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news & views ECOLOGY Moving farther and faster The distributions of terrestrial organisms are shifting in response to climate change. Research shows that these changes are happening at a much faster rate than previously estimated. Joshua J. Tewksbury, Kimberly S. Sheldon and Ailene K. Ettinger F orecasting the ecological effects of climate change is a tricky business, as species tend to move when conditions around them change. The movement of species affects everything from endangered-species management and reserve planning 1 to the spread of invasive species, agricultural pests and disease vectors2 (Fig. 1). Eight years ago, an analysis of published data on the distribution of terrestrial species demonstrated that many species were moving in response to climate change3, and estimated the average range shift per decade to be 6.1 km poleward and 6.1 m upwards in elevation. We have now received a startling update. By compiling data from many studies finished in the past eight years, I-Ching Chen and colleagues4 show the median rate of movement towards the poles is now 16.9 km per decade, almost three times as fast as previously estimated, and the median movement upwards in elevation is 11.0 m per decade, almost twice as fast as previously estimated4. 396 Tracking changes in species’ range limits requires taking detailed measurements at multiple points in time. Almost a decade ago, Camille Parmesan and Gary Yohe3 set out to document a “coherent fingerprint of climate change” in biological systems, yet at that time few ecologists were measuring and re-measuring range boundaries. Parmesan and Yohe thus had only three datasets they could use to examine the rate at which species were moving — one focused on British birds, another on Swedish butterflies and a third documented the shifts in alpine plants in Switzerland. After their paper was published, however, dusty natural history monographs of species’ presence and absence data suddenly became the justification for whole new expeditions to re-measure range boundaries, and the inherent value of systematic observation of the natural world became much clearer to many younger ecologists. Eight years after the work by Parmesan and Yohe3, the results of this work have now been summarized and published in Science 4. This time, instead of just three studies with quantitative estimates of movement, Chen and colleagues4 had 53 different studies — 23 estimating shifts in latitude and 31 estimating shifts in elevation. Yet the reason for the big change in the estimated rate of movement is not the number of studies, or even the location (a north-temperate bias still persists owing to the lack of investment in the natural history of the tropics). Instead, the discrepancy stems from the different time spans investigated in the two analyses, and the fact that the rate of warming is increasing 5. Two of the studies analysed by Parmesan and Yohe compared range boundaries at the beginning of the twentieth century with those at the end, whereas all but two of the 53 estimates used by Chen and colleagues compare range boundaries during the last half of the twentieth century. This makes a startling difference, because climate change during the twentieth century has not NATURE CLIMATE CHANGE | VOL 1 | NOVEMBER 2011 | www.nature.com/natureclimatechange © 2011 Macmillan Publishers Limited. All rights reserved been uniform; the rate of change in global surface temperature from 1970 to present is four times that observed from 1900 to 1970 (ref. 5). Range shifts are estimated per decade, so previous estimates probably included large periods of time during which there was little pressure on species to move. As the pace of climate change increases, so too does the rate at which species relocate.Because the rate of climate change is expected to increase further in the future5, we may expect upward revisions in the rates observed by Chen and colleagues. They were also able to make a critical link that had evaded previous researchers — they showed that the response rate was positively correlated with the amount of warming experienced: the greater the mean temperature change in a location, the farther the average shift in the range of the species measured in that location. This link suggests causality, but aspects of the relationship also point to the complexity of the response to warming. Observed shifts in elevation suggest that species are generally not moving fast enough to track climate up mountains, but this was not seen for latitudinal shifts; here, the median response was about as fast as expected based on the minimum distance needed to keep up with the changing location of annual mean temperatures. In other words, species are moving northwards at about the same pace as mean temperatures are moving. However, species are not ‘keeping up’ with elevational shifts in mean temperatures. This difference is probably caused by two issues, one biological and the other methodological. First, there are fundamental differences between latitudinal and elevational range boundaries. Habitat and substrate availability may strongly constrain upward movement at high elevations. Furthermore, mountains are topographically complex, providing diverse microclimates at a small scale, such that species may not need to shift as far or as fast to find climatically suitable habitat. Both of these issues point to biological differences between latitudinal and elevational range boundaries. However, there are also differences in the regional bias of the studies used in the analysis that could affect range-shift estimates. Most of the latitudinal studies were from northtemperate regions, whereas the majority of elevational studies were from tropical mountains. Differences between temperate and tropical locations extend far beyond differences in the amount of warming they experience. Substantial evidence suggests © FRANK PEAIRS/COLORADO STATE UNIV./BUGWOOD.ORG news & views Figure 1 | The southwestern corn borer, Diatraea grandiosella, is a serious agricultural pest in the United States and Mexico. Recent research by Chen and colleagues4 shows that in response to more rapid warming in the past 40 years, species are shifting their latitudinal range up to three times faster than previously reported. Movement towards higher latitudes and elevations could allow agricultural pests such as the corn borer to expand into new areas and undermine food security. that tropical and temperate communities may also differ in their sensitivity to climate change6 and their capacity to respond by moving 7. Chen and colleagues also found that some groups of species had moved farther poleward than would be needed to track changing mean temperatures. This result probably reflects the fact that range boundaries are not generally set by mean annual temperature, and are instead the result of a complex set of biotic and abiotic processes. Future work will probably provide researchers with a more robust set of expectations for shifting boundaries as we refine our understanding of the climate controls on species’ range limits. The headline from this work will clearly be that current rates of movement are much larger than previously reported, but the danger is that some readers will see this as evidence that most species are actually tracking changes in their climate. Yet, species shift their distribution in response to warming at different rates, and some species in a community may not shift at all8. This variation in movement among members within a community is the metric that will determine the rate at which existing communities unravel or disassemble9. Given the importance of biotic interactions in setting demographic rates and constraining species’ movements, such shifts in community structure may be as important as the movement of individual species. NATURE CLIMATE CHANGE | VOL 1 | NOVEMBER 2011 | www.nature.com/natureclimatechange © 2011 Macmillan Publishers Limited. All rights reserved From a conservation perspective, it may be the species left behind that we should be most focused on. Nevertheless, the current work by Chen and colleagues has broad implications. Many species are moving much faster than previously appreciated. This should give us pause as we design reserves, plan strategies for combating insect pests, and map the spread of communicable diseases, the majority of which spend time in mobile animal hosts10. ❐ Joshua J. Tewksbury, Kimberly S. Sheldon and Ailene K. Ettinger are in the Department of Biology, University of Washington, Box 351800, Seattle, Washington 98195, USA. e-mail: [email protected] References 1. Krosby, M., Tewksbury, J., Haddad, N. M. & Hoekstra, J. Conserv. Biol. 24, 1686–1689 (2010). 2.IPCC Climate Change 2007: Impacts, Adaptation and Vulnerability (eds Perry, M. L., Canzianai, O. F., Palutikof, J. P., van der Lindon, P. J. & Hanson, C. E.) (Cambridge Univ. Press, 2007). 3. Parmesan, C. & Yohe, G. Nature 421, 37–42 (2003). 4. Chen, I. C., Hill, J. K., Ohlemuller, R., Roy, D. B. & Thomas, C. D. Science 333, 1024–1026 (2011). 5.IPCC Climate Change 2007: The Physical Science Basis (eds. Solomon, S. et al.) (Cambridge Univ. Press, 2007). 6. Deutsch, C. A. et al. Proc. Natl Acad. Sci. USA 105, 6668–6672 (2008). 7. Dynesius, M. & Jansson, R. Proc. Natl Acad. Sci. USA 97, 9115–9120 (2000). 8. Moritz, C. et al. Science 322, 261–264 (2008). 9. Sheldon, K. S., Yang, S. & Tewksbury, J. J. Ecol. Lett. http://dx.doi. org/10.1111/j.1461-0248.2011.01689.x (2011). 10.World Health Organization The Control of Neglected Zoonotic Diseases (WHO, 2011); available at http://www.who.int/zoonoses/ control_neglected_zoonoses/en/index.html. 397