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

Changes in climate are impacting biodiversity on a global scale. Recent evidence has suggested that numerous terrestrial species are shifting their geographic ranges to higher elevations and latitudes in response to warming temperatures. However, previous studies have yet to demonstrate a direct link between the warming climate and species’ range shifts. Using a combination of data from several studies, Chen et al. (2011) analyzed the mean latitude range shifts across species of 23 taxonomic groups per region and the mean elevation range shifts across species of 31 taxonomic groups per region. The authors then compared these observed range shifts to expected range shifts necessary for taxonomic groups to remain in the same average temperature zone. The results suggest that the rates of terrestrial range shifts in latitude and elevation are two to three times faster than previously described. Additionally, the authors found that the observed latitudinal and elevation range shifts were correlated with the expected range shifts, suggesting a causal relationship between warming temperatures and terrestrial species’ range shifts. Despite these results, there was variation in the directional range shifts among species, indicating that other internal and external factors influence terrestrial species distributions.—Megan Smith
Chen, I., Hill, J.K., Ohlemuiler, R., Roy, D.B., Thomas, C.D., 2011. Rapid Range Shifts of Species Associated with High Levels of Climate Warming. Science 333, 1024 – 1026.

          Chen et al. collected data from previous studies to analyze the current rates of elevation and latitudinal range shifts of terrestrial taxonomic groups. Though other studies investigated the range shifts of individual species, the authors averaged the response of a taxonomic group in a specific region and used this mean as a single observation (i.e. plants in Switzerland). These authors determined range shifts by comparing the differences between two temporally separated recordings of a taxonomic group’s range margins (the average of a group’s upper/cold and lower/warm temperature range). The latitudinal shift of taxonomic groups was categorized as poleward, stable, or Equatorward while the elevation shift was categorized as up, stable, or down. A figure showing the observed latitudinal shifts of the northern range boundaries of species of four taxonomic groups in Britain was constructed.
          The authors extracted regional temperature increases from previous studies or identified the time periods and locations of the regions using CRU_TS2.0 data at 0.5° resolution. They gridded each region and then averaged the temperature across the grid cells to obtain a mean yearly temperature for the area in question. The regional temperature increase over each time period was then obtained by measuring the change in temperature between two temporally separate recordings. 
          The authors then derived the expected range shifts of the study’s taxonomic groups to assess the possible link between the changing climate and terrestrial species’ range shifts. Chen et al. calculated the expected elevation range shift by first computing the lapse rate, which is the decrease in degrees Celsius per increase in meters. For each region, Chen et al. divided the regional temperature increase by the lapse rate to calculate an estimate of the elevation increase or decrease a taxonomic group would have to make to remain within the same temperature range.
          Latitudinal range shifts were estimated by first calculating the temperature-distance transfer rate, which is the decrease in degrees Celsius per increase in kilometer of latitude, using CRU_CL2.0 data on a global 10’ grid. After gridding each region, the authors identified the nearest cell that was 0.5°C cooler than an original cell. The transfer rate was computed by dividing the temperature difference between the two cells by the latitudinal distance in kilometers between the cells. Then, Chen et al. averaged these measurements across every cell in the region to obtain the final transfer rate. The expected latitudinal shifts were determined by dividing the regional temperature increase by its corresponding transfer rate. These shifts represent an estimate of the latitude increase or decrease that a species would need to make to remain in the same temperature range. A figure comparing the observed and expected elevation and latitudinal range shifts for the taxonomic groups was constructed.
          The authors found that taxonomic groups shifted their boundaries north of the Equator at a median rate of 16.9 kilometers per decade and that species shifted to higher elevations by a median rate of 11.0 meters per decade. A previous meta-analysis study, which looked at individual species rather than taxonomic groups per region, reported that species’ shifts were increasing north of the Equator at a rate of 6.1 kilometers per decade and to higher elevations at a rate of 6.1 meters per decade. Chen et al.’s new rates suggests that species’ range shifts are moving at a much faster rate, indicating that terrestrial species are responding to climate change more rapidly than previously proposed.
          Most significantly, Chen et al. found a correlation between the observed range shifts of the taxonomic groups and their expected range shifts. Although other studies suggest that species lag in their response to warming temperatures, nearly equal amounts of taxonomic groups have exceeded expected range shifts as have fallen below in response to climate change. In contrast, the observed distances moved in elevation by species are much shorter than those proposed by the expected range shifts. This may be due to a variety of factors that include difficulty of movement at higher elevations and directional climate complexities found on mountainsides.
          Interestingly, although 75% of species moved north, 22% of species shifted southwards in latitude against expectations. Similarly, 25% of species shifted to lower elevations instead of following expected range shifts to higher elevations. Chen et al. identified three processes that could account for the diversity of species’ range shifts. These processes include time delays in species’ responses to climate change, physiological limits, and other interacting drivers of change. For example, some species may lag behind in response to climate change if they specialize in a certain habitat or if they are immobile. Other species may exhibit different responses to increasing temperatures at different stages in their life cycles. Species’ ranges may also be determined by non-climatic driving factors such as competition with other species and habitat loss.
          Although further studies investigating the physiological, ecological, and environmental drivers of species boundaries are needed to assess the variation in range shifts found in this study, Chen et al.’s findings overall suggest that species’ ranges are shifting faster than reported and that these range shifts are connected to rising temperatures worldwide. 

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