by Samantha Thompson
Species have widely been affected by changes in global climate but to what extent is uncertain, though predictions of species decline are often urgent. For example, one prominent analysis predicted that 15 to 37% of species would be endangered or extinct by 2050 (Moritz, 2013). Another predicts more than a 50% loss of climatic range by 2080 for some 57% of widespread species of plants and 34% of animals (Moritz, 2013). Montane taxa are expected to lose range area as they shift northward with warming (Moritz, 2013). Craig Moritz et al. point out that fossil records suggests that most species have persisted through past climate change, whereas forecasts of future impacts predict large-scale range reduction and extinction. Moritz et. al. explore the apparent contradiction between observed past and predicted future species responses summarizing salient concepts and theories and by reviewing broad-scale predictions of future response and evidence from paleontological and phylogeographic studies of past responses at millennial or greater time scales. Bringing the two ideas together, the authors consider evidence for species responses to recent twentieth century climate changes and place them in a management context.
The vulnerability of a given species to climate change can be viewed as a combination of exposure and intrinsic sensitivity. These factors are mediated by the capacity of local populations to buffer climatic alterations in situ via plastic reactions (including behavioral responses) or genetic adaptation, or by shifting geographically to track optimal conditions. Exposure is typically measured as shifts in mean precipitation or temperature at the mesoscale (e.g., 1 to 100 km2).
Plastic responses are undoubtedly important for short-term persistence but they can also entail costs and may be insufficient to avoid extinction. Evolutionary rescue requires moderate-to-high heritability of key traits or high potential growth rates of populations, with critical adaptations tracking the rate of climate change.
Forecasts of potential species responses to future climate change come largely in two forms: correlative mechanistic models of individual species, and prediction of higher-level properties such as species richness or turnover. Correlative models are currently the most widespread and scalable method, but they have inherent limits. These models typically apply some form of climate envelope approach, assessing whether the (realized) climate niche occupied by a species continues to exist within the current geographic range and whether it will shift elsewhere or cease to exist. Correlative models are probably a better measure of exposure than of species vulnerability to climate change.
The fossil record and the imprint of history in geographic patterns of DNA diversity (phylogeography) provide information that can be correlated with how species responded to past shifts in global temperature. These sources of information on historical responses have distinct limitations that can be partially overcome by combining types of evidence. The fossil record varies in extent and resolution according to preservation conditions. Phylogeographic analysis, on the other hand, affords higher spatial resolution but typically has low temporal precision compared with fossils.
Comparative phylogeographic studies, often combined with paleoclimatic modeling of geographic ranges, offer another window on past species responses and can identify regions in which taxa persisted through past climate change. When combined with fossil evidence and spatial models, such studies highlight the extent of range shifts but also the importance of scattered microrefugia, which are important for range recovery and perhaps also harbor distinct adaptations. Going further, direct DNA analyses of subfossils provides a much clearer picture of population dynamics through climate change and, for megafauna, highlight differences among species in response to the twin challenges of climate change and human colonization.
The discord between predictions of high extinction under future climate change and relatively high resilience through paleoclimatic change could be partly due to the limitations of the fossil record, but may also reflect the fact that species were previously able to persist in the absence of human-caused impacts on natural systems. In historic times, even though the rate of expected future change may be much faster than that over the past century, there is value in examining how species have responded to climate change over the 20th century.
Shifts in phenology are widely observed in the 20th-century record and could cause temporal mismatch between strongly interacting species, especially where these species employ different environmental cues. As expected with warming, decreasing body size has been observed in several studies of birds and mammals. This response seems to be plastic rather than genetic, or it may be related to extended food availability rather than direct physiological effects.
As yet, no species extinctions are clearly attributable to climate change per se, although several studies recorded local extinctions and population declines. Nevertheless, it is very difficult to establish causative relationships between warming and population declines or extinction, due to the interaction with other anthropogenic factors such as habitat loss or previously unseen pathogens. A recurring message is that we have insufficient knowledge of the proximate cause(s) of observed species declines under global warming: The few examples appear to be more closely related to indirect ecological effects than to demonstrable physiological challenges.
Moritz, C., Agudo, R., 2013. The future of species under climate change: resilience or decline? Science 341, 504-508. http://bit.ly/1zguwEh