1.3.5.2 Changes in species distributions and abundances

Many studies of species abundances and distributions corroborate predicted systematic shifts related to changes in climatic regimes, often via species-specific physiological thresholds of temperature and precipitation tolerance. Habitat loss and fragmentation may also influence these shifts. Empirical evidence shows that the natural reaction of species to climate change is hampered by habitat fragmentation and/or loss (Hill et al., 1999b; Warren et al., 2001; Opdam and Wascher, 2004). However, temperature is likely to be the main driver if different species in many different areas, or species throughout broad regions, shift in a co-ordinated and systematic manner. In particular, some butterflies appear to track decadal warming quickly (Parmesan et al., 1999), whereas the sensitivity of tree-line forests to climate warming varies with topography and the tree-line history (e.g., human impacts) (Holtmeier and Broll, 2005). Several different bird species no longer migrate out of Europe in the winter as the temperature continues to warm. Additionally, many species have recently expanded their ranges polewards as these higher-latitude habitats become less marginal (Thomas et al., 2001a). Various studies also found connections between local ecological observations across diverse taxa (birds, mammals, fish) and large-scale climate variations associated with the North Atlantic Oscillation (NAO), El Niño-Southern Oscillation (ENSO), and Pacific Decadal Oscillation (Blenckner and Hillebrand, 2002). For example, the NAO and/or ENSO has been associated with the synchronisation of population dynamics of caribou and musk oxen (Post and Forchhammer, 2002), reindeer calf survival (Weladji and Holand, 2003), fish abundance (Guisande et al., 2004), fish range shifts (Dul?i? et al., 2004) and avian demographic dynamics (Sydeman et al., 2001; Jones et al., 2002; Almaraz and Amat, 2004).

Changes in the distribution of species have occurred across a wide range of taxonomic groups and geographical locations during the 20th century (Table 1.9). Over the past decades, a poleward extension of various species has been observed, which is probably attributable to increases in temperature (Parmesan and Yohe, 2003). One cause of these expansions is increased survivorship (Crozier, 2004). Many Arctic and tundra communities are affected and have been replaced by trees and dwarf shrubs (Kullman, 2002; ACIA, 2005). In north-western Europe, e.g., in the Netherlands (Tamis et al., 2001) and central Norway (EEA, 2004), thermophilic (warmth-requiring) plant species have become significantly more frequent compared with 30 years ago. In contrast, there has been a small decline in the presence of traditionally cold-tolerant species. These changes in composition are the result of the migration of thermophilic species into these new areas, but are also due to an increased abundance of these species in their current locations.

Table 1.9. Evidence of significant recent range shifts polewards and to higher elevations.

Altitudinal shifts of plant species have been well documented (Grabherr et al., 2001; Dobbertin et al., 2005; Walther et al., 2005a) (Table 1.9). In several Northern Hemisphere mountain systems, tree lines have markedly shifted to higher elevations during the 20th century, such as in the Urals (Moiseev and Shiyatov, 2003), in Bulgaria (Meshinev et al., 2000), in the Scandes Mountains of Scandinavia (Kullman, 2002) and in Alaska (Sturm et al., 2001). In some places, the position of the tree line has not extended upwards in elevation in the last half-century (Cullen et al., 2001; Masek, 2001; Klasner and Fagre, 2002), which may be due to time-lag effects owing to poor seed production/dispersal, to the presence of ‘surrogate habitats’ with special microclimates, or to topographical factors (Holtmeier and Broll, 2005). In mountainous regions, climate is a main driver of species composition, but in some areas, grazing, logging or firewood collection can be of considerable relevance. In parts of the European Alps, for example, the tree line is influenced by past and present land-use impacts (Theurillat and Guisan, 2001; Carnelli et al., 2004). A climate warming-induced upward migration of alpine plants in the high Alps (Grabherr et al., 2001; Pauli et al., 2001) was observed to have accelerated towards the beginning of the 21st century (Walther et al., 2005a). Species ranges of alpine plants also have extended to higher altitudes in the Norwegian Scandes (Klanderud and Birks, 2003). Species in alpine regions, which are often endemic and of high importance for plant diversity (Vare et al., 2003), are vulnerable to climate warming, most probably because of often restricted climatic ranges, small isolated populations, and the absence of suitable areas at higher elevations in which to migrate (Pauli et al., 2003).