November 12, 2008

Beating a Dead Frog

Filed under: Extinctions

Just in case there are still some folks out there who continue to insist that there is a firm cause-and-effect relationship between anthropogenic global warming and the decline of amphibian species around the world despite our pointing out on numerous occasions just how tenuous such a linkage is (pay attention here Al), we present the following abstract of a paper by Jason Rohr and colleagues titled “Evaluating the links between climate, disease spread, and amphibian decline,” published November 11, 2008 in the Proceedings of the National Academy of Sciences (PNAS):

Human alteration of the environment has arguably propelled the Earth into its sixth mass extinction event and amphibians, the most threatened of all vertebrate taxa, are at the forefront. Many of the worldwide amphibian declines have been caused by the chytrid fungus, Batrachochytrium dendrobatidis (Bd), and two contrasting hypotheses have been proposed to explain these declines. Positive correlations between global warming and Bd-related declines sparked the chytrid-thermal-optimum hypothesis, which proposes that global warming increased cloud cover in warm years that drove the convergence of daytime and nighttime temperatures toward the thermal optimum for Bd growth. In contrast, the spatiotemporal-spread hypothesis states that Bd-related declines are caused by the introduction and spread of Bd, independent of climate change. We provide a rigorous test of these hypotheses by evaluating (i) whether cloud cover, temperature convergence, and predicted temperature dependent Bd growth are significant positive predictors of amphibian extinctions in the genus Atelopus and (ii) whether spatial structure in the timing of these extinctions can be detected without making assumptions about the location, timing, or number of Bd emergences. We show that there is spatial structure to the timing of Atelopus spp. extinctions but that the cause of this structure remains equivocal, emphasizing the need for further molecular characterization of Bd. We also show that the reported positive multi-decade correlation between Atelopus spp. extinctions and mean tropical air temperature in the previous year is indeed robust, but the evidence that it is causal is weak because numerous other variables, including regional banana and beer production, were better predictors of these extinctions. Finally, almost all of our findings were opposite to the predictions of the chytrid-thermal-optimum hypothesis. Although climate change is likely to play an important role in worldwide amphibian declines, more convincing evidence is needed of a causal link. [emphasis added].

Admittedly, in our previous WCRs describing why global warming was not killing frogs, we never came up with the link to banana and beer production (although perhaps some prior consumption of those items would have improved our creativity). In any case, by examining the patterns and timing of the amphibian declines compared with the patterns and timing of regional climate changes, Rohr and colleagues present a pretty good case against global warming.

And as far as the last line of the abstract goes, is seems like it was included to increase the odds of publication in PNAS—a more formal expression of the idea that “even though we’ve pretty much dismissed the current global-warming-is-killing-frogs hypothesis in our paper, we can’t discount folks coming up with a different global-warming-is-killing-frogs hypothesis at some point in the future, after all, anthropogenic climate change must be doing bad things, mustn’t it?”

Reference:

Rohr, J. R. et al, 2008. Evaluating the links between climate, disease spread, and amphibian decline. Proceedings of the National Academy of Sciences, 105, 17436-17441.




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