Creating a homogeneous avifauna

Abstract

I compared birds on urban gradients in two ecoregions of the United States by censusing summer resident bird populations at six sites in central California’s coastal chaparral and southwest Ohio’s eastern broadleaf forest. These sites represented comparable gradients of urban land-use which ranged from relatively undisturbed to highly developed and included biological preserves, recreational areas, golf courses, residential neighborhoods, office parks (or apartment complexes), and business districts. Species richness and Shannon diversity peaked at moderately disturbed sites and were significantly correlated between comparable land-use types in the two ecoregions. Bird abundance and biomass peaked at moderately disturbed sites as well but were not significantly correlated between ecoregions. The pre-development bird species (assumed to be those found at the most undisturbed sites) dropped out gradually as the sites became more urban and the number of remaining species was significantly correlated between ecoregions with only three (CA) and one (OH) species remaining in the most urban sites. Taxonomically, the bird communities at the least urbanized sites (the preserves and recreational areas) were very different with an average Jaccard’s index of species similarity of 0.065 while the most disturbed sites (the business district, apartment complexes/office park, residential areas and golf courses) had an average similarity of 0.185. The species assemblages along the gradient shifted gradually, demonstrating local extinction of and local invasion by different species as the sites become more urban.