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:
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):
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 animalssuch as many birds, large mammals, pelagic fish, and some insectsshifts 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.
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