Fig. 1.

Observed variation in the distribution of habitat land cover area, bird species richness, and total bird abundance along the altitudinal heterogeneity gradient in Catalonia (northeastern Spain). Patterns are reported for three bird functional groups: farmland birds, forest birds, and alpine birds. (A–C) Spline fit describing variation along the heterogeneity gradient of farmland, forest, and alpine habitats. (D–F) Observed variation in species richness along the heterogeneity gradient for farmland, forest, and alpine birds (2). Spline fits (thin dark lines) and OLS fits (wide gray lines) are illustrated. Note that species-richness patterns for each group parallel group-specific habitat availability gradients. (G–I) Observed variation of total bird abundance (community size) along the heterogeneity gradient for farmland, forest, and alpine birds. Maximum abundances at low heterogeneity sites, as predicted by the area-heterogeneity tradeoff hypothesis, were only observed in farmland birds (G). All patterns shown are significant at P < 0.0001.