Amphibian Population Declines

Rohr et al. state that 43% of all amphibian species have experienced some form of population decline, most of which were driven by the chytrid fungus Batrachochytrium dendrobatidis or Bd, and that there are two competing hypotheses for the cause of the declines. The first -- championed by Pounds et al. (2006) and known as the chytrid-thermal-optimum hypothesis -- posits 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," which ultimately led to the population declines. The second -- championed by Lips et al. (2008) and known as the spatiotemporal-spread hypothesis -- posits that "Bd-related declines are simply caused by the introduction and spread of Bd from a limited number of introduction sites rather than by any interaction between Bd and climate change."

Rohr et al., as they describe it, "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 Antelopus 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."

The five researchers report that "almost all of our findings are contrary to the predictions of the chytrid-thermal-optimum hypothesis," even noting that "not all of the data presented by Pounds et al. (2006) are consistent with the chytrid-thermal-optimum hypothesis." Most tellingly, they say "there was no regional temperature convergence in the 1980s when extinctions were increasing, and that convergence only occurred in the 1990s when Atelopus spp. extinctions were decreasing, opposite [our italics] to the conclusions of Pounds et al. (2006) and the chytrid-thermal-optimum hypothesis." On the other hand, they report that "there is a 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."

On the basis of Rohr et al.'s findings, it would appear that the chytrid-thermal-optimum hypothesis, which depicts global warming as the unnatural disturbance that ultimately leads to amphibian declines and extinctions, should itself be considered extinct, while the spatiotemporal-spread hypothesis still has a good deal of life in it.

Additional References
Lips, K.R., Diffendorfer, J., Mendelson III, J.R. and Sears, M.W. 2008. Riding the wave: Reconciling the roles of disease and climate change in amphibian declines. PLoS (Public Library of Science) Biology 6(3): e72. doi:10.1371/journal.pbio.0060072, or p. 441-454.

Pounds, J.A., Bustamante, M.R., Coloma, L.A., Consuegra, J.A., Fogden, M.P.L., Foster, P.N., La Marca, E., Masters, K.L., Merino-Viteri, A., Puschendorf, R., Ron, S.R., Sanchez-Azofeifa, G.A., Still, C.J. and Young, B.E. 2006. Widespread amphibian extinctions from epidemic disease driven by global warming. Nature 439: 161-167.