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Global Warming and the Silver-Spotted Skipper Butterfly
Reference
Davies, Z.G., Wilson, R.J., Coles, S. and Thomas, C.D. 2006. Changing habitat associations of a thermally constrained species, the silver-spotted skipper butterfly, in response to climate warming. Journal of Animal Ecology 75: 247-256.

Background
"During the twentieth century," in the words of the authors, "the silver-spotted skipper butterfly [Hesperia comma L.] became increasingly rare in Britain [as] a result of the widespread reduction of sparse, short-turfed calcareous grassland containing the species' sole larval host plant, sheep's fescue grass [Festuca ovina L]." As a result, Davies et al. describe the "refuge" colonies of 1982 as but a "remnant" of what once had been. But the end was not yet; for then came the infamous warming that is said by climate alarmists to have been unprecedented over the past two millennia. Was it the final environmental insult that would ultimately drive the decimated species to extinction?

What was done
The four researchers analyzed population density data together with estimates of the percentage bare ground and the percentage sheep's fescue grass available to the butterflies, based on surveys conducted in Surrey in the chalk hills of the North Downs, south of London, in 1982 (Thomas et al., 1986), 1991 (Thomas and Jones, 1993), 2000 (Thomas et al., 2001>; Davies et al., 2005) and 2001 (R.J. Wilson, unpublished data). In addition, they assessed egg-laying rates in different microhabitats, as well as the effects of ambient and oviposition site temperatures on egg laying, and the effects of sward composition on egg location.

What was learned
Davies et al. report that "in 1982, 45 habitat patches were occupied by H. comma [but] in the subsequent 18-year period, the species expanded and, by 2000, a further 29 patches were colonized within the habitat network." In addition, they found that "the mean egg-laying rate of H. comma females increased with rising ambient temperatures," and that "a wider range of conditions have become available for egg-laying."

What it means
In the words of the scientists who conducted the eye-opening study, "climate warming has been an important driving force in the recovery of H. comma in Britain [as] the rise in ambient temperature experienced by the butterfly will have aided the metapopulation re-expansion in a number of ways."

First, they suggest that "greater temperatures should increase the potential fecundity of H. comma females," and that "if this results in larger populations, for which there is some evidence (e.g. 32 of the 45 habitat patches occupied in the Surrey network experienced site-level increases in population density between 1982 and 2000), they will be less prone to extinction [our italics]," with "larger numbers of dispersing migrant individuals being available to colonize unoccupied habitat patches and establish new populations. Second, they state that "the wider range of thermal and physical microhabitats used for egg-laying increased the potential resource density within each grassland habitat fragment," and that "this may increase local population sizes." Third, they argue that "colonization rates are likely to be greater as a result of the broadening of the species realized niche, [because] as a larger proportion of the calcareous grassland within the species' distribution becomes thermally suitable, the relative size and connectivity of habitat patches within the landscape increases." Fourth, they note that "higher temperatures may directly increase flight (dispersal) capacity, and the greater fecundity of immigrants may improve the likelihood of successful population establishment."

In light of these several real-world observations, Davies et al. conclude that "the warmer summers predicted as a consequence of climate warming are likely to be beneficial to H. comma within Britain," and they suggest that "warmer winter temperatures could also allow survival in a wider range of microhabitats."

References
Davies, Z.G., Wilson, R.J., Brereton, T.M. and Thomas, C.D. 2005. The re-expansion and improving status of the silver-spotted skipper butterfly (Hesperia comma) in Britain: a metapopulation success story. Biological Conservation 124: 189-198.

Thomas, C.D., Bodsworth, E.J., Wilson, R.J., Simmons, A.D., Davies, Z.G., Musche, M. and Conradt, L. 2001. Ecological and evolutionary processes at expanding range margins. Nature 411: 577-581.

Thomas, C.D. and Jones, T.M. 1993. Partial recovery of a skipper butterfly (Hesperia comma) from population refuges: lessons for conservation in a fragmented landscape. Journal of Animal Ecology 62: 472-481.

Thomas, J.A., Thomas, C.D., Simcox, D.J. and Clarke, R.T. 1986. Ecology and declining status of the silver-spotted skipper butterfly (Hesperia comma) in Britain. Journal of Applied Ecology 23: 365-380.

Reviewed 16 August 2006