On the Decline of Coral Reef Ecosystems (II): John Bruno Responds

Coral White Syndrome

As the lead author of the recent articles on coral disease outbreaks on the GBR and Indo-Pacific reef decline, Dr John Bruno raises some valid points regarding the recent critique on the decline of GBR corals by Peter Ridd:

Ove, thanks for posting Peter Ridd’s critique of Pandolfi et al. 2003. I’d be interested in hearing from Peter about whether he had a hard time getting this published and what type of reaction he got from reviewers at journals like Science or Coral Reefs. Although I agreed with some of his specific criticisms of the methodology used by Pandolfi et al., Peter’s overall arguments about the status of the GBR are demonstrably incorrect. Below I briefly comment on some his main problems (in caps) and points (in quotes) and correct some of his major errors about the state of reef-building corals on the GBR. PROBLEM 1: EFFECT OF WEIGHTING OF THE GUILDS
“The problem with equal weighting of the guilds is that the fundamental importance of corals to coral reef ecosystems is not adequately recognized.”I totally agree. This seems like a major flaw. But I imagine that the authors would respond that their intent was to quantify the status of entire reef ecosystems, including food web structure and trophic dynamics. I certainly agree that a reef without large vertebrates is aesthetically and ecologically impoverished. But it is hard to ignore the fact that without corals you have no ecosystem but without top predators you just have an altered and possibly “degraded” ecosystem.As Peter (sorry for the informality, but the Dr. Ridd said, Dr. Bruno replied thing feels a bit stuffy to me in a blog discussion) succinctly states, “coral reefs cannot exist without reef-building corals.” A seemingly obvious point that is sometimes lost.“Because the guilds and species that are subject to human exploitation (e.g. large herbivores) are often in worse state than corals”
As I discuss below, this is not the case due to the indirect and often long distance effects of humans on corals.

PROBLEM 3: FAILURE TO APPRECIATE THAT THE GBR NEVER EXISTED IN PRE-HUMAN TIMESThis is a good point that I have heard many of my colleagues make. It is hard to understand how the gaffe was made or made it through peer review. But this point also undermines an argument Peter recently made on this blog; essentially that corals have survived for tens of millions of years, despite lots of environmental change, therefore they will not be affected or driven extinct by climate change. As Peter points out, coral reefs are a quite ephemeral habitat. During a large majority of the last few million years, the environment was unfavorable to reef accretion, despite the presence of most modern coral species. So corals clearly are sensitive to environmental conditions. The point of reef conservation and management is not to prevent coral extinctions, but instead to preserve the structure and functioning of reef ecosystems. We are worried about loosing reefs not corals themselves. PROBLEM 4: THE CITED LITERATURE OFTEN DOES NOT JUSTIFY CONCLUSIONS ABOUT THE DECLINE IN ECOLOGICAL STATEI agree with Peter that it is hard to find the actual evidence in many of the cited sources in Pandolfi et al. to justify many of the state scorings. The technique relies largely on qualitative observations made in a single point in space and time, that are the extrapolated to regional and millennial scales. But I do think the paper provides more general evidence of reef degradation on the GBR and elsewhere.

Peter concludes by stating; “these references provide no data to support the proposition that the inner GBR reefs are 25% of the way to ecological extinction. Indeed, no such data exists in the literature.” “Furthermore, a large amount of research has occurred since Hopley (1988) was written. None of the subsequent research has unequivocally demonstrated that there has been a significant long-term reduction in coral cover, species diversity, or coral health on a significant part of the outer GBR.” “the general conclusion that the Great Barrier Reef is in excellent state is reasonable.”

The paper we recently published in PLoS One (Bruno and Selig 2007) clearly debunks this argument; there is no doubt that coral cover on the GBR has declined significantly, is now quite low, and based on the best available data, it is in fact no higher than any other subregion in the Indo-Pacific.

He also states that; “but in its [the Pandolfi et al. analysis] present form it provides a misleading impression that the state of some of the reef systems cited are in a much worse situation than they are.” Peter is wrong here and ironically, Pandolfi et al. 2003 in fact underestimated the degradation of the GBR. At least in terms of coral cover (which as Peter says, should be weighted far higher than other metrics), the GBR was the first region in the world that we know of to shows signs of widespread decline. Which by the reasoning in Pandolfi et al. should lead logically to the conclusion that overfishing increases reef resilience.

There are a number of papers quantifying large scale degradation of the GBR going back many decades (e.g., work by Endean and Stablum in the late 60s and early 70s). But interpreting the pattern of coral loss and the historical coral cover baseline on the GBR is quite tricky since subregional coral loss seems to have occurred there in the late 1960s and we don’t have much in the way of pre-decline quantitative records (the recovery in the early 1980s does suggest a higher temporary “baseline” of 45-50%, but the sample size is unfortunately quite small).

We do, however, have fairly good data from other regions such as the Philippines (e.g., Gomez et al. 1982) before substantial coral losses. So we know that coral cover twenty years ago was at least roughly 45% in the Philippines and other subregions. Obviously, substantial degradation probably occurred before these early surveys, which were in fact
performed to document observed declines in the 1970s. My best estimate for historical mean cover in the Indo-Pacific is roughly 50%, but I’d like to hear what others think.

But I do agree with the points Peter makes about the many differences between the GBR and the Caribbean (which are roughly of equal size):

“The Great Barrier Reef is a very large system, 2000 km in length and adjacent to a very small coastal population of less than 0.5 million. The Caribbean Reefs are adjacent to populations of over 50 million. Most of the GBR is rarely visited because it is over 50 km from the coast, in contrast to the Caribbean reefs. The large distance of most of the GBR from the coast greatly mitigates the influence of enhanced river runoff due to poor agricultural practices.”

Although the statement that “Caribbean Reefs are adjacent to populations of over 50 million.” is simply ludicrous; there is no reef in Caribbean basin adjacent to a human population of that size, since there are no cities that have even close to that many people in the Caribbean. In total there are that many people in the Caribbean, but most reefs are 10s or 100s of km from 99% of them.

Given all this and the far more intense management of the GBR, to me at least, the similarity in coral cover between these two systems is quite striking. As we reported in our PLoS article, in 2003 the average coral cover on the GBR was 22.1% (95% CI: 20.7, 23.4, n = 390 reefs). This may come as a surprise to many, but Caribbean cover (including the Florida Keys) that year was only marginally lower at 18.6 % (95% CI: 17.0, 20.1; based on an unpublished meta-analysis of 343 quantitative reef surveys).

The difference in cover between these two vastly different regions is barely statistically significant and possibly not ecologically significant. Why? I really don’t know, but I think it casts some doubt on the broad idea that local, point-source human impacts are important drivers of coral loss. The pattern is also at least concordant with the
argument that regional to global scale phenomena (e.g., climate change, infectious and non-infectious diseases, etc.) are the main cause of coral decline.

Peter concludes the paper by stating that; “In many regards, the GBR is the best protected, most pristine and remote ecosystem on earth with the exception of only Antarctica.”

Unfortunately this sentiment (whether made out of hope, desperation, or national pride) is not even partially true as hundreds of scientists and thousands of sport divers have seen with their own eyes and as we have shown in our PLoS One paper. And despite all the scorn he directs at Pandolfi et al., Peter doesn’t offer a single citation or even attempt
to justify this extraordinary claim. It is just made as a point of fact, or probably more accurately as a deeply held belief.

If you look at a globe, the GBR is actually not all that geographically remote compared to countless other ecosystems. But that point is largely irrelevant. The multi-faceted plague we politely call “Global Change” is the great geographic equalizer: it doesn’t seem to matter how far an ecosystem is from our urban centers. Our footprint is truly global and
our choices and behaviors can have impacts half way around the world that are as strong as those in our own backyards.

Literature cited
Bruno, J. F., and E. R. Selig. 2007. Regional decline of coral cover in the Indo-Pacific: timing, extent, and subregional comparisons. PLoS One.
Gomez, E. D., A. C. Alcala, and A. C. San Diego. 1982. Status of the Philippine coral reefs – 1981. Proceedings of the Fourth International Coral Reef Symposium, Manila 1:275-282.

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