Introduction
Two macroecological patterns of species richness are sufficiently common and occur across such a wide range of taxa and geographic realms that they can be regarded as universal. The first is an increase in the number of species with the area sampled, the species–area relationship (hereafter SAR). The other is the relationship between species richness and the availability of energy that can be turned into biomass – the species–energy relationship (hereafter SER). Both patterns have a long history of exploration (e.g. Arrhenius, 1921; Gleason, 1922; Preston, 1960; Wright, 1983; Williamson, 1988; Currie, 1991; Rosenzweig, 1995; Waide et al., 1999; Gaston, 2000; Hawkins et al., 2003). However, attempts to interpret them within one unifying framework, or at least to relate them to each other, have been surprisingly rare. The most notable exception has been Wright's (1983) attempt to derive both patterns from the assumed relationship between total energy availability (defined as the product of available area and energy input per unit area) and population size. According to this theory, both area and energy positively affect species' population abundances, which decreases probabilities of population extinction, and thus increases the total number of species that can coexist on a site. Then, species richness should increase with increasing area or increasing energy in the same way.
Although this theory can be valid in island situations where the total number of species is determined by the rate of extinctions which are not balanced by immigration events (MacArthur & Wilson, 1967), the situation on the mainland is more complicated.