Response of Vegetation and Breeding Birds to the Removal of Cattle on the San Pedro River, Arizona (U.S.A.)

Abstract

Abstract:In late 1987 cattle were removed from the San Pedro Riparian National Conservation Area ( NCA ) in southeastern Arizona ( U.S.A. ). We monitored vegetation density and abundance of birds during the breeding season during 1986–1990 in riparian, mesquite grassland, and Chihuahuan desert‐scrub communities in the NCA. The density of herbaceous vegetation increased four‐ to six‐fold in riparian and mesquite grassland communities. Little change occurred in herbaceous vegetation in desert scrub, or in the density of shrubs or trees in any of the communities. Of 61 bird species for which sufficient data were collected, mean detections per kilometer increased for 42 species, 26 significantly, and decreased for 19 species, 8 significantly. The number of individuals of all avian species detected on surveys increased each year from 103/kilometer in 1986 to 221/kilometer in 1991, an average annual increase of 23% ( p< 0.001 ). The largest increases occurred in riparian species, open‐cup nesters, Neotropical migrants, and insectivores. Species of the Chihuahuan desert‐scrub, in which vegetation changed the least, showed the smallest increases. Only a few of the species showed increasing regional trends for the same period, as demonstrated by the North American Breeding Bird Survey; thus, increases on the San Pedro Riparian NCA were likely caused by the change in local conditions, not by regional effects. Our results suggest that removing cattle from riparian areas in the southwestern United States can have profound benefits for breeding birds.