Live and let die

A new paper in PLoS One (Maina et al. 2011) presents a new map of threats to the world’s coral reefs (Fig. 1).  The map is based in part on data – spatial patterns – of past stressors such as ocean warming.  For example, Maina used the Coral Reef Temperature Anomal Database (CorTAD) my lab developed with Ken Casey at NOAA.  The CorTAD is based on satellite readings of ocean surface temperatures and goes back to 1984.

In theory, you can map where the ocean has warmed (for either discrete periods or over the longer term) and where it hasn’t and score the reefs in warming regions as being more threatened and less likely to survive.  (This is because especially high temperatures can cause coral bleaching and disease outbreaks).  The team then used these maps of recent stressors to make recommendations to local managers about where very limited resources should be spent:

“We need to focus on the winners, not losers,” study co-author Tim McClanahan adds. “They’re off in the Pacific, southern East Africa, Sri Lanka, and the northern Red Sea. They’re not very well protected and they’ll do much, much better if they had more fish and less sediment.”

Figure 1. Some corals (green spots) are located in places where climatic conditions are likely to allow them to survive global warming, whereas others (red dots) are likely to die early from more adverse regional conditions. Credit: Joseph Maina

According to a story in Science Now “The 1982-2009 temperature data the study is based on is a good indicator of future trends”, McClanahan says, “because many oceanographic physical processes are stable enough to insure consistent patterns in temperature changes.”

I’m not so sure.  I think such threat mapping is valuable and I’ve been part of several similar projects.  But I’ve got a few concerns.

First, the logic of whether focusing protection and resources on “winners” ie, reefs that have experienced less local and regional stress, is less than clear to me.  I’m not arguing we do the opposite, but I think the decision framework is complicated and not well developed. In addition, I don’t see the practicality of this advice.  It isn’t as though “loser” reef nations are going to hand over funds they’ve allocated for reef management to other nations with “winner” reefs.

Second, I am not convinced we can do much locally to mitigate global stressors anyway.

Third, I doubt patterns of past stress (i.e. warming) related to Anthropogenic Global Warming, are indicative of future spatial patterns. In fact, I think there is growing evidence that this isn’t the case.  Reefs and regions that warmed and/or bleached in the past appear less likely to bleach in the (near) future.  We’ve seen this pattern, i.e., negative spatial autocorrelation, in the Caribbean, on the GBR and elsewhere at several spatial scales.  For example, compare the patterns of bleaching in the figure below (from Berkelmans 2004):

Figure 2. From Berkelmans et al 2004. Patterns of warming and bleaching on the GBR.You can see the warming and bleaching were much more coastal in 1998 than in 2002, when more offshore reefs bleached.

A more recent paper from my lab (Selig et al 2010) found the same patterns pretty much everywhere. The warmest year during a 20 year period varied a lot from place to place. See the map (Fig. 3) from the Caribbean below for an example:

Figure 3. Spatial patterns of anomalous warming in the Caribbean. The color coding illustrates the year with the greatest number of Temperature Stress Anomalies. From Selig et al 2010. 

There could also be a biological explanation for why reefs that bleached in the past seem to be less likely to bleach in the future (all things being equal).  As yet we don’t know why this is exactly, but it could be due to the weeding out of temperature sensitive species or genotypes or even the changing of zooxanthellae strains.  I’m not arguing that corals are adapting or acclimating so fast that ocean warming isn’t a big problem.  But a reef that has never experienced really high temperatures is going to fry when it does – sadly inevitably – in the future.

Unfortunately, Global Climate Models don’t help much either as they make predictions on very large spatial scales (grid sizes) and ignore coastal waters where most reefs are found.

I appreciate what Maina et al are trying to do, but I think the messages we should be sending managers are; (1) the signature of Anthropogenic Climate Change is going to get much stronger in the coming decades and (2) in all but a few exceptions, past patterns of change are not indicative of future patterns.

There is a much more far-reaching debate about this precise issue in the broader climate change literature.  For example, Oreskes et al 2010 point out that we simply don’t know enough about how key aspects of weather, eg, rainfall, will change locally to plan for or implement (or even estimate the cost of) local social adaptation.  Most climate change scientists are sending the exact opposite message to local managers as Maina et al; namely not to confuse weather predictions – local perturbations – and climate change.  We know the latter is coming, but we can’t know how this will affect local environmental conditions.

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