Comparing Model Projections with Observations: Mammals
Norment et al. (1999) summarized and compared the results of many surveys of mammal populations observed along the Thelon River and its tributaries in the Canadian Northwest Territories from the 1920s through much of the 1990s. Over this time period, red squirrel, moose, porcupine, river otter, and beaver were found to have established themselves in the area, significantly increasing its biodiversity. The three researchers stated these primarily northward range expansions could be explained by “a recent warming trend at the northern treeline during the 1970s and 1980s.” Alternatively, they noted the influx of new species may have been due to “increasing populations in more southerly areas.” But in either case, we have a situation where several types of mammals appear to have fared quite well in the face of increasing temperatures in this forest-tundra landscape.
Millar and Westfall (2010) studied American pikas, small generalist herbivores that are relatives of rabbits and hares, inhabit patchily distributed rocky slopes of western North American mountains, and are good at tolerating cold. Given the latter characteristic, it is not surprising that pikas are widely believed to have a physiological sensitivity to warming, which when “coupled with the geometry of decreasing area at increasing elevation on mountain peaks,” in the words of the two scientists, “has raised concern for the future persistence of pikas in the face of climate change.” In fact, they note, the species “has been petitioned under California [USA] state and federal laws for endangered species listing.”
In a study designed to investigate the validity of the basis for that classification, Millar and Westfall developed a rapid assessment method for determining pika occurrence and used it “to assess geomorphic affinities of pika habitat, analyze climatic relationships of sites, and evaluate refugium environments for pikas under warming climates.” The researchers gathered data over the course of two field seasons in the Sierra Nevada Mountains of California, the southwestern Great Basin of California and Nevada, and the central Great Basin of Nevada, as well as a small area in the central Oregon Cascades.
In reporting their findings, the two U.S. Forest Service researchers state, “whereas concern exists for diminishing range of pikas relative to early surveys, the distribution and extent in our study, pertinent to four subspecies and the Pacific southwest lineage of pikas, resemble the diversity range conditions described in early 20th-century pika records (e.g., Grinnell and Storer, 1924).” The lowest site at which they detected the current presence of pikas, at an elevation of 1827 meters, “is below the historic lowest elevation of 2350 m recorded for the subspecies by Grinnell and Storer (1924) in Yosemite National Park; below the low elevation range limit for the White Mountains populations given by Howell (1924) at 2440 m; and below the lowest elevation described for the southern Sierra Nevada populations of 2134 m (Sumner and Dixon, 1953).” In addition, they write, “a similar situation occurred for another lagomorph of concern, pygmy rabbit (Brachylagus idahoensis), where a rapid assessment method revealed much wider distribution than had been implied from historic population databases or resurvey efforts (Himes and Drohan, 2007).”
Millar and Westfall thus conclude “pika populations in the Sierra Nevada and southwestern Great Basin are thriving, persist in a wide range of thermal environments, and show little evidence of extirpation or decline.” Moreover, the documentation of a similar phenomenon operating among pygmy rabbits suggests still other animals may also be better able to cope with various aspects of climate change than we have been led to believe possible.
In a study of moose, Lowe et al. (2010) write, “intuitively, we would expect that a large northern ungulate with low tolerance for high temperatures would gradually be pushed out of the southern reaches of its range as the climate continues to warm and temperature conditions become increasingly unfavorable,” the logic being that “persistent temperatures above the upper critical limit will suppress foraging time and consequently cause mass loss during the summer, and that this reduced condition could affect overwinter survival and productivity,” citing the work of Schwartz and Renecker (1998).
The authors thus “tested the hypothesis that climate limits the southern distribution of moose (Alces alces) by documenting space use and behavior of 36 females at the margin of the species’ range in Ontario, Canada.” They did this in 2006, 2007, and 2008 through the use of “global positioning system (GPS) telemetry to study their habitat use and movement,” in an attempt “to document behavioral mechanisms indicative of adaptive responses to warm temperatures.” This work was conducted during periods of the year when ambient temperatures frequently exceeded known critical thresholds (-5°C in winter and 14°C in summer) that had been demonstrated by Dussault et al. (2004) to induce heat stress in moose.
Lowe et al. state they “detected no differences in habitat use relative to thermoregulation thresholds,” which they deemed to be particularly important during the summer, when they report the temperatures of all habitat classes greatly exceeded—by an average of 6°C, and by as much as 19°C in the first week of August 2006—the 14°C threshold for a large extent of the day and partially during the night. As a result, the three Canadian researchers conclude “moose in their southern range either ameliorate heat stress at a finer resolution than we measured or are more resilient to temperature than previously thought.”
In a contemporaneous study, Garroway et al. (2010) write, “many species have responded to contemporary climate change through shifts in their geographic ranges,” and they state “this could lead to increased sympatry [i.e., partially overlapping ranges] between recently diverged species, likely increasing the potential for hybridization.” They further note this phenomenon “can be positive if it increases genetic variability and creates new gene combinations that increase the potential to adapt.”
To test this hypothesis, between 2002 and 2004, Garroway et al. conducted more than 1,600 successful live-trappings of southern (Glaucomys volans) and northern (Glaucomys sabrinus) flying squirrels throughout portions of Ontario, Canada, and Pennsylvania, USA. From the hairs of these squirrels they extracted nuclear and mitochondrial DNA, which they analyzed in ways that allowed them to obtain the following results.
It already had been determined by Bowman et al. (2005) that G. volans had expanded its range from the south beginning in the mid-1990s in concert with a series of warm winters; and now the nine Canadian and U.S. researchers’ new findings indicate “the expansion of G. volans north into the G. sabrinus range in Ontario has resulted in the formation of a new hybrid zone.” In addition, their analyses suggest “the hybridization was recent, coinciding with the recent increase in sympatry.” Thus, they go on to state that, to their knowledge, “this is the first report of hybrid zone formation following a range expansion induced by contemporary climate change.” These unique findings indicate yet another way in which living organisms can both physically (by shifting their ranges) and genetically (by hybridization) successfully confront the challenges that may be presented to them by global warming.
Pockley (2001) reported the results of a survey of the plants and animals on Australia’s Heard Island, a small island located 4,000 kilometers southwest of Perth. Over the prior 50 years, this sub-Antarctic island had experienced a local warming of approximately 1°C that had resulted in a modest (12 percent) retreat of its glaciers. For the first time in a decade, scientists were attempting to document what this warming and melting had done to the ecology of the island.
Pockley began by stating the scientists’ work had unearthed “dramatic evidence of global warming’s ecological impact,” which obviously consisted of “rapid increases in flora and fauna.” He quoted Dana Bergstrom, an ecologist at the University of Queensland in Brisbane, as stating that areas once poorly vegetated had become “lush with large expanses of plants.” And he added that populations of birds, fur seals, and insects also had expanded rapidly. One of the real winners in this regard was the king penguin, which, according to Pockley, had “exploded from only three breeding pairs in 1947 to 25,000.”
Eric Woehler of Australia’s environment department was listed as a source of other equally remarkable information, such as the Heard Island cormorant’s comeback from “vulnerable” status to a substantial 1,200 pairs, and fur seals’ emergence from “near extinction” to a population of 28,000 adults and 1,000 pups. Given such findings, it can be concluded the regional warming experienced at Heard Island actually rescued these threatened animal populations from the jaws of extinction.
Bowman, J., Holloway, G.L., Malcolm, J.R., Middel, K.R., and Wilson, P.J. 2005. Northern range boundary dynamics of southern flying squirrels: evidence of an energetic bottleneck. Canadian Journal of Zoology 83: 1486–1494.
Dussault, C., Ouellet, J.-P., Courtois, R., Huot, J., Breton L., and Larochelle, J. 2004. Behavioural responses of moose to thermal conditions in the boreal forest. Ecoscience 11: 321–328.
Garroway, C.J., Bowman, J., Cascaden, T.J., Holloway, G.L., Mahan, C.G., Malcolm, J.R., Steele, M.A., Turner, G., and Wilson, P.J. 2010. Climate change induced hybridization in flying squirrels. Global Change Biology 16: 113–121.
Grinnell, J. and Storer, T.I. 1924. Animal Life in the Yosemite. Berkeley, CA: University of California Press.
Himes, J.G. and Drohan, P.J. 2007. Distribution and habitat selection of the pygmy rabbit, Brachylagus idahoensis, in Nevada (USA). Journal of Arid Environments 68: 371–382.
Howell, A.H. 1924. Revision of the American Pikas. North American Fauna No. 47. Washington, DC: USDA Bureau of Biological Survey.
Lowe, S.J., Patterson, B.R., and Schaefer, J.A. 2010. Lack of behavioral responses of moose (Alces alces) to high ambient temperatures near the southern Periphery of their range. Canadian Journal of Zoology 88: 1032–1041.
Millar, C.I. and Westfall, R.D. 2010. Distribution and climatic relationships of the American Pika (Ochotona princeps) in the Sierra Nevada and Western Great Basin, U.S.A.: periglacial landforms as refugia in warming climates. Arctic, Antarctic, and Alpine Research 42: 76–88.
Norment, C.J., Hall, A., and Hendricks, P. 1999. Important bird and mammal records in the Thelon River Valley, Northwest Territories: range expansions and possible causes. The Canadian Field-Naturalist 113: 375–385.
Pockely, P. 2001. Climate change transforms island ecosystem. Nature 410: 616.
Schwartz, C.C. and Renecker, L.A. 1998. Nutrition and energetics. In Ecology and Management of the North American Moose, edited by A.W. Franzmann and C.C. Schwartz, C.C., 441–478. Washington, DC: Smithsonian Institution.
Sumner, L. and Dixon, J.S. 1953. Birds and Mammals of the Sierra Nevada. Berkeley, Ca: University of California Press.