Volume 9, Number 45: 8 November 2006
In a provocative paper that analyzes the potential effects of global warming on various animal diseases, Hall et al. (2006) begin their analysis of the subject by asking "Will an increasingly warmer world necessarily become a sicker world?" They pose the question because, in their words, "increased temperatures can accelerate the fitness of parasites, reduce recruitment bottlenecks for parasites during winter, and weaken hosts," noting further that "warmer temperatures may allow vectors of parasites to expand their range," which would enable them to "introduce diseases to novel habitats," which is something climate alarmists claim about mosquitoes and malaria. However, as they continue, "these doom-and-gloom scenarios do not necessarily apply to all taxa or all situations," and they note that "warming does not necessarily increase fitness of all parasites."
Enlarging upon these latter points, the four biologists and their statistician sidekick write that the "virulence of parasites may not change, may decrease or may respond unimodally to increasing temperatures (Stacey et al., 2003; Thomas and Blanford, 2003)," and in this regard they further note that "vital rates increase with temperature until some optimum is reached," and that "once temperature exceeds this optimum, vital rates decline gradually with increasing temperature for some taxa, but rapidly for others," such that "in some host-parasite systems, a parasite's optimum occurs at cooler temperatures than the optimum of its host," citing the work of Carruthers et al. (1992), Blanford and Thomas (1999) and Blanford et al. (2003) on fungus-grasshopper associations in substantiation of this scenario. In such cases, as they describe it, "a host can use warmer temperatures to help defeat its parasites through behavioral modification of its thermal environment."
However, the situation sometimes can be even more complex than this; for Hall et al. write that "warmer temperatures can also select for shifts in temperature optima (Huey and Hertz, 1984; Huey and Kingsolver, 1989, 1993)," and that "the exact evolutionary trajectory of host-parasite systems in a warmer world may depend sensitively upon underlying genetic correlation structures and interactions between host genotypes, parasite genotypes, and the environment (Blanford et al., 2003; Thomas and Blanford, 2003; Stacey et al., 2003; Mitchell et al., 2004)." Consequently, they conclude that "longer-term response of the physiology of host-parasite systems to global warming becomes difficult to predict."
But these considerations are not the end of the story either; for the researchers note that "other species can profoundly shape the outcome of parasitism in host populations," and that "predators provide an important example" because, as they elucidate, predators "can actually inhibit epidemics by selectively culling sick hosts and/or by maintaining host densities below levels required for parasites to persist (Hudson et al., 1992; Packer et al., 2003, Lafferty, 2004; Ostfeld and Holt, 2004; Duffey et al., 2005; Hall et al., 2005)."
When all is said and done, therefore, Hall et al. conclude that "global warming does not necessarily mean that disease prevalence will increase in all systems." Although it likely will increase for some, for others it will likely do just the opposite; and only time will tell which outcome will predominate.
Sherwood, Keith and Craig Idso
References
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