Those of you who routinely spend time outdoors may have noticed some disturbing trends over the last several years: Wildlife cannot always be found in the same place, or at the same time, as in the past. Studies have shown that, in many cases, this is because species are altering the timing of their life cycles and the placement of their home ranges. There is fairly conclusive evidence that changes in the timing of behaviors, such as migration and breeding, are closely related to variations in temperature. Specifically, because climate change has turned up the thermostat, "minimum appropriate temperatures" are reached earlier in the spring and later in the fall, causing a shift in when animals engage in certain behaviors. However, little work has been done to definitively link similar variations in geographic distribution with temperature change--until now.
Collaborators from the University of York, the Academia Sinica, Durham University, and the Centre for Ecology and Hydrology have just published a "meta-analysis" investigating this issue. Meta-analysis is a technique whereby researchers look for broad patterns in the results of multiple different studies that investigated related hypotheses. In this case, the authors looked at research documenting latitudinal and elevational range shifts (or lack thereof) for a variety of wildlife. The original field work was conducted in Europe, North America, Chile, Malaysia, and Marion Island, and covered as many as 1367 species in 31 different taxonomic groups.
The scientists calculated the average range shift across all species of a taxonomic group within a single region. In other words, they produced a single value for all birds in England, or all fish in the US, etc. The resulting analysis indicated that, on average, taxonomic groups have moved away from the Equator at a rate of 16.9 kilometers per decade, and have moved to higher elevations at a rate of 11.0 m uphill/decade. These values are significantly higher than those calculated by a meta-analysis from 2003 in which shifts were investigated at the species level. These differences may partly be due to the new data collected in the 8 years since the previous study, with which different species and habitats have been analyzed, and from which more extreme shifts may have been reported.
When the authors of the current study compared geographical range shifts with variations in local temperature, they found that changes in both latitude and elevation were significantly greater where temperatures had increased. The scientists also calculated the distances that individual species would need to move in order to consistently place themselves in habitats at the same temperature. These distances strongly correlated with shifts actually observed in the wild, indicating that species were "tracking" temperature changes and adjusting their behaviors accordingly.
Interestingly, both of these patterns were stronger for latitude than for elevation, despite the fact that less physical distance is required to achieve elevation-related temperature shifts. This may reflect the fact that temperature-appropriate elevations may be difficult to reach (for instance, require traveling between two mountain peaks separated by a valley), or the fact that elevation is associated with more complex variations in topography and microclimate (for example, cooler locations may be on different-facing slopes, as well as at higher elevations).
The researchers found that different groups of animals varied in how much, and in what way, they responded to increasing temperatures. For instance, while birds responded least in terms of elevation, they responded most in terms of latitude. Further, nearly a quarter of species shifted in the opposite direction as expected. This variation reflects a broader pattern of greater differences among species within a taxonomic group than among taxonomic groups, likely caused by time delays in responses (for instance, among immobile species or those that are not good at colonizing new locations), species-specific physiological constraints (including different sensitivities to temperatures), and alternative or interacting influences on habitat choice (for example, the need to coexist with a particular prey item).
In order to better understand which species are likely to shift their ranges, and why, it will be important to collect additional, finer-grained information on physiological, topographic, climatic, geological, and ecological constraints. These details should allow researchers to predict the consequences of habitat alterations and, potentially, develop plans to help species in need.
Chen, I.-C., Hill, J.K., Ohlemuller, R., Roy, D.B., Thomas, C.D. 2011. Rapid range shifts of species associated with high levels of climate warming. Science 333:1024-1026.
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