Keeping up with climate change: species on the move

Butterfly

Editor's Introduction

Rapid range shifts of species associated with high levels of climate warming

Annotated by
Christine Naspinski

During the past century, many plant and animal species have been moving away from the Equator and to higher elevations. How fast is this happening, and is it getting faster? Are these shifts related to climate change? Can we use this information to predict how species will respond to climate change in a particular area?  To answer these questions, the authors of this paper assembled and analyzed a large set of data on many different plant and animal species from around the world.

Paper Details

Original title
Rapid range shifts of species associated with high levels of climate warming
Authors
I-Ching Chen Chris Thomas
Original publication date
Reference
Vol. 333, Issue 6045, pp. 1024-1026
Issue name
Science
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.

Report

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.

Video. Dr. Chris Thomas discusses the work he and his team did for this study.

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).

table s1a

Table S1a. Latitudinal range shifts of groups of species included in this study.

table s1b

Table S1b. Elevational range shifts of groups of species included in this study.

Overview

These tables summarize the data the authors used in their meta-analysis. Table S1a is for shifts in latitude; Table S1b is for shifts in elevation.

Each row of the table represents a group of species (taxon) in a given region (location). In the text, this is referred to as a "taxonomic group x geographic region combination" or just "taxonomic group."

The range shift data in these tables are also used in the graphs of Figure 1.

Column descriptions

Taxon: Species were grouped together according to shared characteristics. These groups are referred to as "taxons" here and as "taxonomic groups" in the text.

Margin: Range shifts can occur on either boundary of the range, or both. These boundaries are called margins. This column identifies whether the observed range shift for each group was measured at the upper margin, lower margin, or an average of the two.

The upper margin is the boundary of the colder side of the range and the lower margin is the boundary of the warmer side. For latitude shifts, the upper margin is closer to the pole and the lower margin is closer to the equator. For elevation shifts, the upper margin is at a higher elevation and the lower margin is at a lower elevation.

Duration: Most studies took range measurements during two multiyear time periods (shown in the Years of Study column). The duration of the observation is the difference between the midpoints of the two periods. These durations were used to calculate the rate of range shift (kilometers or meters per decade) reported in the text.

Mean of observed range shifts: An observed range shift (between the time periods) was calculated for each species in each study. The mean of those values for a taxonomic group was reported here.

SE of observed range shifts: SE is an abbreviation for standard error of the mean. It is a statistical measure of the variability of the data.

Expected range shifts: The expected range shift is how far a group would have needed to move its range between the two time periods to experience the same annual average temperature.

Temperature changes: The change in the regional annual average temperature between the midpoints of the two time periods. The temperature change was taken from the original studies or calculated by the authors using a global temperature data set from the Climatic Research Unit of the University of East Anglia in the United Kingdom: http://www.cru.uea.ac.uk/about-cru

Question 1

How fast are ranges changing?

The authors calculated a rate of range shift for each taxon/location group by dividing the observed range shift by the duration. These were used to calculate the overall median and mean rates for latitude and elevation. They conducted a one-sample t test to compare the sample (observed) mean rate of range shift to a mean of zero.

The median shift rates were 16.9 kilometers away from the Equator per decade and 11.0 meters uphill per decade. The small p-value results from the t tests indicate that both latitudinal and elevational shift rates were significantly different from zero.

Question 2

Are the range shift rates larger than previously reported?

The observed range shift rates were compared to those from a previous meta-analysis (Reference 14) using a one-sample t test of the current rate versus the previous rate.

Both latitudinal and elevational range shift rates were significantly higher than those reported previously. 

Question 3

Is there a correlation between range shifts and temperature change?

The authors used a statistical correlation test to determine the relationship between observed range shifts and temperature changes. There was a significant positive linear correlation for latitude. The linear correlation for elevation was also positive, but less strong.

Range shifts were significantly larger for groups which experienced larger temperature increases.

Question 4

How would ranges change if they depend only on annual average temperature?

For each of the taxon/location groups, the authors used regional temperature data to model how annual average temperature varied with position (latitude or elevation) in that area. Based on this and the temperature change over the duration of the study, they calculated the expected range shift.

This expected range shift is the distance the organisms would have to move over the duration of the study period to remain within the same climate. The expected range shifts are reported in Table S1 and used in Figure 1.

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 ttest versus 6.1 km decade−1t = 3.99, P = 0.0007 for latitude; N = 30 groups × regions, one-sample t test versus 6.1 m decade−1t = 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 (171317) 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. 1AN = 20 species groups × regions, r = 0.65, P = 0.002 for latitude; Fig. 1BN = 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.

Questions
  1. Are the observed range shifts of terrestrial organisms statistically linked to climate change?
  2. Do the observed range shifts track climate changes?
Hypotheses
  1. The observed range shifts have a positive linear relationship with the expected range shifts.
  2. The observed range shifts are equal to the expected range shifts.
Data analysis

The observed and expected range shifts were calculated as described in the notes for Table S1. The data were graphed in a scatter plot with observed shifts on the y-axis and expected shifts on the x-axis. Both axes start with negative values because some of the shifts were negative (toward the Equator or downhill), though most were positive (toward the pole or uphill).

Each point represents the mean shift for a taxon/location group. The standard error (SE) of the observed shift mean is shown by error bars extending up/down from the point. A diagonal 1:1 line (where observed shift equals expected shift) is provided for comparison.

  1. The authors did a correlation analysis to determine if the observed and expected shifts have a linear relationship. The strength of the relationship is indicated by the value of the Pearson correlation coefficient (r) and its associated p-value.
  2. The authors did a chi-square goodness-of-fit test to determine if the observed range shifts are equal to the expected shifts. The strength of this relationship is indicated by the value of chi-squared and its associated p-value.

The way the test is used here, a large p-value indicates that the 1:1 relationship describes the data well. Therefore, we can infer that species are tracking climate change. A small p-value indicates that the observed range shifts do not match well with the expected range shifts, and the species are not tracking climate change.

Results and conclusions
  1. There was a significant positive linear correlation between observed and expected range shifts for latitude. The linear correlation for elevation was also positive, but less strong. The authors conclude that there is a direct statistical link between range shifts and climate change.
  2. The observed latitudinal shifts are similar to the expected shifts; they appear to be tracking climate change. However, the observed elevation shifts are not well described by the expected shifts; they appear to lag behind climate change.

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.

Questions

For an observed range shift, how much variation is there among species in a taxonomic group? How does this compare to the variation among taxonomic groups?

Data analysis

Four taxonomic groups were selected as exemplars (typical examples) of latitude shifts. In Table S1a, these are groups #13 (spiders), #7 (ground beetles), #3 (butterflies), and #6 (grasshoppers and allies).

For each group, the observed range shift data were graphed in a relative frequency histogram with percentage of species on the y-axis and observed shift class on the x-axis. Vertical lines mark zero shift, median shift, and predicted (expected) shift.

Each column in a histogram represents the percentage of species that had an observed upper margin range shift in that class. For example, in Panel A, the left-most column indicates that approximately 1% of the spider species made a range shift between –100 km and –75 km.

Results and conclusions

All four groups had overall positive median and mean observed shifts, and were expected to move toward the pole based on the temperature change. However, there was considerable variability in each group, including species with negative (away from the pole) range shifts. The SE for these groups ranged from 7.9 km to 9.7 km, which are larger than the overall SE of 2.9 km for all latitudinal taxonomic groups.

The authors conclude that, although overall average range shifts correlate with climate warming, there is much variation in response among individual species. Therefore, the average range shifts do not provide enough information to predict how a single species will react to climate change.

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 (172123), 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 (2128). 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

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  52. 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.