Functional homogenization of bird communities along habitat gradients: accounting for niche multidimensionality

Aim To test whether functional homogenization of bird communities is promoted by anthropogenic landscape transformation, using specialization and habitat preference indices that account for the multidimensionality of niches. Location Catalonia, north-east Iberian Peninsula. Methods We used data on bird species occurrences and landscape features in 2834 1-km2 squares. Three orthogonal landscape gradients, which were taken as niche dimensions, were defined by means of principal components analysis (PCA). Specialization and habitat preference indices were created for 103 terrestrial bird species on the basis of their frequency of occurrence variation along the landscape gradients. These indices, together with species rarity, were then averaged for bird communities. We then analysed the patterns of variation of communities' mean specialization, mean rarity and mean habitat preference values along a gradient of agricultural-forest habitat mosaics. Results Wherever we found a significant variation in the degree of specialization along the agricultural-forest gradient, agricultural habitats held more specialized bird communities than did forest ones and bore, on average, rarer species. Thus, results contradicted our initial hypothesis that humanized areas would bear more functionally homogenized bird communities. Higher alpha-diversity values tended to be associated with generalist communities and with those having rarer species. Main conclusions Estimations of bird community specialization for different niche dimensions can behave differently along certain landscape gradients, and some of these differences can be explained by the variation of mean habitat preferences. Thus, we argue that a multidimensional approach to assess average niche breadth of communities can be more informative than a unidimensional measure. Our results suggest that widespread land abandonment and current secondary forest expansion throughout the Mediterranean area are promoting functional homogenization of bird communities. It would be desirable to construct larger-scale indicators of functional homogenization in order to monitor communities' responses to widespread landscape changes.