Why the existence of ‘heat tolerant’ corals does not mean that coral reefs will be able to resist climate change.

ofuA recent study published by Tom Oliver and Stephen Palumbi from Stanford University in the journal ‘Marine Ecology Progress Series‘ seems to suggest yet another miraculous and novel mechanism by which corals will ‘escape’ the pressures of global warming. In a nutshell, the researchers found that corals from ‘warm pools’ at Ofu Island (American Samoa) hosted ‘heat tolerant’ types of symbiotic algae, whereas corals from cooler lagoons hosted more ‘heat sensitive’ types of algae. When combined with regional data, Oliver & Palumbi suggest that in regions where annual maximum temperatures reached 29 – 31C, coral ‘avoided bleaching’ by hosting higher proportions of ‘heat tolerant’ algal symbionts.  Whilst these findings are interesting, the study is a long way from the suggestion in the paper and accompanying press release that coral reefs are ‘adapting’ and ‘may survive global warming’, and relies mainly on over interpreting their results. There are several issues at hand:

  1. Whilst these results highlight both the diversity of bleaching responses at a community level and the array of algal symbionts, the finding of heat tolerant corals has been shown throughout the Indo-Pacific and Great Barrier Reef on a number of previous occasions. The suggestion that this pattern results from a correlation in local scale heating based upon a limited sampling regime is far from proving causality
  2. The identification of a few remnant tough (‘heat resistant’) corals does not equate to these corals spreading out and maintaining coral reef ecosystems under rapid climate change.  Such coral types are rare, and are likely to have minimal impacts in sustaining reef populations under future climate change scenarios.  In making this argument, the authors are leaping across a myriad of issues that would need to be proven before we could pin our hopes on a few odd-ball corals for building and maintaining functional reef ecosystems into the future.
  3. Although corals have been shown to be able to ‘shuffle’ symbionts (change the proportion of ‘heat sensitive’ to ‘heat tolerant’ types), to date it has never been shown that corals can uptake ‘novel’ symbiont types from external sources. So, corals that are ‘heat sensitive’ can’t acquire ‘heat tolerant’ types from the environment. In light of rapid increases in sea surface temperatures under future climate scenarios, these ‘heat sensitive’ corals will undergo mass mortality, as they are unable to simply ‘adapt’ or switch to more resistant types.
  4. Any successful proliferation of these heat resistant genotypes will depend on a stabilised climate. Continual increases in temperatures means that these genotypes will have a harder time proliferating and stabilising, given that selection pressures will continue to intensify. This is akin to the bar in a high jump competition being placed ever higher.  As time goes on, fewer and fewer ecotypes from the population will be able to pass beyond the barrier.
  5. The authors seem to imply that functional reef ecosystems (and countless ecosystem services) will be ‘saved’, based upon a specific niche of ‘heat resistant’ corals.  The issue here is not the survival of corals species – many of whom will be resistant to extinction under global climate change (albeit as rare organisms) – but the destruction of functional coral reef ecosystems that millions of people depend on. Unfortunately, a few corals in a warm rock pool in Samoa will not save the day or the planet.
  6. These results do not address ocean acidification – the ‘other CO2’ issue along with temperature that threatens all calcifying organisms. To somehow imply that coral reefs are not facing problems from climate change because Oliver and Palumbi found a few tough coral genotypes in a rock pool, verges on the incredible.

These points aside, the study is an interesting one in terms of exploring heat stress in corals. My main issue is that Oliver & Palumbi have massively overextended their conclusions, which is particularly apparent in the associated press release.  Needless to say, these sorts of overblown claims are less than useful in the lead-up to the critically important COP15 negotiations in Copenhagen at the end of year.

Update, 25th May

And here is a classic example of why such media releases are less than useful, courtesy of the detractor Andrew Bolt:


4 thoughts on “Why the existence of ‘heat tolerant’ corals does not mean that coral reefs will be able to resist climate change.

  1. i agree ove, it looks like the authors have stretched just a little too far.

    how is ‘swapped out’ defined? i thought that ‘exogenous acquisition’ of entirely ‘new’ symbionts was still to be proven? if so, this means that the corals that don’t contain ‘heat sensitive’ algae are selectively weeded out under warming seas? what proportion of corals contain ‘heat sensitive’ algae?

  2. Agreed. Encouraging results, but I doubt that many would have suggested that corals will never adapt to higher temperatures, but I think the important message is (from the press release) “It comes down to a calculation of the rates of environmental change versus the rates of adaptation” which it always has.

    “Remember” the Permian-Triassic event 250mya – the largest extinction event ever known killed off 96% of all marine and 70% terrestrial species, but took hundreds of thousands years to do so – quite a lot longer than the entirety of human history! The scales are hard to conceptualise, but current human activity is probably resulting in one of the fastest ever mass extinctions.

    It is unlikely that coral reefs will become completely extinct, but it is also looking increasingly unlikely that they will persist as viable, productive ecosystems of the next few hundred years without huge changes in human behaviour.

  3. Ove: Have you had this conversation (blog) with Steve or Tom directly? I too am not a fan of the current tendency for press-release science. Stephen

  4. Good point Stephen. I am also not a fan of press release science as you probably know. Reducing science to flashy sound bites can quickly lead to mythologies and misunderstandings. This can have important ramifications. I was just told by someone who visited Capitol Hill recently that there are a number of senators who now think that coral reefs will survive climate change. They cited the Oliver and Palumbi paper.

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