In discussing the first of these "untested assumptions," the three Australian researchers say that "predictions that reefs will disappear as a result of global warming are based, at least in part, on the assumption that corals are living close to their maximum thermal limits." However, they note that "the severity of bleaching responses varies dramatically within and among taxa," citing McClanahan et al. (2009), and that "such variable bleaching susceptibility implies that there is a considerable variation in the extent to which coral species are adapted to local environmental conditions."

The three scientists further report that little is known about the sensitivity of coral population response to climate-induced changes in vital rates; but they state that a large body of evidence "supports temperature tolerance varying among species, populations, communities, and reef regions (Marshall and Baird, 2000; Coles and Brown, 2003)." Hence, they conclude that "even in the absence of an adaptive response, a change in the relative abundance of species is a far more likely outcome of climate change than the disappearance of reef corals," citing the work of Loya et al. (2001), McClanahan (2002) and Hughes et al. (2003).

But is there "an adaptive response," in contradiction of the second untested assumption Maynard et al. (2008b) discuss? The three researchers clearly believe there is, stating that "a number of studies suggest that bleaching mortality rates have declined and thermal tolerance has increased in some regions." As one example, they report that "mortality rates in the Eastern Pacific were significantly lower in 1998 when compared with 1982 and 1983 (Glynn et al., 2001)," while as another example they write that "Maynard et al. (2008a) found thermal tolerance of three common coral genera on the Great Barrier Reef to be greater in 2002 than that expected from the relationship between temperature stress and bleaching severity observed in 1998."

So how is this adaptation accomplished? The Australian scientists say there is "circumstantial evidence for ongoing evolution of temperature tolerance between both species and reefs," citing the review of Coles and Brown (2003). In addition, they suggest that "symbiont shuffling from less to more stress-resistant clades is another mechanism by which corals may increase the thermal tolerance of the holobiont." And they declare "there is growing evidence that such shuffling can increase thermal tolerance, at least in the short term (Berkelmans and van Oppen, 2006)."

When all is said and done, therefore, Maynard et al. (2008b) conclude that "it is premature to suggest that widespread reef collapse is a certain consequence of ongoing bleaching, or that this will inevitably lead to fisheries collapses."

If they are right -- and we believe they are -- nature is far more resilient than many people give it credit for being.

Sherwood, Keith and Craig Idso

References
Berkelmans, R. and van Oppen, M. 2006. The role of zooxanthellae in the thermal tolerance of corals: a 'nugget of hope' for coral reefs in an era of climate change. Proceedings of the Royal Society of London B 273: 2305-2312.

Coles, S.L. and Brown, B.E. 2003. Coral bleaching - capacity for acclimatization and adaptation. Advances in Marine Biology 46: 183-223.

Glynn, P.W., Mate, J.L., Baker, A.C. and Calderon, M.O. 2001. Coral bleaching and mortality in Panama and Ecuador during the 1997-1998 El Nino-Southern Oscillation event: Spatial/temporal patterns and comparisons with the 1982-1983 event. Bulletin of Marine Science 69: 79-109.

Hughes, T.P., Baird, A.H., Bellwood, D.R., Card, M., Connolly, S.R., Folke, C., Grosberg, R., Hoegh-Guldberg, O., Jackson, J.B.C., Kleypas, J., Lough, J.M., Marshall, P., Nystrom, M., Palumbi, S.R., Pandolfi, J.M., Rosen, B. and Roughgarden, J. 2003. Climate change, human impacts, and the resilience of coral reefs. Science 301: 929-933.

Loya, Y., Sakai, K., Yamazato, K., Nakano, Y., Sambali, H. and van Woesik, R. 2001. Coral bleaching: the winners and the losers. Ecology Letters 4: 122-131.

Marshall, P.A. and Baird, A.H. 2000. Bleaching of corals on the Great Barrier Reef: differential susceptibilities among taxa. Coral Reefs 19: 155-163.

Maynard, J.A., Anthony, K.R.N., Marshall, P.A. and Masiri, I. 2008a. Major bleaching events can lead to increased thermal tolerance in corals. Marine Biology 155: 173-182.

Maynard, J.A., Baird, A.H. and Pratchett, M.S. 2008b. Revisiting the Cassandra syndrome; the changing climate of coral reef research. Coral Reefs 27: 745-749.

McClanahan, T., Maina, J. and Pet-Soede, L. 2002. Effects of the 1998 coral mortality event on Kenyan coral reefs and fisheries. Ambio 31: 543-550.

McClanahan, T., Weil, E., Cortes, J., Baird, A.H. and Ateweberhan, M. 2009. Consequences of coral bleaching for sessile reef organisms. In: van Oppen, M.J.H. and Lough, J.M., Eds. Ecological Studies: Coral Bleaching: Patterns, Processes, Causes and Consequences. Springer-Verlag, Berlin, Germany.