Species Density and Taxonomic Diversity of Texas Amphibians and Reptiles

Abstract

Rogers, J. S. (Department of Biological Sciences, University of New Orleans, New Orleans, Louisiana 70122) 1976. Species density and taxonomic diversity. of Texas amphibians and reptiles. Syst. Zool. 25:26-40.-Species densities of Texas amphibians and . reptiles are highly correlated with several components of the physical environment. Multiple regression and partial correlation analyses of these relationships lead to several conclusions: 1) species densities of salamanders, frogs, turtles, lizards, and snakes increase significantly with increasing topographic relief, probably because of the greater number of habitat types that occur in uneven terrain versus flat terrain; 2) altitude, per se, has a negative effect upon species densities of all five groups, especially lizards and snakes; 3) species densities of salamanders and turtles increase with increasing mean annual precipitation, either because of increasing productivity or increasing numbers of aquatic habitats; 4) mean annual temperature and growing season have significant effects on species densities only for salamanders, temperature having a positive effect and growing season a negative effect; 5) with a few exceptions, seasonality and year-to-year variability of climatic factors are not correlated with species densities; and 6) species densities of snakes and, to a lesser degree, lizards increase with increasing species density of small mammals. The mean number of species per genus in each of the five groups of Texas amphibians and reptiles also exhibits considerable geographical variation. Much of this variation is caused by variation in species densities, since species per genus and species density are strongly positively -correlated. However, even after this correlation is statistically removed there is extensive geographical variation of numbers of species per genus. Some of this variation can be accounted for by environmental influences: 1) species per genus of frogs and snakes increase significantly with increasing mean annual precipitation, possibly due to increasing uniformity of habitats and 2) species per genus of frogs and, to a smaller degree, snakes increase with increasing altitude. This may be related to decreasing productivity of food resources with increasing altitude. Dauring the past decade geographical variation in species density (numbers of species per unit area) of North American vertebrates has received considerable attention (Simpson, 1964; Cook, 1969; Kiester, 1971; Wilson, 1974). Each of the researchers who have studied this problem has proposed causal relationships between species density and various environmental factors, including topographic relief, altitude and climate. However, only one (Wilson, 1974) attempted to analyze these possible relationships statistically. The lack of statistical analyses would seem a serious deficiency, because of strong correlations between many environmental variables. Another possible complicating factor is history. This might be especially serious when considering a large area such as North America, where northem parts were glaciated rather recently and southern extremes have probably never been glaciated. Is the paucity of species of some vertebrate groups in the north due to present climatic conditions, to insufficient time having elapsed for recovery from the.. last glacial advance, or to a combination of these factors? In the present .study I attempt to, deal with the first problem by analyzing relationships between species density and several environmental vari-ables using multivariate statistical techniques. Secondly, to reduce the effect of history, I confined the study to a much smaller geographical area (the state . of Texas), but one with sufficient variation in species density and environmental parameters to yield interesting re-