When a coral reef dies without a lot of plant-eating fish around to save it, there is usually one direction it tends to move toward: total annihilation.
Coral reefs can begin to die for a lot of reasons. Low oxygen level in the water is one example. When coral polyps are ailing, they tend to expel the symbiotic single-celled plants called zooxanthellae that give corals their color and the nutrients they need to survive. The corals take on a white, bleached appearance. If they don't manage to get their zooxanthellae healthy within a few weeks, the corals will usually die.
This is part of the problem that has notoriously threatened Australia's Great Barrier Reef.
A dead, bleached reef can come back to life as the skeleton of its remains becomes a substrate for new baby corals to grow on. But the reef's new enemy will be the growth of blanketing algae that prevents new, living coral from growing. Overfishing by humans tends to reduce the number of big, herbivorous fish that would otherwise eat the algae and give the reef a chance at rebirth.
Andrew Altieri, a staff scientist, was doing research at Bocas del Toro on the Atlantic coast of Panama when he noticed something strange on a reef that had experienced a mass die-off five years earlier. “We saw widespread coral mortality,” Altieri says. “The expectation was that it's going to turn into a big algal garden. Here it is a few years later and I'm with my students and we're looking around like, 'where's all the algae?'”
“The thought has always been that there's two dominant grazers,” Altieri says. “Diadema [a widely-distributed genus of sea urchins] and the parrotfish. But there was a history of overfishing on the reefs” and the larger species of parrotfish that ate the algae disappeared. To make matters worse, disease had decimated a population of large sea urchins, Diadema antillarum—meaning there were no obvious creatures to keep algae growth under control.
Altieri and Caitlin D. Kuempel, a PhD fellow from the University of Queensland in Australia, set out to find out why the bleached coral hadn't been covered in algae. The missing piece of the puzzle turned out to be creatures so small that they had been overlooked in the past. Sea urchins the size of ping-pong balls (Echinometra viridis) and a diminutive species of parrotfish (Scarus iseri) were grazing algae on the reef. The finger-sized parrotfish was thriving because it was too small for local fishermen to successfully spear—let alone bother to eat. “But when you add up the biomass, there's quite a lot," says Altieri.
The ability of these tiny grazers to step in and fill the role of their larger relatives is somewhat analogous to taking bison off of a prairie, only to learn that groundhogs were able to maintain it.
The fossil record of past coral reef communities in the Caribbean is not well understood. But some existing research suggests that small species of sea urchins may have become ecologically important in the past, taking on the key role of algae clean-up in the absence of larger grazers. These diminutive grazers might be part of a historical redundancy in the evolution of the region's coral reefs.
“There's starting to be some paleontological work examining urchin dynamics in the Caribbean,” says Altieri. “It appears that in some cases these smaller urchin species might have been important in the past.”
It isn't clear yet whether the coral reef will fully return to life. But now we know that it has the opportunity. New coral polyps take years to establish themselves and become easily observed. They may already be growing. Ongoing research at the Smithsonian Tropical Research Institute will monitor the reef for new corals. Conditions since the big die-off five years ago seem to be right for new growth.
“In general on the reefs there are two approaches to active restoration,” says Altieri. “These hatcheries, or grow-out farms, where they grow corals up to size and then literally glue them to the reef. And the other approach is to move an artificial structure and wait for that structure to be colonized. These approaches are time consuming and costly, so that's not something that we've initiated on these particular reefs.”
Altieri and other scientists at the Smithsonian Tropical Research Institute are waiting for the corals to re-colonize the reef naturally. He expects to know whether this is happening by the end of 2017.
The research gives some new hope to the well-known plight of The Great Barrier Reef, which has recently entered the bleaching phase of a die-off. While the exact mix of species on the Great Barrier Reef is different, the ecological principle of tiny herbivores eventually expanding their population enough to do the job of their larger cousins could potentially be repeated.