Comparing Model Projections with Observations: Butterflies

From ClimateWiki

The butterfly is another animal species the IPCC claims to be at risk of extinction due to global warming. In this section, we analyze how butterflies respond to rising air temperatures and CO2 concentrations.

Over the prior century of global warming, Parmesan et al. (1999) examined the distributional changes of non-migratory butterfly species whose northern boundaries were in northern Europe (52 species) and whose southern boundaries were in southern Europe or northern Africa (40 species). This work revealed the northern boundaries shifted northward for 65 percent of of the first group, remained stable for 34 percent, and shifted southward for 2 percent. The southern boundaries shifted northward for 22 percent of of the second group , remained stable for 72 percent, and shifted southward for 5 percent. Thus “nearly all northward shifts,” according to Parmesan et al., “involved extensions at the northern boundary with the southern boundary remaining stable.”

This behavior is precisely what we would expect to see if the butterflies were responding to shifts in the ranges of the plants upon which they depend for their sustenance, because increases in atmospheric CO2 concentration tend to ameliorate the effects of heat stress in plants and induce an upward shift in the temperature at which they function optimally. These phenomena tend to cancel the impetus for poleward migration at the warm edge of a plant’s territorial range, yet they continue to provide the opportunity for poleward expansion at the cold edge of its range. Hence it is possible the observed changes in butterfly ranges over the past century of warming and rising atmospheric CO2 concentration are related to matching changes in the ranges of the plants upon which they feed. Alternatively, this similarity could be a result of some more complex phenomenon, possibly even some direct physiological effect of temperature and atmospheric CO2 concentration on the butterflies themselves. In any event, and in the face of the 0.8°C of warming that occurred in Europe over the twentieth century, the consequences for European butterflies were primarily beneficial.

Across the Atlantic in the United States, Fleishman et al. (2001) used comprehensive data on butterfly distributions from six mountain ranges in the U.S. Great Basin to study how butterfly populations of that region may respond to IPCC-projected climate change. Their work revealed “few if any species of montane butterflies are likely to be extirpated from the entire Great Basin (i.e., lost from the region as a whole).” In discussing their results, the three researchers noted “during the Middle Holocene, approximately 8000–5000 years ago, temperatures in the Great Basin were several degrees warmer than today.” They note “we might expect that most of the montane species—including butterflies—that currently inhabit the Great Basin would be able to tolerate the magnitude of climatic warming forecast over the next several centuries.”

Thomas et al. (2001) documented an unusually rapid expansion of the ranges of two butterfly species (the silver-spotted skipper and the brown argus) along with two cricket species (the long-winged cone-head and Roesel’s bush cricket). They observed the warming-induced “increased habitat breadth and dispersal tendencies have resulted in about 3- to 15-fold increases in expansion rates.” In commenting on these findings, Pimm (2001) remarked that the geographical ranges of these insects were “expanding faster than expected” and the synergies involved in the many intricacies of the range expansion processes were “unexpected.”

Three years later, Crozier (2004) noted “Atalopedes campestris, the sachem skipper butterfly, expanded its range from northern California into western Oregon in 1967, and into southwestern Washington in 1990,” where she reports temperatures rose by 2–4°C over the prior half-century. Thus intrigued, and in an attempt to assess the importance of this regional warming for the persistence of A. campestris in the recently colonized areas, Crozier “compared population dynamics at two locations (the butterfly’s current range edge and just inside the range) that differ by 2–3°C.” Then, to determine the role of over-winter larval survivorship, she “transplanted larvae over winter to both sites.”

This work revealed, in Crozier’s words, “combined results from population and larval transplant analyses indicate that winter temperatures directly affect the persistence of A. campestris at its northern range edge, and that winter warming was a prerequisite for this butterfly’s range expansion.” Noting “populations are more likely to go extinct in colder climates,” Crozier indicated “the good news about rapid climate change [of the warming type] is that new areas may be available for the introduction of endangered species.” The species she studied responded to regional warming by extending its northern range boundary, thereby expanding its range, which should enable it to move further back from the “brink of extinction” that some advocates associate with rapid global warming.

Davies et al. (2006) introduced their study of the silver-spotted skipper butterfly (Hesperia comma) by noting that during the twentieth century it “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,” while describing the “refuge” colonies of 1982 as but a “remnant” of what once had been. But that was not the end of the matter, for then came the warming claimed by some to have been unprecedented over the past two millennia, which could ultimately drive the already decimated species to extinction.

The four researchers analyzed population density data, estimates of the percent of bare ground, and the percent of sheep’s fescue 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). They also 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 multifaceted work of Davies et al. (2006) revealed, in their words, 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 “the mean egg-laying rate of H. comma females increased with rising ambient temperatures” and “a wider range of conditions have become available for egg-laying.”

In discussing their findings, Davies et al. write, “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 “greater temperatures should increase the potential fecundity of H. comma females,” and “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,” with “larger numbers of dispersing migrant individuals being available to colonize unoccupied habitat patches and establish new populations.” Second, they state “the wider range of thermal and physical microhabitats used for egg-laying increased the potential resource density within each grassland habitat fragment,” and “this may increase local population sizes.” Third, they argue “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 “higher temperatures may directly increase flight (dispersal) capacity, and the greater fecundity of immigrants may improve the likelihood of successful population establishment.” Consequently, Davies et al. conclude “the warmer summers predicted as a consequence of climate warming are likely to be beneficial to H. comma within Britain,” and they suggest “warmer winter temperatures could also allow survival in a wider range of microhabitats.”

In a concurrent study, Menendez et al. (2006) provided what they called “the first assessment, at a geographical scale, of how species richness has changed in response to climate change,” concentrating on British butterflies. They tested “whether average species richness of resident British butterfly species has increased in recent decades, whether these changes are as great as would be expected given the amount of warming that has taken place, and whether the composition of butterfly communities is changing towards a dominance by generalist species.” By these means they determined “average species richness of the British butterfly fauna at 20 x 20 km grid resolution has increased since 1970–82, during a period when climate warming would lead us to expect increases.” They also found, as expected, “southerly habitat generalists increased more than specialists,” which require a specific type of habitat that is sometimes difficult for them to find, especially in the modern world where habitat destruction is commonplace. In addition, they were able to determine that observed species richness increases lagged behind those expected on the basis of climate change.

These results “confirm,” according to the nine U.K. researchers, “that the average species richness of British butterflies has increased since 1970–82.” Some of the range shifts responsible for the increase in species richness take more time to occur than those of other species, and the researchers state their results imply “it may be decades or centuries before the species richness and composition of biological communities adjusts to the current climate.”

Also working in Britain, Hughes et al. (2007) examined evolutionary changes in adult flight morphology in six populations of the speckled wood butterfly—Pararge aegeria—along a transect from its distribution core to its warming-induced northward-expanding range margin. The results of this exercise were then compared with the output of an individual-based spatially explicit model developed “to investigate impacts of habitat availability on the evolution of dispersal in expanding populations.” This work indicated the empirical data the researchers gathered “were in agreement with model output” and “showed increased dispersal ability with increasing distance from the distribution core.” This included 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, the researchers indicate “increased dispersal ability was evident in populations from areas colonized >30 years previously.”

In discussing their findings, Hughes et al. suggest “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.” Put another way, it would appear global warming is proving not to be an insurmountable problem for the speckled wood butterfly, which is evolving physical characteristics that allow it to keep up with the poleward migration of its current environmental niche, whereas direct destructive assaults of humanity upon its natural habitat could still end up driving it to extinction.

Analyzing data pertaining to the general abundance of Lepidoptera in Britain over the period 1864–1952, based on information assembled by Beirne (1955) via his examination of “several thousand papers in entomological journals describing annual abundances of moths and butterflies,” Dennis and Sparks (2007) reported, “abundances of British Lepidoptera were significantly positively correlated with Central England temperatures in the current year for each month from May to September and November” and “increased overall abundance in Lepidoptera coincided significantly with increased numbers of migrants,” having derived the latter data from the work of Williams (1965). In addition, they report Pollard (1988) subsequently found much the same thing for 31 butterfly species over the period 1976–1986, and Roy et al. (2001) extended the latter investigation to 1997 and found “strong associations between weather and population fluctuations and trends in 28 of 31 species which confirmed Pollard’s (1988) findings.” All of these observations indicate the warming-driven increase in Lepidopteran species and numbers in Britain has been an ongoing phenomenon since the end of the Little Ice Age.

In another analysis from Britain, Gonzalez-Megias et al. (2008) investigated species turnover in 51 butterfly assemblages by examining regional extinction and colonization events that occurred between the two periods 1976–1982 and 1995-2002. The five researchers found regional colonizations exceeded extinctions: “over twice as many sites gained species as lost species,” such that “the average species richness of communities has increased.” They also found 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. found “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 they report, “as with butterflies, most of these taxonomic groups have fewer northern than southern representatives, so climate-driven colonisations are likely to exceed extinctions.” They suggest “most of these taxa will also be experiencing slight community-level increases in species richness.”

White and Kerr (2006) “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,” describing the latter factor as “a reasonable proxy for land use change.” In this category they included such elements 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 state that to their knowledge, “this is the broadest scale, longest term dataset yet assembled to quantify global change impacts on patterns of species richness.”

The two researchers discovered butterfly species richness “generally increased over the study period, a result of range expansion among the study species.” They further found 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 increase, for they also found species richness was “negatively correlated with human population density change.”

In another study conducted in Canada, Westwood and Blair (2010) measured the responses of 19 common butterfly species of the boreal forests of Manitoba to temperature changes experienced there during 1971–2004, focusing on each species’ date of first appearance, week of peak abundance, and length of flight period. Autumn temperatures were found to have warmed significantly, and the two Canadian researchers observed “13 of 19 species showed a significant increase in flight period extending longer into the autumn,” when “flight period extensions increased by 31.5 ± 13.9 days over the study period.” They note “two species, Junonia coenia and Euphydryas phaeton, increased their northerly ranges by ~150 and 70 km, respectively.”

Westwood and Blair state, “warmer autumns and winters may be providing opportunities for range extensions of more southerly butterfly species held at bay by past climatic conditions.” In addition, they cite other investigators who have obtained similar results, stating “northward expansions in butterfly species range correlating with northward shifts in isotherms have been documented in both Europe and North America (Karl et al., 1996; Parmesan, 1996; Parmesan et al., 1999; Hill et al., 1999; Hickling et al., 2006),” while indicating “in Canada, the Gorgone checkerspot (Chlosyne gorgone, Hubner) and the Delaware skipper (Anatryone logan, W.H. Edwards) have recently expanded their northern ranges significantly (Kerr, 2001).” These results confirm the observations of White and Kerr and contradict the negative prognostications about climate impacts from warming.

One additional means by which butterflies can cope with high temperatures is through the production of heat-shock proteins (HSPs). According to Karl et al. (2008), HSPs “are thought to play an important ecological and evolutionary role in thermal adaptation,” where “the up-regulation of stress-inducible HSPs may help organisms to cope with stress thus enhancing survival (Sorensen et al., 2003; Dahlhoff, 2004; Dahlhoff and Rank, 2007).”

Working with Lycaena tityrus, a widespread temperate-zone butterfly that ranges from western Europe to central Asia, Karl et al. tested this hypothesis by comparing expression patterns of stress-inducible HSPs across replicated populations originating from different altitudes and across different ambient temperatures. Their observations revealed “a significant interaction between altitude and rearing temperature [that] indicates that low-altitude animals showed a strongly increased HSP70 expression at the higher compared with at the lower rearing temperature.” This is exactly where one would expect to see such a response in light of its obvious utility in warmer conditions.

In discussing their findings, Karl et al. state their observation that “HSP70 expression increased substantially at the higher rearing temperature in low-altitude butterflies ... might represent an adaptation to occasionally occurring heat spells,” which further suggests this response should serve these organisms well in the days and years to come, especially if the dramatic warming and increase in heat spells predicted by the IPCC should come to pass.

Most recently, Forister et al. (2010) analyzed 35 years of butterfly presence-absence data collected by a single observer at ten sites approximately every two weeks along an elevation gradient stretching from sea level to an altitude of 2,775 meters in the Sierra Nevada Mountains of Northern California (USA). During the data-collection period (1) both maximum and minimum temperatures rose, (2) low-altitude habitat was negatively affected by encroaching land development, and (3) there was no systematic variation in precipitation. This effort revealed, in the words of the eight researchers, that over this period, species richness “declined at half of the sites, with the most severe reductions at the lowest elevations,” where “habitat destruction [was] greatest.” At intermediate elevations, they report, there were “clear upward shifts in the elevational ranges of species, consistent with the influence of global warming.” And at the highest site, they found species richness actually increased, and “in addition to an increase in richness, abundance has also generally increased at the highest-elevation site.”

References

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Comparing Model Projections with Observations: Butterflies

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