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Range Expansion (Animals: Butterflies) -- Summary
Could global warming decimate earth's biosphere? Climate alarmists have long claimed that the increase in temperature predicted to result from the ongoing rise in the air's CO2 content will be so great and so rapid that many species of plants and animals will not be able to migrate either poleward in latitude or upward in elevation rapidly enough to avoid extinction, as they are forced to seek cooler living conditions. However, as we explain in our major report The Specter of Species Extinction, there are many reasons for rejecting this contention, one of the most powerful being that increases in the air's CO2 content generally enable plants to endure warmer weather, with the result that there is little impetus for them to shift the heat-limiting warm-temperature boundaries of their ranges as temperatures rise. At the cold-limiting cool-temperature boundaries of their ranges, on the other hand, warming enables plants to push poleward and upward, which results in expansions of their ranges and reductions in their risk of extinction; and we here review the findings of several studies of butterflies, which indicate that they respond in like manner.

Parmesan et al. (1999) analyzed distributional changes, broadly spread over the past century, of non-migratory butterflies whose northern boundaries were in northern Europe and whose southern boundaries were in southern Europe or northern Africa. A northern boundary analysis of the ranges of 52 species revealed that northern boundaries shifted northward for 65% of them, remained stable for 34% of them, and shifted southward for 2% of them, while a southern boundary analysis of the ranges of 40 species revealed that their southern boundaries shifted northward for 22% of them, remained stable for 72% of them, and shifted southward for 5% of them. The consequences of the 0.8C of warming that occurred in Europe over the course of the 20th century, therefore, had to have been largely beneficial to the butterflies, because "nearly all northward shifts involved extensions at the northern boundary with the southern boundary remaining stable," as the thirteen researchers explained it, so that "most species effectively expanded the size of their range when shifting northwards," which should have reduced various pressures on their populations.

Noting that the sachem skipper butterfly expanded its range from northern California into western Oregon in 1967 and into southwestern Washington in 1990, where it had warmed 2-4C over the prior 50 years, Crozier (2004) compared population dynamics at two locations (the butterfly's current range edge and just inside its range) that differed by 2-3C. Then, to determine the role of over-winter larval survivorship, she transplanted larvae over winter to both sites. This work revealed that winter temperatures directly affect the persistence of the sachem skipper butterfly at its northern range edge, and that winter warming was a prerequisite for the butterfly's range expansion. Noting that butterfly populations "are more likely to go extinct in colder climates," Crozier says "the good news about rapid climate change [of the warming type] is that new areas may be available for the introduction of endangered species." Her work also demonstrates that the species she studied has responded to regional warming by extending its northern range boundary and thereby expanding its range, which should enable it to move further back from the "brink of extinction" that so many climate alarmists associate with rapid global warming.

Across the continent, and across the Atlantic Ocean, Davies et al. (2006) reported that during the 20th century "the silver-spotted skipper butterfly [Hesperia comma] 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." As a result, they 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?

To find out, 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.

This work revealed 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." As a result, Davies et al. concluded that "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. concluded 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."

White and Kerr (2006), as they describe it, report "butterfly species' range shifts across Canada between 1900 and 1990 and develop spatially explicit tests of the degree to which observed shifts result from climate or human population density," the latter of which factors they describe as "a reasonable proxy for land use change," within which category they include such things as "habitat loss, pesticide use, and habitat fragmentation," all of which anthropogenic-driven factors have been tied to declines of various butterfly species. In addition, they say that to their knowledge, "this is the broadest scale, longest term dataset yet assembled to quantify global change impacts on patterns of species richness."

What the two researchers found was that butterfly species richness "generally increased over the study period, a result of range expansion among the study species," and that this increase "from the early to late part of the 20th century was positively correlated with temperature change," which had to have been the cause of the change, for they also found that species richness was "negatively correlated with human population density change." Thus, contrary to the doom-and-gloom prognostications of the world's climate alarmists, the supposedly unprecedented (and dreaded) global warming of the 20th century has been nothing but beneficial for the butterfly species that inhabit Canada, as their ranges have expanded and greater numbers of species are now being encountered in most parts of the country.

Hughes et al. (2007) examined evolutionary changes in adult flight morphology in six populations of the speckled wood butterfly along a transect from its distribution core to its warming-induced northward expanding range margin in Britain, comparing the results of this exercise with the output of an individual-based spatially explicit model that was developed "to investigate impacts of habitat availability on the evolution of dispersal in expanding populations." In doing so, they found that the empirical data they gathered were in agreement with the model output, and that they "showed increased dispersal ability with increasing distance from the distribution core," including favorable changes in thorax shape, abdomen mass and wing aspect ratio for both males and females, as well as thorax mass and wing loading for females. In addition, they say that "increased dispersal ability was evident in populations from areas colonized >30 years previously."

In discussing their findings, Hughes et al. say "evolutionary increases in dispersal ability in expanding populations may help species track future climate changes and counteract impacts of habitat fragmentation by promoting colonization." However, they report that in the specific situation they investigated, "at the highest levels of habitat loss, increased dispersal was less evident during expansion and reduced dispersal was observed at equilibrium, indicating that for many species, continued habitat fragmentation is likely to outweigh any benefits from dispersal [our italics]." Put another way, it would appear that global warming is proving not to be an insurmountable problem for the speckled wood butterfly, which is evolving physical characteristics that allow it to better keep up with the poleward shifting of its current environmental niche. However, it would appear that the direct destructive assaults of humanity upon its habitat could well end up driving it to extinction.

Last of all, Gonzalez-Megias et al. (2008) investigated species turnover in 51 butterfly assemblages in Britain by examining regional extinction and colonization events that occurred between the two periods 1976-1982 and 1995-2002, over which time interval the world's climate alarmists claim the planet experienced a warming they contend was unprecedented over the past millennium or more. In doing so, the five researchers found that regional colonizations exceeded extinctions, as "over twice as many sites gained species as lost species," such that "the average species richness of communities has increased." They also found that species abundances following colonization likewise increased, due to "climate-related increases in the [land's] carrying capacity."

In comparing their results with those of a broader range of animal studies, Gonzalez-Megias et al. write that "analyses of distribution changes for a wide range of other groups of animals in Britain suggest that southern representatives of most taxa are moving northwards at a rate similar to -- and in some cases faster than -- butterflies (Hickling et al., 2006)," and that "as with butterflies, most of these taxonomic groups have fewer northern than southern representatives, so climate-driven colonisations are likely to exceed extinctions." Hence, they suggest that "most of these taxa will also be experiencing slight community-level increases in species richness."

In light of these many positive real-world observations, plus a number of others that are highlighted in the second DVD of our Carbon Dioxide and the "Climate Crisis" series (Avoiding Plant and Animal Extinctions), it would appear that the world of nature is behaving just the opposite of how climate alarmists typically contend it should behave with respect to plant and animal biodiversity on a CO2-enriched and warming earth.

References
Crozier, L. 2004. Warmer winters drive butterfly range expansion by increasing survivorship. Ecology 85: 231-241.

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.

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.

Gonzalez-Megias, A., Menendez, R., Roy, D., Brereton, T. and Thomas, C.D. 2008. Changes in the composition of British butterfly assemblages over two decades. Global Change Biology 14: 1464-1474.

Hickling, R., Roy, D.B., Hill, J.K., Fox, R. and Thomas, C.D. 2006. The distributions of a wide range of taxonomic groups are expanding polewards. Global Change Biology 12: 450-455.

Hughes, C.L., Dytham, C. and Hill, J.K. 2007. Modelling and analyzing evolution of dispersal in populations at expanding range boundaries. Ecological Entomology 32: 437-445.

Parmesan, C., Ryrholm, N., Stefanescu, C., Hill, J.K., Thomas, C.D., Descimon, H., Huntley, B., Kaila, L., Kullberg, J., Tammaru, T., Tennent, W.J., Thomas, J.A. and Warren, M. 1999. Poleward shifts in geographical ranges of butterfly species associated with regional warming. Nature 399: 579-583.

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.

White, P. and Kerr, J.T. 2006. Contrasting spatial and temporal global change impacts on butterfly species richness during the 20th century. Ecography 29: 908-918.

Last updated 24 June 2009