Climate Change 2001: Working Group II: Impacts, Adaptation and Vulnerability

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2.2.1.2. Predicted Biological Responses to Climatic Warming Trends

All organisms are influenced by climate and weather events. Physiological and ecological thresholds shape species distributions (i.e., where species can survive and reproduce) and the timing of their life cycles (i.e., periods of growth, reproduction, and dormancy) (Uvarov, 1931; MacArthur, 1972; Precht et al., 1973; Weiser, 1973; Brown et al., 1996; Hoffman and Parsons, 1997; Saether, 1997) In the face of a local environmental change, such as a systematic change in the climate, wild species have three possible responses:

• Change geographical distribution to track environmental changes
• Remain in the same place but change to match the new environment, through either a plastic or genetic response [a plastic response is a reversible change within an individual, such as a shift in phenology (timing of growth, budburst, breeding, etc.); a genetic response is an evolutionary change within a population over several generations, such as an increase in the proportion of heat-tolerant individuals]
• Extinction.

In many individual studies, careful experimental design or direct tests of other possible driving factors make attribution of response to climate change possible with medium to high confidence. These studies address three questions (see Sections 5.4 and 19.2):

• Are changes observed in natural systems during the 20th century in accord with predictions from known effects of climate and from bioclimatic theory?
• Given that species, communities, and ecosystems are responding to a complex function of factors, do statistical analyses identify climatic components that statistically explain most of the observed change?
• If so, how can these results guide future predictive models of biotic response to climate change?

Studies of responses to past large-scale climatic changes during the Pleistocene ice ages and the early Holocene provide a good basis for predicting biotic responses to current climate change. Overwhelmingly, the most common response was for a species to track the climatic change such that it maintained, more or less, a species-specific climatic envelope in which it lived or bred. Typically, a species' range or migratory destination shifted several hundreds of kilometers with each 1°C change in mean annual temperature, moving poleward and upward in altitude during warming trends (Barnosky, 1986; Woodward, 1987; Goodfriend and Mitterer, 1988; Davis and Zabinski, 1992; Graham, 1992; Baroni and Orombelli, 1994; Coope, 1995; Ashworth, 1996; Brandon-Jones, 1996). Extinctions of entire species, as well as observable evolutionary shifts, were rare. Phenological shifts may have occurred but cannot be detected with Pleistocene data.

For very mobile or migratory animals—such as many birds, large mammals, pelagic fish, and some insects—shifts of species range occur when individuals move or migration destinations change. Thus, these movements actually track yearly climatic fluctuations. In contrast, most wild species, especially plants, are sedentary, living their lives in a single spot because they have limited mobility or because they lack behavioral mechanisms that would cause them to disperse from their site of birth. Rather than occurring by individual movements, range changes in sedentary species operate by the much slower process of population extinctions and colonizations. Intertidal organisms represent a mix of these two extremes: Adults frequently are completely sedentary, but many species have free-floating planktonic larvae. The dispersal of this early life-history phase is heavily governed by ocean currents. As a result, changes in distribution are driven by a combination of changes in strength and pathways of currents as well as general changes in sea temperature.