Smaller corals take the heat

New Australian research has found coral reefs will survive warmer ocean temperatures brought on by climate change - but they will be very different.

Professor Peter Mumby and Dr Laith Yakob from the University of Queensland report on their findings this week in the Proceedings of the National Academy of Sciences that small short lived corals which are taking over from large corals in some parts of the world are more resistant to disease.

For several decades, marine researchers have observed warmer sea temperatures devastate large, ancient corals such as staghorns (Acropora - at right) and boulder or dome corals (Montastraea), particularly in the Caribbean.

In many areas, these corals have been replaced by smaller, faster growing corals such as Porities and Agaricia.

"It's like having an oak tree forest replaced by a forest of scrubby young plants," says Mumby.

But a model developed by the two researchers, and based on a 10-year study in the Caribbean, has shown diseases would not spread as quickly or kill as extensively in the small fast lived corals in that area.

Why do small corals cope better with disease? Mumby says that for an outbreak to occur, a coral colony must survive for long enough to become infected and in turn infect other colonies. In short-lived colonies, the disease does not have enough time to spread.

The model used data on a series of outbreaks of 'white plague' in the Caribbean. This disease can kill in a matter of days as the infection creeps across the surface of the coral, destroying tissue as it goes.

The researchers emphasize that the research is only based on Caribbean corals. The small corals of the Indo-Pacific, including the Great Barrier Reef in Australia, have high rates of disease transmission and so the findings may not apply there.

"It almost sounds like a good news story but it isn't really," says Mumby. "Having reefs built by these small corals is not a good thing - they support less fish.

"We don't want reefs dominated by these corals. But we want to make the most accurate predictions we can so we can give policy makers the right advice on how things are going to change."

Commenting on the paper, Professor Bette Willis from James Cook University says large, complex corals provide better habitats for associated reef fish and invertebrates.

"We know that following bleaching, we see a dramatic decline in coral-associated fish and invertebrates that depend on large branching corals. They live among the branches and seek protection, so you need that three dimensional complexity for habitat diversity.

"One question is whether [new] coral assemblages will provide the same habitat for reef fish (right) and invertebrates. If small encrusting corals take over, the lack of habitat complexity means that we are likely to see very different coral reef communities. [They] will also have implications for other ecosystem services like coastline protection."

Mumby says their findings are a 'cautionary tale' to other scientists.

"We can't blindly apply what we know and extrapolate into the future. If we want to give realistic predictions and protect the benefits that people derive from those sorts of reefs, we have to have a good understanding of how those reefs are going to behave," he says. (Left: large reef fish)

"As we transform ecosystems through climate change, they become completely new and novel ecosystems. We can't apply the lessons of the past."

Source:
ABC Science, "Smaller corals take the heat", accessed January 19, 2011