What’s killing corals?
Scientists behind a new study into coral mortality suggest that research should focus on simple everyday processes as well as more dramatic events such as cyclones.
The new study suggests that although corals, that can build spectacular structures such as the Great Barrier Reef, can be killed in extreme ways, subtle everyday processes such as strong currents are important factors.
The research, published by Ecology Letters, was carried out by scientists at the University of St Andrews alongside fellow ecologists in Australia.
Following decades of study in which scientists tended to focus on extreme and rare causes of coral death, the new study instead looked at the effects of everyday currents, waves and wind. The findings suggest that these subtle and predictable reef processes ‘far outweighs’ mortality caused by more dramatic events.
Dr Maria Dornelas, a lecturer at the School of Biology at the University of St Andrews, worked alongside the Australian team in the study. Dr Dornelas is an expert on global biodiversity and tropical systems such as coral reefs.
She said, “For decades, researchers looking into coral mortality have concentrated efforts on extreme events such as tropical cyclones, thermal bleaching and outbreaks of the coral-eating crown-of-thorns starfish. However, although the effects of bleaching or cyclones are dramatic and worrying given climate change predictions, our study shows that day-to-day mortality should not be ignored.”
The project involved painstakingly tracking hundreds of individual coral colonies every year at Lizard Island on Australia’s Great Barrier Reef. The international effort was led by Dr Joshua Madin of Australia’s Macquarie University alongside researchers at James Cook University.
Dr Madin explained, “The most important finding was that physical forces have a strong signature on coral death during normal conditions. This “background” mortality, although traditionally understood to be important, has fallen off the radar in recent times due to a focus on occasional extreme disturbances, which tend to only affect local areas for short periods of time.
“We found that a coral’s physical strength is really important for surviving the rigours of living on shallow-water reefs. Strong currents and large waves occur quite frequently on the reef, not just during cyclones, and if you think about the enormous area of reef out there, death caused by water motion during the typical summer storms or other periods of strong winds and high waves, which occur on a regular basis, far outweighs death caused by very rare extreme events, like cyclones.”
The study’s other key finding is that corals with similar shapes tend to have similar chances of dying as they grow larger.
The authors say that the results greatly simplify their jobs as ecologists because they can make predictions based on a few simple and easily measured features of corals rather than have to get to know each species intimately.
It is hoped that the findings of the new study will help environmental planners work out better models for coral reef ecology.
Professor Sean Connolly of James Cook University added, “Targeted studies that track changes in the vital rates – birth, growth, and mortality – in corals is vitally important for anticipating how they will respond in the future to threats like local-scale pollution and large-scale climate change.”
The paper ‘Mechanical vulnerability explains size-dependent mortality of reef corals’ is published by Ecology Letters. It is available Open Access at: http://dx.doi.org/10.1111/ele.12306
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