon Witman typically doubt that they’ll see predators in diverse tropical ecosystems have meaningful impacts on species even just two links down the line. But after six years of meticulous experimentation and observation off the coast of the famed Galápagos Islands, Witman and two colleagues have amassed direct evidence that just such an important “trophic cascade” is happening there.

The researchers report in the journal PLOS ONE that two species of triggerfish are overcoming consistent pestering by sharks, sea lions and especially hogfish to gobble up enough pencil urchins to reduce the urchins’ consumption of algae. Determining such interactions matters, Witman and his co-authors wrote, because while it is clear that humans disrupt the normal functioning of tropical ecosystems across the world, it is often unclear exactly how. Understanding when and how trophic cascades occur, and who is involved, is the only way to prevent or fix such problems.

As human exploitation is depleting large predators in the ocean, there is a growing appreciation that many predators have important indirect effects on species lower in the food web,” wrote Witman and co-authors Franz Smith of Brown and Mark Novak of Oregon State University.

In the Galápagos, which remain relatively untouched, the new study provides a clear example of a mid-level predator that’s proving crucial to the coastal ecosystems’ vitality.


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