Land Use and Avian Species Diversity Along an Urban Gradient

TLDR

The study sites spanned an urban land‑use gradient from a biological preserve to a business district, ranging from relatively undisturbed to highly developed. The authors surveyed summer resident bird populations at six former oak woodland sites in Santa Clara County to examine how species distribution and community structure vary along this urban gradient. They compared each site to four additional sites of similar development and used canonical correspondence analysis of habitat variables (pavement, buildings, lawn, grassland, trees/shrubs) to link bird community patterns to habitat structure. Bird communities shifted from native species dominance in the undisturbed preserve to invasive species in the business district, with species richness, diversity, and biomass peaking at moderately disturbed sites, while pre‑development species gradually disappeared as urbanization increased.

Abstract

I examined the distribution and abundance of bird species across an urban gradient, and concomitant changes in community structure, by censuring summer resident bird populations at six sites in Santa Clara County, California (all former oak woodlands). These sites represented a gradient of urban land use that ranged from relatively undisturbed to highly developed, and included a biological preserve, recreational area, golf course, residential neighborhood, office park, and business district. The composition of the bird community shifted from predominantly native species in the undisturbed area to invasive and exotic species in the business district. Species richness, Shannon diversity, and bird biomass peaked at moderately disturbed sites. One or more species reached maximal densities in each of the sites, and some species were restricted to a given site. The predevelopment bird species (assumed to be those found at the most undisturbed site) dropped out gradually as the sites became more urban. These patterns were significantly related to shifts in habitat structure that occurred along the gradient, as determined by canonical correspondence analysis (CCA) using the environmental variables of percent land covered by pavement, buildings, lawn, grasslands, and trees or shrubs. I compared each formal site to four additional sites with similar levels of development within a two‐country area to verify that the bird communities at the formal study sites were representative of their land use category.