1.3.5.3 Climate-linked extinctions and invasions

Key indicators of a species’ risk of extinction (global loss of all individuals) or extirpation (loss of a population in a given location) include the size of its range, the density of individuals within the range, and the abundance of its preferred habitat within its range. Decreases in any of these factors (e.g., declining range size with habitat fragmentation) can lower species population size (Wilson et al., 2004). Each of these factors can be directly affected by rapid global warming, but the causes of extinctions/extirpations are most often multifactorial. For example, a recent extinction of around 75 species of frogs, endemic to the American tropics, was most probably due to a pathogenic fungus (Batrachochytrium), outbreaks of which have been greatly enhanced by global warming (Pounds et al., 2006). Other examples of declines in populations and subsequent extinction/extirpation are found in amphibians around the world (Alexander and Eischeid, 2001; Middleton et al., 2001; Ron et al., 2003; Burrowes et al., 2004). Increasing climatic variability, linked to climate change, has been found to have a significant impact on the extinction of the butterfly Euphydryas editha bayensis (McLaughlin et al., 2002a, 2002b). Currently about 20% of bird species (about 1,800) are threatened with extinction, while around 5% are already functionally extinct (e.g., small inbred populations) (Sekercioglu et al., 2004). The pika (Ochotona princeps), a small mammal found in mountains of the western USA, has been extirpated from many slopes (Beever et al., 2003). New evidence suggests that climate-driven extinctions and range retractions are already widespread, which have been poorly reported due, at least partly, to a failure to survey the distributions of species at sufficiently fine resolution to detect declines and to attribute such declines to climate change (Thomas et al., 2006).

A prominent cause of range contraction or loss of preferred habitat within a species range is invasion by non-native species. Fluctuation in resource availability, which can be driven by climate, has been identified as the key factor controlling invasibility (Davis et al., 2000). The clearest evidence for climate variability triggering an invasion occurs where a suite of species with different histories of introduction spread en-masse during periods of climatic amelioration (Walther, 2000; Walther et al., 2002). Climate change will greatly affect indigenous species on sub-Antarctic islands, primarily due to warmer climates allowing exotic species, such as the house mouse (Mus musculus) and springtails (Collembola spp.), to become established and proliferate (Smith, 2002). A prominent example is that of exotic thermophilous plants spreading into the native flora of Spain, Ireland and Switzerland (Pilcher and Hall, 2001; Sobrino et al., 2001). Elevated CO2 might also contribute to the spread of weedy, non-indigenous plants (Hattenschwiler and Korner, 2003).