Road Density as a Proxy for Urbanization Effects on Trachemys Scripta Elegans in the Lower Rio Grande Valley
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Trachemys scripta elegans (red-eared slider) is one of many turtle species often encountered in our lakes and streams and seen crossing the roads of Texas. Turtles are regularly killed while attempting to travel across roadways (Ashley and Robinson 1996). The expansion of roadways in the United States has been linked with an increase in male biased turtle populations beginning as early as the 1930s (Gibbs and Steen 2005). The Lower Rio Grande Valley is an appropriate locality to currently examine this phenomenon since this region of Texas has and is currently experiencing extreme levels of urbanization. I conducted this study at 36 sites within three counties (Willacy, Cameron and Hidalgo) of the lower Rio Grande Valley; these three counties vary in their level of urbanization. I selected an even number of sites within each county with variable road densities (i.e. high, medium, or low) within a 1 km buffer of the trapping site. I sampled each site for 50 trap days (1 trap day = 1 trap in the water for 24 hours). I recorded morphological measurements including carapace length, carapace width, plastron length, plastron width, body depth, and mass. I analyzed urbanization effects by comparing capture rates and sex ratios among counties and among road density classes. I used single factor ANOVAs with mean carapace length and mean Fulton-type condition factor to detect changes in populations among road density classes. Differences in capture rates were not detected among counties or road density classes but sex ratios were significantly male biased in Cameron County and the high road density class. Single factor ANOVAs revealed that for both males and females mean carapace length was smallest in the high road density class and increased as road density class decreased. This result was significantly different in females between high and low road density classes. No differences in Fulton-type condition factors were detected among road density classes. I conclude that roads are contributing to changes in the population structure of wild T. s. elegans; however, I cannot simply attribute these changes to roads. Historically, high levels of market hunting of these animals and broad land use changes in this region are also likely contributing to these changes.