The Australian newspaper published an article this weekend entitled “Great Barrier Reef could adapt to climate change, scientists say”.
THE prediction of a prominent marine biologist that climate change could render the Great Barrier Reef extinct within 30 years has been labelled overly pessimistic for failing to account for the adaptive capabilities of coral reefs.
University of Queensland marine biologist Ove Hoegh-Guldberg said yesterday that sea temperatures were likely to rise 2C over the next three decades, which would undoubtedly kill the reef.
But several of Professor Hoegh-Guldberg’s colleagues have taken issue with his prognosis.
Andrew Baird, principal research fellow at the Australian Research Council’s Centre for Excellence for Coral Reef Studies, said there were “serious knowledge gaps” about the impact rising sea temperatures would have on coral.
“Ove is very dismissive of coral’s ability to adapt, to respond in an evolutionary manner to climate change,” Dr Baird said.
“I believe coral has an underappreciated capacity to evolve. It’s one of the biological laws that, wherever you look, organisms have adapted to radical changes.”
Dr Baird acknowledged that, if left unaddressed, climate change would result in major changes to the Great Barrier Reef.
“There will be sweeping changes in the relative abundance of species,” he said. “There’ll be changes in what species occur where.
“But wholesale destruction of reefs? I think that’s overly pessimistic.”
Dr Baird said the adaptive qualities of coral reefs would mitigate the effects of climate change.
I must say I’m a little amazed that Andrew Baird has come out with such poorly supported statements. In fact, his conclusions seem to depend almost entirely on his personal opinion! The argument that corals are able to magically “adapt” over one or two decades to climate change (even though their generation times are often longer) has come up many times over the years – always, with a complete dearth of evidence to support it.
I wrote to Andrew Baird yesterday, to try and understand if there was something that he knew that I might have missed in the scientific lecture. In response, Andrew sent me a recent article published by Jeff Maynard and himself (Maynard et al 2008).
Unfortunately, the article is an opinion piece (a bit like the newspaper article) that is poorly supported by anything but the most scant evidence (if you could actually call it that) from literature. I have responded to these types of articles before, but frustrated, here we go again:
Maynard et al (2008) state the following as important evidence that corals can adapt to changes in the environment, and therefore that they can adapt to the current very rapid changes in ocean temperature and acidity.
“..geographic variation in bleaching thresholds within species, sometimes over scales <100km, provides circumstantial evidence for ongoing evolution of temperature tolerance between both species and reef”
Let me start by saying that no credible biologist would doubt the role of evolution in the shaping of the physiology and ecology of corals with respect to temperature. Biological populations evolve in response to stress. However, the mere observation of geographic variation in thermal tolerance, does not give any hint about the rates or the length of time that these changes have taken to occur. Importantly, this statement does not equate to evidence that thermal tolerance can evolve in ecological time. The only way that Andrew Baird could convince anyone of this particular somewhat fanciful leap of logic is to present data that show that coral populations can rapidly evolved in the period of years. They can’t, and they haven’t.
Moving on to the issue of community change, there is no doubt about the consequence of increased selective pressure in shaping community structure. And hence, in the early phases of the current rapid rate of climate change, we will see community shifts (and we have). Therefore, the following statement by Maynard et al (2008) is correct, but only with respect to the early stages of climate change impacts on coral reefs.
“As a consequence, even in the absence of an adaptive response, a change in the relative abundance of species is a far more likely outcome of climate change than the disappearance of reef corals”
At this point, two things strike me. The first is the complete absence of data that Maynard et al (2008) used to support their conjecture that climate change (with sea temperature changes of between 2 and 4°C, and a steadily acidifying ocean) will only affect the community composition of corals (and not the abundance of corals on coral reefs). If we are already seeing mass mortality is like that of 1998 (with minimal recovery in most areas) at only 0.7°C increase in global temperature, why are they are so confident that an increase of between 2 and 4° C does not represent a problem? The second is the amazing and seemingly baseless confidence these to authors have that the culling of many often abundant species from coral reefs isn’t going to have other consequences for coral reef ecosystems. Haven’t the authors heard about the problems that populations face when it comes to reduced diversity?
One of the most simplistic notions that Maynard et al (2008) push in the article is the idea that evidence of variation in temperature tolerance is prime faci evidence that coral communities can rapidly evolve new temperature tolerance.
A large body of evidence, however, supports temperature tolerance varying among species, populations, communities, and reef regions (Marshall and Baird 2000; Coles and Brown 2003).
One of the most concerning aspects here is the conclusion that appears to pervade the entire article: that in the absence of evidence, there is evidence of absence (or words to this effect) – this is simply ignoring a vast quantity of literature linking thermal thresholds and coral bleaching events. Perhaps it is time for Baird and Maynard to propose a mechanism (with solid evidence) for how physiological traits such as thermal tolerance are able to evolve fast enough to keep rate with oceans that are warming and acidifying at rates which dwarf even the most rapid changes over the last several million years. Two can play the game of saying that “perhaps the white crow does exist even though we’ve never seen one!”
Following a brief (and somewhat unconvincing) dialogue on generation times and larval dispersal, Maynard et al (2008) also cite two pieces of evidence that mortality rates may be decreasing following repeated bleaching events:
For example, mortality rates in the Eastern Pacific were significantly lower in 1998 when compared with 1982 and 1983 (Glynn et al. 2001)
I have commented previously that the problem with the Glynn et al (2001) is that their assessment that the two bleaching events had the same amount of stress is poorly supported. For example, we know that light levels are an important cofactor of bleaching, determining the amount of damage and mortality, yet we don’t have any information on how 1998 compare it to 1982/3 with respect to solar radiation. We also have a sampling design which is not compatible between the two studies. That is, the ability to detect a subtle difference in the tolerance of corals between the two events is not possible and hence the conclusions are completely speculative.
Similarly, Maynard et al. (2008b) found thermal tolerance of three common coral genera on the Great Barrier Reef to be greater in 2002 than that expected from the relationship between temperature stress and bleaching severity observed in 1998.
The Maynard et al (2008b) article suggests that “Major bleaching events can lead to increased thermal tolerance in corals”, although fail to provide any real conclusive evidence to back up such a statement. Whilst several other potential mechanisms (photo-protective mechanisms, selective mortality, symbiont shuffling, shifts to heterotrophic feeding), the suggestion of “rapid evolution” or “long-term physiological memory” simply are not supported by the data presented. Moreover, the suggestion of “acclimation” or “adaptation” seems more realistically be an artifact of analyzing community responses at the level of genus level (e.g. “Acropora”) rather than understanding the variability and species loss (differential mortality within genera), and selective mortality at sites (as acknowledged by the authors).
While there must be upper limits to rates of adaptation, these examples suggest that the evolutionary capacity of corals to respond to thermal stress is far from exhausted.
This is quite a leap of faith given the absence of a plausible mechanism! So poorly supported, yet such a rich and fanciful statement. You would have to wonder!
There is one curious point (under “the changing climate of coral reef research”) in which the authors present an odd critique based upon comments of Hoegh-Guldberg et al (2007):
Ocean acidification has been hypothesised to affect energy allocation within coral colonies. In particular, acidification may force corals to invest more energy in calcification to produce skeletons of similar strength (Hoegh-Guldberg et al. 2007), leaving less energy for allocation to reproduction. However, it has yet to be shown that corals are capable of expending more energy on producing skeletons under lower aragonite saturation states. On the contrary, if calcification rates decrease, more energy could become available for reproduction, maintenance, or storage—a slight net positive benefit, at least in the short term, for corals within low-energy environments.
Apart from some logical flaws here, let me refer to some recent research which shows quite clearly that this isn’t true. In a recent and complex study published in the Proceedings of the National Academy of Science, my laboratory group has shows that ocean acidification impacts directly on the energy budgets of coral colonies. Furthermore, this research strongly suggests that ocean acidification has a much stronger impact upon bleaching and productivity than on calcification rates. This is in stark contrast to the suggestions of Maynard et al (2008), who fancifully imply that a decrease in calcification will result in more energy available for reproduction (with no data!).
Overall, while the interest of Andrew Baird and his colleague Jeff Maynard are to be encouraged, one hopes that such loose and unsupported perspectives on such an important issue will be better thought out next time. No one doubts that evolution occurs on organisms like corals, and hence if adaptation if given time will occur, I think it is important that we realise how unusual the current situation is. I together with 16 other leading scientific experts recently calculated the rates of change over the past 420,000 years and found that the current rates of changes in temperature and important aspects such as the carbonate ion concentration increased at rates that were as much as three orders of magnitude higher than even the most rapid rates of climate change over this period. Given the huge changes that did occur over this period in response to ice age and other global transitions, I think we need to be extremely careful in jumping to the conclusion that:
“adaptive qualities of coral reefs would mitigate the effects of climate change.”