An article in the journal Current Biology this week reported that biodiversity may not provide security for complex ecosystems such as coral reefs: one particular species can make or break its regeneration.

Working on a patch of Great Barrier Reef off the southern coast of Australia, Dr. David Bellwood simulated overfishing conditions by excluding big herbivorous fish from a quadrant of the reef. This induced algae to grow. Bellwood and his co-authors wanted to know which fish could rejuvenate the reef by eating the algae.

Bellwood says, “The assumption was that the fishes that prevent algae are the ones that remove it.”

But the researchers found something different. The 43 herbivorous fish that live on that part of the reef—including the parrotfishes and surgeonfishes that prevent the algae from taking over in the first place—played almost no role in cleaning the reef.

Bellwood compares the reef to your lawn. If you stop using a lawnmower, the grass grows up, along with shrubs and trees. And once the shrubs and trees move in, your lawnmower doesn’t work anymore. You need a chainsaw instead. The same is true for algae on reefs. The reef’s lawnmowers—in this case parrotfishes and surgeonfishes—can’t do much to save the reef once it’s overgrown.

The algae destruction was driven by a single species of batfish, Platax pinnatus—a double surprise for the researchers because batfishes were thought to dine mostly on invertebrates.

But the finding didn’t come easy. Bellwood only saw batfishes eat algae after he installed underwater cameras. “Batfishes are shy. If you sit in the water, they won’t perform,” Bellwood says. “As soon as you get out, the fish start doing their jobs.”