Biodiversity: Some reefs are better than others

Maintaining healthy populations of algae-eating species like parrotfish may be the key to helping coral reefs recover from disturbance. PHOTO BY LEONARD LOW, VIA THE CREATIVE COMMONS.

Scientists identify keys to coral reef protection and recovery after study dramatic recoveries in Moorea reefs

By Summit Voice

SUMMIT COUNTY — Scientists conducting a long-term study on the Pacific island of Moorea, in French Polynesia, say they’ve figured why coral reefs surrounding the island are able to discover from disturbances like cyclones and predators.

The key is maintaining a healthy population of algae-eating fish, not just in the primary reef system, but also in nursery reefs that harbor juvenile populations of the fish, including species like parrotfish and surgeonfish.

“We wanted to know why Moorea’s reefs seem to act differently than other reefs,” said Tom Adam, a postdoctoral fellow at University of Santa Barbara’s Marine Science Institute. “Specifically, we wanted to know what ecological factors might be responsible for the dramatic patterns of recovery observed in Moorea,” said Adam, lead author of the paper that was published in a recent issue of he journal PLoS ONE.

The UCSB research team is part of the Moorea Coral Reef Long-Term Ecological Research project, funded by the National Science Foundation.

Coral reefs worldwide are increasingly disturbed by environmental events that are causing their decline, yet some coral reefs recover. But in the case of severely damaged reefs in the Caribbean, coral reefs that suffer large losses of live coral often become overgrown with algae and never return to a state where the reefs are again largely covered by live coral.

A major reason the reefs in the Caribbean do not recover after serious disturbances is because these reefs lack healthy populations of parrotfishes and surgeonfishes, due to the effects of overfishing, Adam said.

“Without these species to help crop the algae down, these reefs quickly become overgrown with algae, a situation that makes it very hard for corals to re-establish themselves,” he said.

In contrast, the reefs surrounding Moorea experienced large losses of live coral in the past –– most recently in the early 1980’s –– and have returned each time to a system dominated by healthy, live corals.

The research team was surprised by its findings. The biomass of herbivores on the reef — fish and other animals that eat plants like algae — increased dramatically following the loss of live coral.

“What was surprising to us was that the numbers of these species also increased dramatically,” said Andrew Brooks, co-author and deputy program director of the long-term reef study center. “We were not simply seeing a case of bigger, fatter fishes, We were seeing many more parrotfishes and surgeonfishes, all of whom happened to be bigger and fatter. We wanted to know where these new fishes were coming from.”

The researchers also found that not all of the coral reefs around Moorea were affected equally by an outbreak of predatory crown-of-thorns sea stars or by cyclones. The crown-of-thorns sea stars did eat virtually all of the live coral on the barrier reef — the reef that separates the shallow lagoons from the deeper ocean. However, neither the sea stars nor the cyclones had much impact on the corals growing on the fringing reef — the reef that grows against the island.

“We discovered that these fringing reefs act as a nursery ground for baby fishes, most notably herbivorous fishes,” said Brooks. “With more food available in the form of algae, the survivorship of these baby parrotfishes and surgeonfishes increased, providing more individuals to help control the algae on the fore reef.

In effect, the large numbers of parrotfishes and surgeonfishes are acting like thousands of fishy lawnmowers, keeping the algae cropped down to levels low enough that there is still space for new baby corals to settle onto the reef and begin to grow.”

Managers have tried to reverse the trend of overfishing through the creation of Marine Protected Areas, where fishing is severely restricted or prohibited.

“Our results suggest that this strategy may not be enough to reverse the trend of coral reefs becoming algal reefs,” said Brooks. “Our new and very novel results suggest that it also is vital to protect the fringing reefs that serve as nursery grounds. Without these nursery grounds, populations of parrotfishes and surgeonfishes can’t respond to increasing amounts of algae on the reefs by outputting more baby herbivores.”

In short, the research team found that by using MPAs, managers can help protect adult fish, producing bigger, fatter fish. “But if you don’t protect the nursery habitat — the babies produced by these bigger fish, or by fish in other, nearby areas — you can’t increase the overall numbers of the important algae-eating fish on the reef,” said Brooks.

According to the scientists, it appears that Moorea’s reefs may recover. “One final bit of good news is that we are seeing tens of thousands of baby corals, some less than a half-inch in diameter, on the fore reefs surrounding Moorea,” said Brooks.

MCR researchers will follow the coral reef recovery process over the next decade or more, in search of additional information that can aid managers of the world’s coral reefs.

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One thought on “Biodiversity: Some reefs are better than others

  1. So, with all the changes taking place on this earth, man made or natural, it’s enlightening to know, that regardless of the changes taking place, there are still areas of growth that are surviving any adverse environmental detriments, imposed upon them. Things we wouldn’t know about, if the science was stuck back in the 20th century mindset.

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