Human activity can't always be blamed for coral reef decline
10 March 2011, by Tamera Jones
The decline of coral reefs over the last few decades is often squarely blamed on human activity. But a recent study suggests the picture is in fact a little more complex.
Researchers have found that our actions aren't always responsible for the decline of coral reefs; some reefs stop growing simply because they've reached the end of their natural lifecycle.
The UK and Australian researchers found that in some parts of Australia's Great Barrier Reef, different reefs have grown at varying rates and times over the last 10,000 years. Some essentially stopped growing without any kind of human influence.
'There's an assumption that degraded reef states are a function of environmental stress, linked to anthropogenic activity,' says Professor Chris Perry, an expert in coral reefs at Manchester Metropolitan University, who led the latest study, published in Global Change Biology.
'They might look a bit worse for wear, but this is normal; you'd expect them to look like that.'
Professor Chris Perry, Manchester Metropolitan University
'Whilst this is often the case, the picture is not as simple as that. Some degradation is natural and not the result of anthropogenic stress. It may simply be that a reef has gone through its natural evolutionally life cycle and then shutdown.'
Perry and Scott Smithers from James Cook University in Australia found that some reefs within the nearshore parts of the Great Barrier Reef stopped growing between 4,000 and 5,000 years ago. But in other places, new reefs have established themselves only in the last 1,000 to 2,000 years.
'These older reefs are now in a state of 'senility'. If you scratch beneath the surface, you find these dead foundations,' explains Perry.
These old coral reef structures are unlikely to grow any further and may have less actual living coral cover, which many conservationists could easily see as a sign of poor coral reef health, caused mainly by human activities.
'Some of these reefs clearly have degraded surfaces and patchy veneers of living coral,' explains Perry. 'They might look a bit worse for wear, but this is normal; you'd expect them to look like that.'
In many areas of the world, reef degradation is clearly down to human activities like over-fishing, pollution and rising sea temperatures. In the Caribbean, 80 per cent of reefs have declined since the mid-1970s and this is almost certainly down to pollution and over-fishing.
But this latest study reveals that, in some cases, it's normal for reefs to decline.
The team's findings mean that, 'conservationists should start to include considerations of the evolutionary state of a reef and its age, as well as focusing efforts on younger and actively growing reefs that can harbour a wider range of habitat types,' says Perry.
Coral reefs are made up of thousands of individual coral colonies, which can live from just a few years to hundreds of years. When they die, their skeletons are left behind. New corals then start colonising these dead structures, slowly building up a reef structure over time.
Two main things can slow their growth - lack of sunlight penetrating the water and lack of space. The shallow and muddy waters of the inner-shelf of the Great Barrier Reef mean that the space into which reefs can grow is relatively limited. This means they can pass through their different evolutionary stage rapidly - young to mature to senile in only a couple of 1,000 years.
'All reefs have a finite timescale within which they can grow under the prevailing sea level,' says Perry.
'Younger reefs may support a greater range of habitat types and experience faster rates of growth. We need to think about reef age and evolutionary state when thinking about reef health,' he adds.
CHRIS T. PERRY, SCOTT G. SMITHERS, Cycles of coral reef 'turn-on', rapid growth and 'turn-off' over the past 8500 years: a context for understanding modern ecological states and trajectories, Global Change Biology Volume 17, Issue 1, pages 76-86, January 2011, DOI: 10.1111/j.1365-2486.2010.02181.x
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