Avian Species Richness along the Urban-Rural Gradient of the San Marcos River
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Increasing urbanization and the resulting degradation of native vegetation cause a decrease in overall avian species richness. Riparian areas are important in urban areas because they may be the only suitable habitat available for many wildlife species. In Central Texas, ideal riparian habitat is characterized as a cottonwood-willow community. The San Marcos River, one of the most popular recreational rivers in Texas, has a narrow riparian corridor because of recreational activities and agriculture. This study was prompted by two important factors: the loss of riparian habitat along the San Marcos River with a resultant loss of bird species, and the possible economic impact that might occur from bird watchers attracted to restored habitat of the urban section of the San Marcos River. I conducted a baseline study of resident summer birds from the headwaters at Aquarena Center in San Marcos to the confluence of the San Marcos River with the Guadalupe River in Gonzales 152 km to the southeast. My initial hypothesis was that there would be greater species richness in more secluded areas of the river than in areas more heavily impacted by human activity. It was also my hypothesis that because of a lack of a natural floodplain, cottonwood-willow habitat would be minimal. I compared bird assemblages at 12 point-count stations along the San Marcos River in urban to more rural, secluded and protected areas. I recorded qualitative data on the diversity of trees and vegetation. Observations were made in summer 2001 and 2002 between 12 June and 15 July. The observation technique was a modification of the pointcount method. I assigned bird species to foraging guilds and assemblages (urban exploiters, suburban adapted and urban avoiders) related to their tolerance of human habitation. I observed 65 bird species along urban and rural portions of the San Marcos River and a portion of the Blanco River at the confluence with the San Marcos River. The urban section alone had 22 insectivores. I classified 33 bird species as urban avoiders, 30 species as suburban adapted, and two as urban exploiters. The urban sites included 19 urban avoiders. In the rural segment of the river, I recorded the most species (42) at Palmetto State Park, which also had the highest number of species for any station. My results contradicted the notion that avian richness declines in relation to an increasing urban gradient. The most impoverished sites for species richness were the most outlying urban site at John Stokes Park and two rural sites, US 90A and CR 101. My data also indicated that specific physical characteristics of a site influenced species presence and abundance.
The urban site with the least adjacent lawn and canopy had the most insectivores. No stations along the San Marcos River had a natural floodplain that supported new growth of cottonwood and willow or sycamore saplings. I have no conclusive evidence that supports the notion that the summer residents were hindered necessarily by the lack of this type of vegetation alone.
CitationSmythe, V. A. (2004). Avian species richness along the urban-rural gradient of the San Marcos River (Unpublished thesis). Texas State University-San Marcos, San Marcos, Texas.
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