Fig. 2.

Test of the predictions of the area-heterogeneity tradeoff hypothesis in forest birds of Catalonia. Observed variation of species richness and mean abundance along the heterogeneity gradient for three functional groups: forest generalists (A and B), wide-niche forest specialists (C and D), and narrow-niche forest specialists (E and F). Following previous analyses (2, 5), we considered a species as a forest generalist if it inhabited both forest and farmland habitats and a forest specialist if it was restricted to forest habitats. The forest-specialist group was, in turn, subdivided in two groups, wide-niche and narrow-niche forest specialists, using quantitative metrics (see details in ref. 5). The observed trends were exactly in the opposite direction of the predictions of the area-heterogeneity hypothesis.