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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.

Coral reef

Coral reef.

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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Your comments

Good for you Perry, My research led me to the same conclusion years ago but as you know it's not a popular conclusion and can insure that you do not obtain funding for further research. Your conclusions certainly counter the herd instinct of most biologists and NGOs that need to keep the myth alive to obtain tax exempt contributions.

After years of study (more than 40 years) doing coral reef drilling and seismic profiling we learned that the so called Florida reef tract has accumulated at most around 3ft of coral over the past 6,000 years. Except for isolated reefs (the ones that began on pre existing rock highs, have lighthouses, and are named on charts) these reefs amount to less than 2 percent of the Florida Reef Tract. The rest of the area fronting the Gulfstream lies under 20 to 35ft of water. Clearly the corals were not able to keep pace with rise in sea level over the past 6,000 to 7,000 years when there was no human activity.

The slowest growing of the corals could have easily kept pace with rising sea level if there had not been some natural influences that periodically knocked them back. We also published a paper in coral reefs that showed that Staghorn coral (probably the fasting growing coral) did not exist for about 500 years centered at 4,500 years ago. The same was true about 3,000 years ago. Trust me no biologist is going to listen to you. It would retard their ability to obtain research funds. Good luck. PS: I think you will enjoy this short video

Eugene A. Shinn, PhD., University of South Florida, St. Petersburg, Florida
Saturday, 12 March 2011 - 17:22

I found this article quiet interesting, and forgive me for my ignorance, but I was wondering if there was any correlation in coral reef decline in relation to how close to the surface the coral grew? As the coral gets closer to the surface, is it affected in any negative way from UV exposure?

Simon Cush, Southsea, United Kingdom
Monday, 14 March 2011 - 10:23

Hi Gene, Yes I'm aware of a lot of the Florida reef growth data of course... but you are right to flag it here. We'll see how the paper is more widely viewed in time I guess, but I think the key point is that all reefs exist at different points along their natural evolutionary (growth wise) trajectories. Hopefully the main message that some reefs are now sea level constrained and thus relatively senescent (and have been for several 1000 years in some cases) will come across to one and all... and as clearly as does the message that many reefs have been hammered by human activities in one way or another. I'd also like to think that studies like these show the key role of reef geoscience in providing a context for contemporary ecological and ecological timescale issues. Cheers. Chris

Chris Perry, Manchester Met Uni
Tuesday, 15 March 2011 - 21:34

Hi Simon ... a good question. The paper itself is really about larger scale reef growth and what happens when the whole structures reach sea level (and thus can't grow vertically anymore). Essentially a reef will run out of space to grow up into and are eventually (unless sea level rises) exposed on the low tides. This will lead to increased stress for the individual corals that still colonise that surface and eventually (in many cases) mortality.

So really it is a case of the systems (and the corals that build them) progressively shutting down. Under such conditions only an increase in sea level can provide new space for the reefs to renew vertical growth. A large number of reefs, especially across the Indo-Pacific region (where sea level has been close to its present level for several 1000 years in many cases) have these well developed, and more or less sea level constrained, upper reef surfaces - others are approaching such states. So, as with my response to Gene Shinn it is really a case that different reefs are at different points on their nature accretion pathways. Hope that answers the question. Chris

Chris Perry, Manchester Met Uni
Tuesday, 15 March 2011 - 21:46

Of course human activities don't cause every case of declining reef structures. Hurricanes, wave action, coral eaters ie crown of thorns starfish, and many other forms of bio erosion are just a few of the factors that can lead to certain reefs declining. So you state that humans aren't to blame in every case. I don't believe any scientists or ecologists has stated otherwise. What troubles me is that you are marine scientists, you should really watch what you say, because people might be listening. If a reef grows up to the surface, then it will die because of lack of space...okay. So tell me this because I am no phd, but if the corals stop growing then the reef will begin to erode. As the reef erodes there is more space for new colonies to populate the area. A healthy reef is always growing and always declining through bio erosion. This isn't the real issue though. The real question is what is your point? You can't really believe that humans don't impact coral reefs at all. So why are you minimizing the real issue? Take a look at a healthy reef system, say in Raja Ampat, Indonesia then take another look at the state of many other reef areas of the coral triangle and tell me that they are just growing old. You are missing the point, and as someone who cares deeply about the future of coral reefs, this makes me sick. I believe that you either help the cause or remain silent because policy makers only need the reassurances of a few careless scientist to justify continuing business as usual. I have no idea what positive goal this article can make besides stating the obvious. To someone who cares it really seems to imply that the massive decline of coral reef ecosystems isn't linked to human impacts. We both know what the truth is, but to someone who isn't as informed it is an invitation the go dynamite fishing in a cyanide leaking boat dragging anchor on the great barrier reef because it is just growing old.

Colby Deyoung, Indiana USA
Thursday, 31 March 2011 - 03:33