The GBR is losing coral even faster than assumed

Elizabeth Selig and I took a lot of heat when we published a paper in 2007 (Bruno and Selig at PLoS One) in which we found the state of western Pacific reefs in general and the Great Barrier Reef (GBR) in particular was much worse than assumed:

The results of our analysis of 6001 quantitative reef surveys indicate that the degree, geographic extent, and duration of the Indo-Pacific coral decline have been significantly underestimated.

The general absence of quantitative data on reef health has led to several misconceptions about the causes, patterns, and best remedies for global coral decline. For example, in 2003, coral cover on the Great Barrier Reef (GBR), considered the ‘‘best managed’’ [25] and ‘‘one of the most ‘pristine’ coral reefs in the world’’ [21], was not significantly greater than on reefs in the Philippines and other subregions that are often thought to be highly threatened and poorly managed [13].

Several years later, my frequent collaborator Hugh Sweatman and some of his AIMS colleagues published a rebuttal of sorts, not just to us but also of Bellwood et al 2004, another paper that had found a striking loss of coral cover on what many believed was a reef that was resilient to human influence due to it’s great size, high species richness and relative isolation (most of the GBRs 3000 or so individual reefs are tens of miles off the coast of Australia).

Australia’s Great Barrier Reef (GBR) is unusual in that direct human pressures are low and the entire system of 2,900 reefs has been managed as a marine park since the 1980s. In spite of these advantages, standard annual surveys of a large number of reefs showed that from 1986 to 2004, average live coral cover across the GBR declined from 28 to 22%….much of the apparent long-term decrease results from combining data from selective, sparse, small-scale studies before 1986 with data from both small-scale studies and large-scale monitoring surveys after that date. The GBR has clearly been changed by human activities and live coral cover has declined overall, but losses of coral in the past 40–50 years have probably been overestimated.Sweatman et al 2011 (emphasis added)

I blogged about some of my disagreements with the Sweatman et al paper here and I collaborated with Terry Hughes and four other colleagues to formally respond in the literature.  Our rebuttal, Hughes et al 2011 PDF argued that:

Using monitoring data from manta tows along and across the Great Barrier Reef, Sweatman et al. (Coral Reefs 30:521–531, 2011 ) show that coral cover after these outbreaks declined further from 28 to 22% between 1986 and 2004. Pointing to the current levels of protection of theGreat Barrier Reef, they state that earlier estimates of losses of coral cover since the early 1960s have been exaggerated…While coral cover has increased for short periods on some individual reefs, it has declined sharply on many more to produce the observed system-wide trend of declining cover. Close to 40% of coral cover on inner reefs has been lost since 1986…This result, and other clear evidence of widespread incremental degradation from overfishing, pollution, and climate change, calls for action rather than complacency or denial.

Now, a new paper from AIMS scientists (De’ath et al 2012 in PNAS) including Dr Sweatman indicates the GBR is in fact rapidly losing coral, at least as fast as the rest of the world’s reefs.

…from 1985 to 2012, mean coral cover declined nonlinearly from 28.0% [95% confidence interval (CI) = (26.6, 29.4)] to 13.8% (95% CI = 12.4, 15.3) (Fig. 2A), a total decline of 14.2% (0.53% y-1). This is equivalent to a loss of 50.7% of the initial cover. Two-thirds of that decline has occurred since 1998, the current rate of decline is 1.51% y-1, and from 2006 to 2012, the rate of decline has consistently been >1.4% y-1 (Fig. 2A) – De’ath et al 2012

The analysis is based on reef surveys using the “manta tow” technique; snorkelers are towed behind a boat, over a reef and visually estimate coral cover within 10m wide swaths to categories such as 0-5%, >5-10%, 10-20%, 20-30%, etc.  I have used this data set in several papers, and although I have concerns about some of the inherent biases (e.g., I think it can underestimate coral cover), it is generally sound for this type of relative change analysis (*however, the end point values should be interpreted with caution).

Their reported decline rates are similar to values I and others have reported for other locations.  For example, we reported:

The annual loss based on repeated measures regression analysis of a subset of reefs that were monitored for multiple years from 1997 to 2004 was 0.72 % (n = 476 reefs, 95% CI: 0.36, 1.08).
Our results suggest that average Indo-Pacific coral cover declined from 42.5% during the early 1980s (95% CI: 39.3, 45.6, n= 154 reefs surveyed between 1980 and 1982) to 22.1% by 2003 (Fig. 3A); an average annual cover loss of approximately 1% or 1,500 km2. However, coral cover fluctuated somewhat throughout the 1980s and the regional average was still 36.1% in 1995 (95% CI: 34.2, 38.0, n =487), subsequently declining by 14% in just seven years (or 3,168 km2 year). – from Bruno and Selig

But it is important to note that our analysis was based on survey data only through 2005, so we don’t really know what coral loss rates are for the last seven or so years for the rest of the Pacific. And although the values in De’ath et al 2012 are higher (~ 1.5% a year) they may not be statistically so given the lack of precision inherent in the coral survey technique they employed .

I also suspect the authors may be putting too much confidence in their assignment and partitioning of the causes of loss, which is essentially determined for each reef/year combination using a best-educated guess approach.  The sites are only surveyed annually, so it can be difficult to reliably assign causality (although I don’t have any reason to dispute their findings per se).

But lets go back to the key finding:

from 1985 to 2012, mean coral cover declined nonlinearly from 28.0% to 13.8%”

This is an incredible if sobering finding. It suggests that the mean coral cover of the GBR is no higher than the Caribbean average* (which according to Schutte et al 2010, was 16.0 ± 0.4% (n = 1547) from 2001 to 2005 (although a more recent non-peer reviewed assessment suggests that the Caribbean has lost still more coral cover since then).

Importantly, the GBR had already lost coral and was far from pristine by 1985. We don’t know precisely what the natural baseline was for average coral cover, but the available science suggests roughly 50 to 60%.  This means the GBR has lost far more than half it’s coral.  A closer estimate would be 75%.

Overall, I am truly stunned by this new finding. The GBR, which only 10 years ago was considered “the world’s most pristine and resilient coral reef” is clearly no better off and no less threatened (and no more resilient) than any other reef.  I am bullish on the long-term survival of reefs, but science like this is challenging that outlook.

UPDATE: Nice to see the MSM covering this story so broadly, e.g., here and here.  I’ll be on the Weather Channel tomorrow at 8:20 AM talking about coral reefs and climate change.






One response to “The GBR is losing coral even faster than assumed”

  1. Sweatnam et al sounds like the standard complaint about inconvenient statistics.

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