When people talk about “keystone” species, they’re generally thinking about predators that shape the behavior of every other creature in their habitat, or about prey that serve as dinner for the entire neighborhood. But a new report on the collapse of coral reefs across the Caribbean is a reminder that entire ecosystems can depend on species that do little more than graze.
The report, Status and Trends of Caribbean Coral Reefs: 1970-2012, published by a consortium of global conservation groups, focuses on the 50 percent decline in Caribbean coral reefs over the past four decades.
It concludes that protecting and restoring populations of two competing grazers—parrotfish and sea urchins—could be the key to saving what’s left of one of the most beloved and economically important seascapes on the planet.
Other studies have generally assumed that climate change and coral bleaching were the major causes of coral reef decline—and they are clearly a part of the problem. But “this study brings some very encouraging news,” says Carl Gustaf Lundin of the International Union for Conservation of Nature.
“The fate of Caribbean corals is not beyond our control, and there are some very concrete steps that we can take to help them recover.”
Why on earth would a couple of humble grazers make such a big difference? In the past, relentless feeding by parrotfish and sea urchins on any form of plant life kept habitat open for corals and prevented algae from smothering them. “Perhaps the most striking aspect of plant life on a coral reef is the general lack of it,” marine biologist Sylvia Earle declared in a 1972 article about the Caribbean.
But Earle was describing what has become a “forgotten world,” according to the new report. That’s because a two-stage attack has dramatically altered the coral reef ecosystem. First, the uncontrolled human harvesting of parrotfish has driven this major coral reef grazer to the brink of extinction in many areas.
Nobody recognized the devastating effect of this loss at first. Then, in 1983, the second stage of the attack hit: An unidentified disease killed off 97 percent of the remaining major grazer, the sea urchins. (They have begun to recover, but a 2011 study in Puerto Rico found that sea urchin densities were still substantially below what they had been before 1983.)
Without these two main grazers, algae and large seaweeds proliferated—call it “the sliming of the Caribbean”—and corals declined.
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