Studying the effects of climatic gradients within anthropogenic environments improves biogeographical inferences

Abstract

news and update ISSN 1948‐6596 commentary Studying the effects of climatic gradients within anthropogenic environments improves biogeographical inferences Due in large part to global climate change (Karl and Trenberth 2003), there is increasing interest among biogeographers in exploring and under‐ standing how species’ distributions are delineated by climatic conditions. Over the past several dec‐ ades, species have been responding to increasing temperatures by shifting their distributions to cooler climes at higher latitudes or elevations (Parmesan 2006). As a result, research efforts have increasingly focused on assessing how cli‐ mate defines species’ current and future distribu‐ tional limits along elevational and latitudinal gra‐ dients (Thomas 2010). A key consideration when studying eleva‐ tional and latitudinal gradients is that tempera‐ ture changes in a very different spatial fashion along each gradient. The tropospheric lapse‐rate, which is an estimate of the relationship between temperature and elevation within the tropo‐ sphere, averages 6.2 °C km ‐1 over the continents, with a range of 4.5 to 6.5 °C km ‐1 from the polar latitudes to the equator, respectively (Mokhov and Akperov 2006). In contrast, the relationship between temperature and latitude is nearly uni‐ form in the tropics, and declines roughly 0.007 °C km ‐1 on average north of the Tropic of Cancer and 0.005 °C km ‐1 on average south of the Tropic of Capricorn (F.A. La Sorte, unpublished). One would expect these unique spatial relationships with temperature to result in equally unique associa‐ tions with species’ distributional limits along these gradients. However, it is often assumed that ele‐ vational and latitudinal gradients function in a similar manner in defining distributional limits. When considering how dispersal processes are likely to differ along these gradients, one pre‐ diction is that the spatially compressed elevational gradient should result in species’ distributions ex‐ tending further into colder environments through more frequent and successful dispersal events (Halbritter et al. 2013). A primary challenge in testing this prediction is that experimentation is not always feasible along these gradients, and es‐ pecially latitudinal gradients, limiting the ability of investigators to control for the influence of non‐ climatic factors. Climate acts in combination with an array of abiotic and biotic factors in determin‐ ing species’ distributional limits (Sexton et al. 2009), and isolating the effect of climate is not always a simple task. For example, the role of cli‐ mate in defining species’ latitudinal responses to global warming may be surpassed by more inten‐ sive local or regional processes, such as land‐use change or management activities (La Sorte and Thompson 2007). When these processes cannot be identified and moderated, an alternative is to incorporate influential local and regional factors directly into the study design. A recent study in Global Ecology and Biogeography used such an approach to assess the comparability of species’ distributions along elevational and latitudinal gra‐ dients. Halbritter et al. (2013) documented the dis‐ tribution of 155 common ruderal herbs along an elevational gradient in the Swiss Alps and along a latitudinal gradient in Northern Europe. The two gradients vary in distance but both cover a similar range of temperatures. To minimize the effect of non‐climatic factors, plant surveys were con‐ ducted within disturbed habitats along roadsides and in human settlements, and species were se‐ lected that were strong dispersers and geographi‐ cally abundant. By incorporating an anthropogenic perspective into the study design, Halbritter et al. (2013) were able to sample plant communities at sites containing similar physical environments, thus providing inferences focused more exclu‐ sively on the effects of external climatic processes. Moreover, because these anthropogenic environ‐ ments were characterized by high disturbance frequencies and high levels of biotic interchange, distributions and climatic associations of these highly dispersive, ruderal taxa were likely to be estimated in a more accurate fashion along both gradients. Halbritter et al. (2013) found that tempera‐ 212 frontiers of biogeography 5.4, 2013 — © 2013 the authors; journal compilation © 2013 The International Biogeography Society