STEVE INSKEEP, host:

Scientists say the world's coral reefs are in danger. Some of the risks are obvious, like global warming, which results in overheated seas. Others are more subtle, as NPR's Richard Harris discovered when he visited a coral reef surrounding the Caribbean island of Bonaire.

RICHARD HARRIS: Looking off the coast of Bonaire, the glistening, aquamarine sea looks peaceful. But just under the waves, there's a war going on between the fragile and colorful corals and a creeping menace: algae. It's a turf battle for precious real estate. And it's a fight to the death.

Both corals and the algae have natural allies. Corals get help from parrot fish, which eat the intruding algae from the reef. And the algae get help from certain species of damselfish.

Dr. MARK VERMEIJ (Marine Biologist): Damselfish are these little fish, and what they do, they make little yards on the bottom of the reef.

HARRIS: Yards or gardens or pastures, marine biologist Mark Vermeij says call them what you want. These small fish are actually farming algae right on the coral.

Dr. VERMEIJ: And in order to make place for one of these yards, they basically kill the coral. So they go to the coral, they start sucking on little polyps until they die. And then when that happens, little algae establish in that died-off patch.

HARRIS: Once the algae starts growing, the damselfish becomes not just a farmer, but a warrior.

Dr. VERMEIJ: They're little feisty guys. I mean, there are rainbow parrot fish that are almost a meter long, and then this damselfish that's not much bigger than a goldfish just comes out. And this parrot fish is sort of aiming at feeding in the guy's little meadow. The little damselfish comes out and just scares that thing away. They're aggressive as hell.

HARRIS: And the more successful they are, the more the coral is killed off and replaced by their algae gardens.

Now, this drama has been going on since time immemorial. But now, welcome onto the scene hooks, lines and sinkers. People don't eat these little damselfish, but Vermeij says people do eat bigger fish on the reef. And that's where the story takes a twist.

Dr. VERMEIJ: Twenty years ago, life on these reefs for little fish was basically annoying, because there was all these big fish around that would eat you if you weren't paying attention. And then because a lot of people took the big fish out, it's a much safer environment for these little fish.

HARRIS: As a result, the damselfish population has boomed. As Mark Vermeij tells the story, Henry DeBey listens in. He's a graduate student at Yale University. He's here on Bonaire to get a better handle on the damselfish story. And he says it turns out that one predatory fish in particular goes after the damselfish. It's called a graysby.

Mr. HENRY DEBEY (Graduate Student, Yale University): Graysbys, which are a type of grouper, are a local favorite, a local specialty. So Bonarians like to fish graysbys.

HARRIS: As a result, graysby populations have been in severe decline along Bonaire's reef. DeBey and a fellow graduate student from Yale are here to quantify just how much graysby overfishing has led to a surge in damselfish and their algae gardens. They want to provide ironclad evidence that the marine park manager can use to give local fishermen a Hobson's choice.

Mr. DEBEY: Feel free to fish graysbys, but corals will decline as a result, indirectly. And everything else that depends on those corals, including the graysbys, ultimately, will also go down.

HARRIS: While algae form a natural ecosystem, it's not nearly as interesting and diverse as a coral reef. And it's the thriving coral reef that attracts divers to Bonaire from far and wide. They keep the local economy humming. So there's a lot at stake in the story of the little damselfish.

Richard Harris, NPR News.

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INSKEEP: Researchers want to make coral reefs more resilient to global warming. And you can hear that story later today on ALL THINGS CONSIDERED. And as you check headlines throughout the day at the new npr.org, you can also take a tour of a damselfish's algae garden.

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