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.
The new report is the work of 90 experts who spent three years analyzing more than 35,000 surveys—including studies of corals, seaweeds, grazing sea urchins, and fish—conducted since 1970 at 90 locations around the Caribbean. Past studies of coral reef decline have generally looked only at the corals themselves. They also tended to lump together data from shallow lagoons and deep-sea reefs. The new report aimed instead to parse out the effects in distinct reef locations and consider the effects of multiple species.
To the surprise of the researchers, the healthiest remaining coral reefs turned up where there were still vigorous populations of grazing parrotfish. Those are mainly areas—like Bermuda, Bonaire, and the U.S. Flower Garden Banks National Marine Sanctuary—where governments have banned or restricted fishing practices that harm parrotfish, including spearfishing and the use of traps. Areas that failed to protect parrotfish—including Jamaica, the U.S. Virgin Islands, and the entire Florida Reef Tract from Miami to Key West—suffered severe coral reef losses.
The report proposes a series of actions to slow or stop coral reef decline. They come down to a single basic idea: Bring back the grazers. But the Caribbean coral reefs span 38 separate and more or less quarrelsome countries, and getting them to act in unison will require a major effort. To encourage action, the report points out that coral reefs generate more than $3 billion a year for local economies from tourism and fisheries, and more than a hundred times that amount in other goods and services.
Maybe, though, acting in unison isn’t what it will take to deslime the Caribbean. The forward-thinking countries may just recognize that the coral reefs—and their grazers—are key to their economies and take action on their own. Barbuda, for instance, is now considering a ban on all harvesting of parrotfish and sea urchins, with one-third of its waters to be set aside as marine reserves.
Countries that act to restore their grazers are likelier to end up with the coral reefs, the tourists, and the money. But what about the other countries?
They’ll have the slime.