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The Downward Spiral of Global Amphibian Populations
Volume 11, Number 24: 11 June 2008

Lips et al. (2008) begin an analysis of the possible role of historical climate change in triggering disease outbreaks of chytridiomycosis -- an emerging infectious disease of amphibians caused by the fungal pathogen Batrachochytrium dendrobatidis (Bd) -- with the statement that "amphibian populations are declining across the globe at an alarming rate, with over 43% of species in a state of decline." Noting that the role of Bd in these population declines "has been linked to interactions with climate change" via the climate-linked epidemic hypothesis (CLEH) of Pounds et al. (2006) and Bosch et al. (2007), they indicate they have some serious reservations about this idea, because, as they continue, "both field studies on amphibians (Briggs et al., 2005; Lips et al., 2006) and on fungal population genetics (Morehouse et al., 2003; Morgan et al., 2007) strongly suggest that Bd is a newly introduced invasive pathogen." Consequently, and "from an ethical standpoint," as they put it, they cite as the primary reason for their further study of the subject, the "need to understand, as quickly as possible, the global patterns and causes of amphibian declines to prevent further losses of biodiversity."

In pursuit of the basic knowledge required to achieve this important goal, the four researchers evaluated data pertaining to population declines of frogs of the genus Atelopus, as well as similar data from other amphibian species, in Lower Central America and Andean South America, based on their own work and that of others recorded in the scientific literature, seeking to determine if the documented population declines were more indicative of an emerging infectious disease or a climate-change-driven infectious disease.

In discussing their findings, Lips et al. (2008) say they reveal "a classical pattern of disease spread across native populations, at odds with the CLEH proposed by Pounds et al. (2006)," emphasizing that their "analyses and re-analyses of data related to the CLEH all fail to support that hypothesis." Quite to the contrary, they say their analyses "support a hypothesis that Bd is an introduced pathogen that spreads from its point of origin in a pattern typical of many emerging infectious diseases," reemphasizing that "the available data simply do not support the hypothesis that climate change has driven the spread of Bd in our study area."

Although the U.S. scientists make it clear that disease dynamics are indeed "affected by micro- and macro-climatic variables," and that "such synergistic effects likely act on Bd and amphibians," their work clearly demonstrates that the simplistic scenario represented by the CLEH -- which posits, in their words, that "outbreaks of chytridiomycosis are triggered by a shrinking thermal envelope" -- paints an unrealistic picture of the role of global climate change in the much more complicated setting of real-world biology, where many additional factors may play even greater roles in determining amphibian wellbeing. With Lips et al.'s demonstration of this fact, perhaps appropriate conservation groups will now be able to do a better job of preventing "further losses of biodiversity," which they opined in the introductory paragraph of their paper might possibly occur in response to a more realistic understanding of "global patterns and causes of amphibian declines."

Sherwood, Keith and Craig Idso

Bosch, J., Carrascal, L.M., Duran, L., Walker, S. and Fisher, M.C. 2007. Climate change and outbreaks of amphibian chytridiomycosis in a montane area of Central Spain: Is there a link? Proceedings of the Royal Society B 274: 253-260.

Briggs, C.J., Vredenburg, V., Knapp, R.A. and Rachowicz, L.J. 2005. Investigating the population-level effects of chytridiomycosis, an emerging infectious disease of amphibians. Ecology 86: 3149-3159.

Lips, K.R., Brem, F., Brenes, R., Reeve, J.D. Alford, R.A., Voyles, J., Carey, C., Livo, L., Pessier, A.P. and Collins, J.P. 2006. Emerging infectious disease and the loss of biodiversity in a Neotropical amphibian community. Proceedings of the National Academy of Sciences USA 103: 3165-3170.

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.

Morehouse, E.A., James, T.Y., Ganley, A.R.D., Vilgalys, R., Berger, L., Murphy, P.J. and Longcore, J.E. 2003. Multilocus sequence typing suggests the chytrid pathogen of amphibians is a recently emerged clone. Molecular Ecology 12: 395-403.

Morgan, J.A.T., Vredenburg, V., Rachowicz, L.J., Knapp, R.A., Stice, M.J., Tunstall, T., Bingham, R.E., Parker, J.M., Longcore, J.E., Moritz, C., Briggs, C.J. and Taylor, J.W. 2007. Enigmatic amphibian declines and emerging infectious disease: population genetics of the frog-killing fungus Batrachochytrium dendrobatidis. Proceedings of the National Academy of Sciences USA 104: 13,845-13,850.

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.