Effects of Flow Restoration, Vegetation Structure, and Bridges on Birds of the Lower Santa Cruz River

Abstract

Riparian areas in the arid Southwest are important refuges for many taxa that rely onthem for foraging, breeding, as corridors, and as a source of water. These areas are also disappearing though, and urbanization plays a major part in the degradation of these areas. As cities grow, natural vegetation is replaced with impermeable surfaces, resources are depleted, and aquifers are often drawn to support city growth. However, urbanization also has the potential to restore riparian areas. In this study, we focused on the use of effluent (treated wastewater) to restore riparian areas along the Santa Cruz River in Tucson, Arizona and how flow restoration affected bird assemblages. We identified and counted birds weekly at nine plots in each of four study reaches from February to April 2022. We measured environmental characteristics at each plot, including average flow of surface water, density of shrubs, amount of canopy cover, and distance to the nearest road bridge. Using these data, we set out to answer the following question: how do changes in stream flow, vegetation structure, and proximity to bridges affect bird communities overall and the population densities of individual species in an urban riparian system? We found that flow had a positive relationship with bird species richness, but no other measured environmental factor had a strong relationship with richness. Bird communities within each reach generally were distinct from one another, and changes in assemblage composition were correlated with flow, the density of shrubs, and canopy cover. Densities of five focal bird species varied among plots. Densities of two species were positively associated with stream flow while density of a third species was negatively associated with flow. Densities of other species varied with shrub density and canopy cover. Our results showcase how restoration of riparian areas using effluent has the potential to support unique species and assemblages of birds, and that a mosaic of different riparian areas support the highest diversity at a regional scale. We believe that collaborations between urban planners and ecologists when undertaking this type of project have the potential for significant positive impacts on riparian birds.