Qualitative Responses to Climate Change

In the absence of evolutionary change, populations have four possible responses to climate change: expand, decline, move, or go extinct (see Cohn 1989, Peters and Lovejoy 1992, Gates 1993, Kareiva et al. 1993). If climate change improves the quality or total area of suitable habitat (e.g., removing abiotic or biotic barriers to occupancy), populations can expand or increase in density and new populations can be established. Alternatively, if climate change impacts a species negatively, populations can decline or individuals can disperse from affected areas to non- or less-affected habitat (including unoccupied sites or areas that were uninhabitable prior to climate change). Declines can also lead to extinction, especially in those species with limited dispersal capabilities or strict habitat requirements.

We already have observed evidence of species range shifts due to climate change (MacDonald and Brown 1992, Root and Schneider 1993, Pollard and Eversham 1995, Parmesan 1996, Hill et al. 1999, Parmesan et al. 1999; in mountain regions see IPCC 1996b, Murphy and Weiss 1992, Fleishman et al. 1998), and theory suggests that climate change could also drive range contractions (Lawton 1995). Central populations in the core of a species' range tend to have the highest abundance, and population sizes at the edges of a range are often smaller and more variable than those at the core. This phenomenon may place edge populations at greater risk of extinction than core populations (Watt et al. 1979, Brown 1984). Local extinction and range contraction can lead to population isolation. Genetic isolation, divergence, or inbreeding depression can also result from limited dispersal between isolated populations (Hewitt 1996, Saccheri et al. 1998).

One critical fact we have learned from basic ecology, paleoeco-logical records, and new evidence of shifting populations is that species often respond to climate change independently of other community members (Graham and Grimm 1990, Overpeck et al. 1992, Harrison 1993, Coope 1995).Therefore, even if every species were able to shift its range in response to global warming, climate change would impact the structure and composition of species assemblages. In addition, chains of climatic effects can cascade through a community, affecting a wide range of taxa (Pimm 1980, Coley 1998). For this reason, it is important to understand not only the direct impacts of climate change on a species of interest, but also how a species may be indirectly affected via impacts on other community members.

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