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Global Warming: Will It Cause Multiple Species Extinctions?
Volume 9, Number 48: 29 November 2006

With the publication of Camille Parmesan's recent review of 866 papers that address the subject of ecological and evolutionary responses to the global warming of the past few decades (Parmesan, 2006), new concerns have been raised about the ability of earth's many species of plants and animals to maintain a viable foothold on the planet if temperatures continue to rise. However, much of the evidence cited by Parmesan actually weighs heavily against this concern.

For starters, Parmesan notes that "by far [our italics], most observations of climate-change responses have involved alterations of species' phenologies." She reports, for example, that many species have exhibited "advancement of spring events," such that there has been "a lengthening of vegetative growing season in the Northern Hemisphere," which is something most people would consider a positive phenomenon. She also reports that "summer photosynthetic activity increased from 1981-1991" - another positive phenomenon - and that the growing season throughout the United States "was unusually long during the warm period of the 1940s," but that "since 1996, growing season length has increased only in four of the coldest, most-northerly zones (42-45 N latitude), not in the three warmest zones (32-37 N latitude)," which makes one wonder if it was not warmer throughout most of the United States in the 1940s than it was at the end of the 20th century. But we digress.

The negativity that Parmesan associates with warming-induced phenological changes arises from the possibility that there may be "mismatches" across different trophic levels in natural ecosystems, such as between the time that each year's new crop of herbivores appears and the time of appearance of the plants they depend upon for food. Eleven plant-animal associations have been intensively studied in this regard; and in seven of them Parmesan says "they are more out of synchrony now than at the start of the studies."

It must be noted, however, that there will always be winners and losers (some big and some small) in such animal-plant matchups during periods of climate change, and maybe a whole lot of "draws." In addition, it must be emphasized that the current paucity of pertinent data precludes a valid determination of which of the three alternatives is the most likely to predominate. As one example of a "big loser" in the face of recent global warming, Parmesan reports that "field studies have documented that butterfly-host asynchrony has resulted directly in population crashes and extinctions." But population extinctions are not the same as species extinctions; and she acknowledges that the local extinctions to which she refers have merely resulted in "shifting [the] mean location of extant populations northward [in the Northern Hemisphere] and upward."

The second of the major biological responses to global warming addressed by Parmesan is that of species migration, which is often touted as leading to range restrictions that make it difficult for species to maintain the "critical mass" required for their continued existence. For example, it is often claimed that global warming will be so formidable and fast that many species will not be able to migrate poleward in latitude or upward in altitude rapidly enough to avoid extinction, or that if located on mountaintops they will actually run out of suitable new habitat to which they can flee when faced with rising temperatures. Here Parmesan essentially rehashes the earlier findings of the meta-analyses of Root et al. (2003) and Parmesan and Yohe (2003), which predominantly portray species ranges as expanding in the face of rising temperatures, since warming provides a huge opportunity for species to expand their ranges at their cold-limited boundaries while often providing a much reduced impetus for them to retreat at the heat-limited boundaries of their ranges. An example of this phenomenon that Parmesan cites occurred in the Netherlands between 1979 and 2001, where she reports that "77 new epiphytic lichens colonized from the south, nearly doubling the total number of species for that community." For more details about this positive impact of global warming - which typically leads to increases in the biodiversity of natural ecosystems - see our major report The Specter of Species Extinction: Will Global Warming Decimate Earth's Biosphere?

Also on the positive side of things, Parmesan says that "increasing numbers of researchers use analyses of current intraspecific genetic variation for climate tolerance to argue for a substantive role of evolution in mitigating negative impacts of future [our italics] climate change," additionally noting that the fossil record contains "a plethora [our italics] of data indicating local adaptation to climate change at specific sites." On the other hand, she states that during earlier periods of dramatic climate change there is also evidence that many existing species "appeared to shift their geographical distributions as though tracking the changing climate." Interestingly, in both situations the outcomes were clearly positive.

The greatest push by Parmesan for a supremely negative consequence of global warming occurs when she says that "documented rapid loss of habitable climate space makes it no surprise that the first extinctions of entire species attributed to global warming are mountain-restricted species," that "many cloud-forest-dependent amphibians have declined or gone extinct on a mountain in Costa Rica (Pounds et al., 1999, 2005)," and that "among harlequin frogs in Central and South American tropics, an astounding 67% have disappeared over the past 20-30 years," citing Pounds et al. (2006) as authority for this latter contention. In carefully reviewing these claims, however, they appear to be far from conclusive.

In the first place, all of the extinctions and disappearances of the amphibian species to which Parmesan refers appear to have nothing at all to do with "rapid loss of habitable climate space" at the tops of mountains. In fact, as noted by Pounds et al. (2006), the loss of these species "is largest at middle [our italics] elevations, even though higher-elevation species generally have smaller ranges." In addition, as noted in an earlier review of the subject by Stuart et al. (2004), many of the amphibian species declines "took place in seemingly pristine [our italics] habitats," which had not been lost to global warming nor even modestly altered. Last of all, the extinctions and species disappearances appear not to be due to rising temperatures per se, but to the fungal disease chytridiomycosis, which is caused by Batrachochytrium dendrobatidis, as noted by both Stuart et al. (2004) and Pounds et al. (2006).

In a final attempt to circumnavigate these several dilemmas, Pounds et al. (2006) strove mightily to implicate global warming as the cause of Batrachochytrium's increased virulence in recent years. So convoluted and tenuous was their reasoning, however, that they repeatedly refer to their view of the subject as being but a hypothesis. In addition, in their paper's Supplementary Information they say that their goal was merely "to stimulate thought and generate ideas concerning the altitudinal patterns of thermal environments, the recent temperature shifts, and the interactions between Batrachochytrium and its amphibian hosts," with the hope that "future experimental studies should examine these ideas, while also considering the influence of other climatic changes such as shifts in precipitation and humidity." Last of all, and most damaging to their thesis, is the almost unbelievable fact, as reported by Bosch et al. (2006), that "Pounds et al. (2006) did not focus on showing whether the pathogen was present, or causing disease, in the species studied, raising questions as to whether infection by B. dendrobatidis [was] actually involved in the observed species declines."

Clearly, the last word on this subject has yet to be written; but Pounds et al. (2006) nevertheless state as factual that "with climate change promoting infectious disease and eroding biodiversity, the urgency of reducing greenhouse-gas concentrations is now undeniable [our italics]," making them what we could well call "climate change undeniers," in that they appear totally unwilling to even entertain the possibility that a different point of view might have some merit. Likewise, Parmesan (2006) states that "range-restricted species, particularly polar and mountaintop species, show more-severe range contractions than other groups and have been the first groups in which whole species have gone extinct due to recent climate change," in a claim that is patently inconsistent with known facts, as indicated above.

Sherwood, Keith and Craig Idso

Bosch, J., Carrascal, L.M., Duran, L., Walker, S. and Fisher, M.C. 2006. Climate change and outbreaks of amphibian chytridiomycosis in a montane area of Central Spain; is there a link? Proceedings of the Royal Society B:10.1098/rspb.2006.3713.

Parmesan, C. 2006. Ecological and evolutionary responses to recent climate change. Annual Review of Ecology, Evolution, and Systematics 37: 637-669.

Parmesan, C. and Yohe, G. 2003. A globally coherent fingerprint of climate change impacts across natural systems. Nature 421: 37-42.

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.

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.

Pounds, J.A., Fogden, M.P.L. and Masters, K.L. 2005. Responses of natural communities to climate change in a highland tropical forest. In: Lovejoy, T. and Hannah, L., Eds. Climate Change and Biodiversity, Yale University Press, New Haven, Connecticut, USA, pp. 70-74.

Root, T.L., Price, J.T., Hall, K.R., Schneider, S.H., Rosenzweig, C. and Pounds, J.A. 2003. Fingerprints of global warming on wild animals and plants. Nature 421: 57-60.

Stuart, S.N., Chanson, J.S., Cox, N.A., Young, B.E., Rodrigues, A.S.L., Fischman, D.L. and Waller, R.W. 2004. Status and trends of amphibian declines and extinctions worldwide. Science 306: 1783-1786.