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Rapid Range Shifts of Species Associated with High Levels of Climate Warming

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Science  19 Aug 2011:
Vol. 333, Issue 6045, pp. 1024-1026
DOI: 10.1126/science.1206432

Abstract

The distributions of many terrestrial organisms are currently shifting in latitude or elevation in response to changing climate. Using a meta-analysis, we estimated that the distributions of species have recently shifted to higher elevations at a median rate of 11.0 meters per decade, and to higher latitudes at a median rate of 16.9 kilometers per decade. These rates are approximately two and three times faster than previously reported. The distances moved by species are greatest in studies showing the highest levels of warming, with average latitudinal shifts being generally sufficient to track temperature changes. However, individual species vary greatly in their rates of change, suggesting that the range shift of each species depends on multiple internal species traits and external drivers of change. Rapid average shifts derive from a wide diversity of responses by individual species.

Threats to global biodiversity from climate change (1-8) make it important to identify the rates at which species have already responded to recent warming. There is strong evidence that species have changed the timing of their life cycles during the year and that this is linked to annual and longer-term variations in temperature (912). Many species have also shifted their geographic distributions toward higher latitudes and elevations (1317), but this evidence has previously fallen short of demonstrating a direct link between temperature change and range shifts; that is, greater range shifts have not been demonstrated for regions with the highest levels of warming.

We undertook a meta-analysis of available studies of latitudinal (Europe, North America, and Chile) and elevational (Europe, North America, Malaysia, and Marion Island) range shifts for a range of taxonomic groups (18) (table S1). We considered N = 23 taxonomic group × geographic region combinations for latitude, incorporating 764 individual species responses, and N = 31 taxonomic group × region combinations for elevation, representing 1367 species responses. For the purpose of analysis, the mean shift across all species of a given taxonomic group, in a given region, was taken to represent a single value (for example, plants in Switzerland or birds in New York State; table S1) (18).

The latitudinal analysis revealed that species have moved away from the Equator at a median rate of 16.9 km decade−1 (mean = 17.6 km decade−1, SE = 2.9, N = 22 species group × region combinations, one-sample t test versus zero shift, t = 6.10, P < 0.0001). Weighting each study by the √(number of species) in the group × region combination gave a mean rate of 16.6 km decade−1. For elevation, there was a median shift to higher elevations of 11.0 m uphill decade−1 (mean = 12.2 m decade−1, SE = 1.8, N = 30 species groups × regions, one-sample t test versus zero shift, t = 7.04, P < 0.0001). Weighting elevation studies by √(number of species) gave a mean rate of uphill movement of 11.1 m decade−1.

A previous meta-analysis (14) of distribution changes analyzed individual species, rather than the averages of taxonomic groups × regions that we used, and also included data on latitudinal and elevational shifts in the same analysis (18). It concluded that ranges had shifted toward higher latitudes at 6.1 km decade−1 and to higher elevations at 6.1 m decade−1 (14), whereas the rates of range shift that we found were significantly greater [N = 22 species groups × regions, one-sample t test versus 6.1 km decade−1, t = 3.99, P = 0.0007 for latitude; N = 30 groups × regions, one-sample t test versus 6.1 m decade−1, t = 3.49, P = 0.002 for elevation (18)]. Our estimated mean rates are approximately three and two times higher than those in (14), for latitude and elevation respectively, implying much greater responses of species to climate warming than previously reported (18). Most of the data we analyzed are from the temperate zone and from tropical mountains (table S1), where ecosystems are at least partly temperature-limited; different rates of change might be observed in moisture-limited ecosystems (19).

Published studies have shown nonrandom latitudinal and elevational changes (1, 7, 1317) but have not previously demonstrated a statistical linkage between range shifts and levels of warming. We found that observed latitudinal and elevational shifts (the latter more weakly) have been significantly greater in studies with higher levels of warming (mean latitudinal shift versus average temperature increase; N = 23 species groups × regions, Pearson correlation coefficient (r) = 0.59, P = 0.003; mean elevational shift versus temperature increase; N = 31, r = 0.37, P = 0.042). Temperature gradients differ across the world, so a given level of warming leads to different expected range shifts of species in different regions (20), assuming that species track climate changes. To estimate the expected shifts, we calculated the distances in latitude (kilometers) and elevation (meters) that species in a given region would have been required to move to track temperature changes and thus to experience the same average temperature at the end of the recording period as encountered at the start (18) (table S1). We found that both observed latitudinal and elevation range shifts were correlated with predicted distances (Fig. 1A, N = 20 species groups × regions, r = 0.65, P = 0.002 for latitude; Fig. 1B, N = 30 groups × regions, r = 0.39, P = 0.035 for elevation), so our analyses directly link terrestrial range shifts to regional and study differences in the warming experienced.

Fig. 1

Relationship between observed and expected range shifts in response to climate change, for (A) latitude and (B) elevation. Points represent the mean responses (±SE) of species in a particular taxonomic group, in a given region. Positive values indicate shifts toward the pole and to higher elevations. Diagonals represent 1:1 lines, where expected and observed responses are equal. Open circles, birds; open triangles, mammals; solid circles, arthropods; solid inverted triangles, plants; solid square, herptiles; solid diamond, fish; solid triangle, mollusks.

Despite reports that many species lag behind climate change (2123), nearly as many studies of observed latitudinal changes fall above as below the observed = expected line in Fig. 1A (9 points above, 11 below; χ2 = 0.20, 1 df, P = 0.65), suggesting that mean latitudinal shifts are not consistently lagging behind the climate. The lag in elevation response (Fig. 1B; 2 points above the 1:1 line, 28 below; χ2 = 22.53, 1 df, P < 0.001) is equally surprising because the required distances to track climate are much shorter than for latitudinal shifts (20). Real and apparent elevation lags may arise if suitable new conditions at higher elevations occur only in locations that cannot be reached easily (for example, on other mountain peaks), or they may reflect the topographic and microclimatic complexity of mountainous terrain [for example, cooler locations may be on poleward-facing slopes rather than higher (24)]; the need for finer-resolution analyses (25); and additional topographic, climatic, geological, and ecological constraints [for example, causing declines in cloud forest species (2628)].

Taxonomic differences are not consistent predictors of recent response rates. For example, birds seem to have responded least in terms of elevational shifts but had a slightly greater than expected latitudinal shift (Fig. 1). Much greater variation is associated with differences among species within a taxonomic group than between taxonomic groups (Fig. 2 and table S2). For latitudinal studies, on average 22% (average of N = 23 species groups × regions) of the species actually shifted in the opposite direction to that expected. Similarly, 25% of species shifted downhill rather than to higher elevations (average of N = 29 species groups × regions). Thus, despite an overall significant shift toward higher latitudes and elevations, which is greatest where the climate has warmed the most, and despite around three-quarters of species shifting poleward and to higher elevations, we found that species have exhibited a high diversity of range shifts in recent decades.

Fig. 2

Observed latitudinal shifts of the northern range boundaries of species within four exemplar taxonomic groups, studied over 25 years in Britain. (A) Spiders (85 species), (B) ground beetles (59 species), (C) butterflies (29 species), and (D) grasshoppers and allies (22 species). Positive latitudinal shifts indicate movement toward the north (pole); negative values indicate shifts toward the south (Equator). The solid line shows zero shift, the short-dashed line indicates the median observed shift, and the long-dashed line indicates the predicted range shift.

At least three processes are likely to generate the high diversity of range shifts among species: time delays in species’ responses, individualistic physiological constraints, and alternative and interacting drivers of change. Species may lag behind climate change if they are habitat specialists or immobile species that cannot colonize across fragmented landscapes (17, 2123), or if they possess other traits associated with low extinction or colonization rates (29). Species may also show individualistic physiological responses to different aspects of the climate, such as different sensitivities to maximum and minimum temperatures at critical times of their life cycles. These sensitivities will combine with variable wait times for different novel climatic extremes to take place (30). Species are also affected to different extents by nonclimatic factors and by multispecies interactions, which themselves depend on a diversity of environmental drivers (21, 28). For example, a species might retreat toward the Equator at its poleward margin if it contracts with habitat loss faster than it expands through climate warming; whereas the poleward range margin of a species that thrives in novel agricultural landscapes may spread at a rate exceeding that expected, were warming the sole driver.

We found that rates of latitudinal and elevational shifts are substantially greater than reported in a previous meta-analysis, and increase with the level of warming. We conclude that average rates of latitudinal distribution change match those expected on the basis of average temperature change, but that variation is so great within taxonomic groups that more detailed physiological, ecological and environmental data are required to provide specific prognoses for individual species.

Supporting Online Material

www.sciencemag.org/cgi/content/full/333/6045/1024/DC1

Materials and Methods

Tables S1 and S2

References (3151)

References and Notes

  1. Materials and methods are available as supporting material on Science Online.
  2. Acknowledgments: We thank A. Bergamini, R. Hickling, R. Wilson, and B. Zuckerberg for data; H.-J. Shiu for statistical assistance; S.-F. Shen, the Ministry of Education in Taiwan, a UK Overseas Research Scholarship Award, and the Natural Environment Research Council for support; and anonymous referees for comments on the manuscript. We are particularly grateful to the many thousands of volunteers responsible for collecting most of the original records of species. All data sources are listed in the supporting online material.
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