Effects of human land use on avian functional and taxonomic diversity within the upland coastal zone of the North American Great Lakes

Breeding birds in North America’s eastern and boreal forests have experienced significant population declines in recent decades due largely to habitat loss. The North American Great Lakes coastal zone provides critical stopover and breeding habitat for millions of migratory and resident birds with diverse life history traits. Extensive human land use in this region has resulted in significant forest loss. To understand how functional and taxonomic diversity of breeding bird communities have been affected by agriculture and urbanization in this region, we calculated measures of functional diversity, Shannon’s diversity index, and species richness along a gradient of human land use intensity. We analyzed bird survey data collected at 2982 locations within 1 km of the shoreline where upland forest was the dominant habitat. We used hierarchical partitioning to determine the extent to which spatial confounding factors (i.e., broad climatic and biogeographic gradients across the five Great Lakes) accounted for observed differences in diversity measures, and then used fourth-corner analysis to describe the relationship between species, functional traits, and land cover types. Although spatial confounding factors accounted for some variation, functional evenness, species richness, and Shannon’s diversity index declined significantly with increasing human land use. Species negatively associated with increasing human land use were those that eat primarily invertebrates by foraging on foliage or bark (75%) and species categorized as long-distance migrants (58%), which suggests increased vulnerability of forest-dependent species to habitat modification and reductions in habitat availability. Functional evenness declined with increasing human land use, while other measures of functional diversity (functional dispersion and Rao’s quadratic entropy) remained relatively constant. Understanding the disconnect between functional and taxonomic diversity of bird assemblages and quantifying the degree of resilience of functional community properties is critical for predicting long-term effects of both habitat loss and ecological restoration on biodiversity and ecosystem function.