Several factors have been postulated to account for the decline in species richness, e.g., the reduction of primary production with decreasing temperature, at least as long as enough water is available (species-energy hypothesis; Wright, 1983; Turner et al., 1988; Currie, 1991; Lennon et al., 2000; Currie et al., 2004; Evans et al., 2005; Hawkins et al., 2003). Higher energy availability also allows larger population sizes, which in turn reduces the risk of local extinction and could therefore result in larger numbers of species surviving at a particular altitude level (Evans et al., 2005; Carnicer et al., 2007).