Let’s not blame climate change for all biogeographic change
Large numbers of biogeographical studies have demonstrated recent poleward range shifts and movement up altitudinal gradients of a variety of organisms (Bebber, Marriott, Gaston, Harris, & Scotland, 2007; Chen, Hill, Ohlemüller, Roy, & Thomas, 2011; Devictor, Julliard, Couvet, & Jiguet, 2008; Forero-Medina, Terborgh, Socolar, & Pimm, 2011; Groom, 2013; Hickling, Roy, Hill, Fox, & Thomas, 2006; Holzinger, Hülber, Camenisch, & Grabherr, 2007; Kelly & Goulden, 2008; Lenoir, Gégout, Marquet, de Ruffray, & Brisse, 2008; Leonelli, Pelfini, Morra di Cella, & Garavaglia, 2010; Parmesan & Yohe, 2003; Root et al., 2003; Smith, 1994; Sturm et al., 2001; Thomas & Lennon, 1999; Velásquez-Tibatá, Salaman, & Graham, 2012). The results of these studies are often referred to as the fingerprint of climate change on biodiversity. However, there are many manmade and environmental factors that have undergone far larger recent change than climate. These changes, either individually or in combination, can also explain these range shifts and we should not be so quick to implicate the climate.
Among the environmental factors that have changed significantly in the last 50-100 years are atmospheric nitrogen deposition; changes in grazing patterns, particularly in mountains; pollution changes, particularly acid rain and salt spreading; a doubling of the CO2 concentration of the atmosphere; a multifold increase in the distances and volume of the international horticultural trade; changes in land management; extensive greenhouse horticulture; heat islands caused by urbanization. In comparison to these factors, the climate has changed very little in the same period. All of these factors have been shown to affect organisms directly and many are known drivers of migration and/or extinction. Yet, papers continue to be published that stress the climate as an explanation.
Mankind is by far the most important disperser of plants and probably also animals and microorganisms (Mack & Lonsdale, 2001). Manmade habitat disturbance impinges on practically all ecosystems of the earth. Even in the Artic, which has experienced the greatest climate change so far, acid rain and atmospheric nitrogen deposition have a significant impact on the vegetation (Bobbink et al., 2010; Sarah J. Woodin, 1997). Manmade disturbance is often portrayed as a destructive influence on natural habitats but in reality its influence is more often for change rather than destruction.
In my own research on the native plants of Great Britain, northerly range shifts could be seen in many species since 1978 (Groom, 2013). However, these changes cannot be explained by the plant’s preferred climate envelope, but are more easily explained by other factors such as habitat change and pollution.
We should be careful about jumping to conclusions about the causes of biogeographic range shifts. It makes sense to look for explanations of change among the factors that have changed the most and only once these factors have been eliminated should one start looking for explanations elsewhere. While climate change will eventually have a large impact on the distribution of organisms, the focus on it as an explanation for all range shifts is obscuring other possible explanations and distorting our view of the changes in biogeography.
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Update 13 Aug. 2014: An example of a publication implicating grazing changes in tree line movements Aakala, T, Hari P, Dengel S, Newberry SL, Mizunuma T, Grace J (2014) prominent stepwise advance of the tree line in North-East Finland DOI: 10.1111/1365-2745.12308
This work by Quentin Groom is licensed under a Creative Commons Attribution 3.0 Unported License.