Anchukaitis, K.J. and Evans, M.N. 2010. Tropical cloud forest climate variability and the demise of the Monteverde golden toad. Proceedings of the National Academy of Sciences, USA 107: 5036-5040.
The authors write that "widespread amphibian extinctions in the mountains of the American tropics have been blamed on the interaction of anthropogenic climate change and a lethal pathogen," as we have discussed at length in reviews of papers we have archived under the heading of Extinction (Real-World Observations -- Animals: Amphibians) in our Subject Index; and they note, in this regard, that "limited meteorological records make it difficult to conclude whether current climate conditions at these sites are actually exceptional in the context of natural variability," questioning once again the original climate-alarmist contention that modern global warming was the primary culprit in the demise of the Monteverde golden toad (Bufo periglenes).
What was done
In an attempt to shed significant new light on the subject, Anchukaitis and Evans developed annual proxy records of hydroclimatic variability over the last century within the Monteverde Cloud Forest of Costa Rica, based on measurements of the stable oxygen isotope ratio (δ18O) made on trees lacking annual rings, as described in the papers of Evans and Schrag (2004) and Anchukaitis et al. (2008).
What was learned
The two researchers report that "contrary to interpretations of the short instrumental record (Pounds et al., 1999), no long-term trend in dry season hydroclimatology can be inferred from our δ18O time series at Monteverde (1900-2002)." Instead, they find that "variability at the interannual scale dominates the isotope signal, particularly during the period of increased ENSO variance since the late 1960s," and they add, in this regard, that "there is no evidence of a trend associated with global warming." Hence, they emphasize that "the extinction of the Monteverde golden toad appears to have coincided with an exceptionally dry interval caused by the 1986-1987 El Niņo event," which they describe as "one of the longest driest periods in the last 100 years," based on their δ18O chronology. In addition, they report there is currently no consensus on how anthropogenic climate change might influence the El Niņo Southern Oscillation, while indicating that "ENSO anomalies in the most recent decades are not beyond the range of natural variability during the instrumental period (Rajagopalan et al., 1997)."
What it means
Anchukaitis and Evans say their analysis suggests that "the cause of the specific and well-documented extinction of the Monteverde golden toad was the combination of the abnormally strong ENSO-forced dryness and the lethality of the introduced chytrid fungus, but was not directly mediated by anthropogenic temperature trends, a finding from paleoclimatology that is in agreement with statistical reanalysis (Rohr et al., 2008; Lips et al., 2008) of the 'climate-linked epidemic hypothesis'," which two reanalyses had also revealed the chytrid-thermal-optimum hypothesis, as it has alternatively been described, to be devoid of merit.
Anchukaitis, K.J., Evans, M.N., Wheelwright, N.T. and Schrag, D.P. 2008. Stable isotope chronology and climate signal in neotropical montane cloud forest trees. Journal of Geophysical Research 113: G03030.
Evans, M.N. and Schrag, D.P. 2004. A stable isotope-based approach to tropical dendroclimatology. Geochimica et Cosmochimica Acta 68: 3295-3305.
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.
Pounds, J.A., Fogden, M.P.L. and Campbell, J.H. 1999. Biological response to climate change on a tropical mountain. Nature 398: 611-615.
Rajagopalan, B., Lall, U. and Cane, M.A. 1997. Anomalous ENSO occurrences: An alternate view. Journal of Climate 10: 2351-2357.
Rohr, J.R., Raffel, T.R., Romansic, J.M., McCallum, H. and Hudson, P.J. 2008. Evaluating the links between climate, disease spread, and amphibian declines. Proceedings of the National Academy of Sciences USA 105: 17,436-17,441.Reviewed 16 June 2010