Rohr Jason R, Raffel Thomas R, Romansic John M, McCallum Hamish, Hudson Peter J
Department of Biology, University of South Florida, Tampa, FL 33620, USA.
Proc Natl Acad Sci U S A. 2008 Nov 11;105(45):17436-41. doi: 10.1073/pnas.0806368105. Epub 2008 Nov 5.
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.
人类对环境的改变可以说是推动地球进入了第六次物种大灭绝事件，而两栖动物作为所有脊椎动物类群中受威胁最严重的，首当其冲。全球范围内许多两栖动物数量的减少是由壶菌（蛙壶菌，Batrachochytrium dendrobatidis，简称Bd）引起的，针对这些减少现象，人们提出了两种截然不同的假说。全球变暖与Bd相关的数量减少之间的正相关引发了壶菌热最适假说，该假说认为全球变暖导致温暖年份云量增加，促使白天和夜间温度趋向于Bd生长的热最适温度。相比之下，时空传播假说认为，Bd相关的数量减少是由Bd的引入和传播导致的，与气候变化无关。我们通过评估以下两点对这些假说进行了严格检验：（i）云量、温度趋同以及预测的温度依赖性Bd生长是否是阿氏蟾蜍属两栖动物灭绝的显著正向预测因子；（ii）在不假设Bd出现的位置、时间或数量的情况下，是否能检测到这些灭绝时间的空间结构。我们发现阿氏蟾蜍属物种灭绝时间存在空间结构，但这种结构的成因仍不明确，这凸显了对Bd进行进一步分子特征分析的必要性。我们还表明，此前报道的阿氏蟾蜍属物种灭绝与上一年热带平均气温之间长达数十年的正相关确实很稳健，但认为其具有因果关系的证据薄弱，因为包括区域香蕉和啤酒产量在内的许多其他变量对这些灭绝现象的预测性更好。最后，我们几乎所有的发现都与壶菌热最适假说的预测相反。虽然气候变化可能在全球两栖动物数量减少中发挥重要作用，但仍需要更有说服力的因果关系证据。
