Factors Limiting the Upstream Distribution of the Sabine Map Turtle

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Abstract
Several environmental variables and population densities of Graptemys ouachitensis sabinensis were measured in a transect along Whisky Chitto Creek, Louisiana (USA), from downstream localities where turtles were abundant to upstream localities without turtles. Correlations among environmental variables and population density were calculated over six study sites and over 72 subdivisions of five of those sites. A causal model for the effects of the environmental variables on population density was hypothesized, and its consequences were investigated by path analysis. Algal density on logs, and basking site area emerged from this analysis as the most important proximate determinants of turtle density. Of the variables measured, stream width appeared to be the ultimate determinant of density, and thus of the upstream limit of the distribution, with positive indirect effects of width being somewhat greater than the direct effect. A discriminant function was used to classify patches of habitat as either favorable or poor and the distribution of favorable patches along the transect was investigated. Patches of favorable habitat tend to become smaller and more isolated upstream and use of these patches decreases with distance upstream. This information supports the hypothesis that distributional termination results from increasingly difficult dispersal and progressively reduced dispersal pools upstream, as well as from changing environmental factors.