Reassessing a lizard survery in Big Bend National Park, Brewster County, Texas

Long-term surveys of species density and community composition are essential to understanding and interpreting ecological changes. The goal of this research was to reinvestigate a lizard survey conducted in Big Bend National Park, Brewster County, Texas, from 1955 through 1960 and from 1968 through 1969. The original study quadrats and methods established by Degenhardt (1960), lizard/acre index, percent vegetative cover, and plant species composition for 2005 and 2006 were compared to previous survey years. Laboratory methods were used to investigate dietary overlap of the two most dominant lizards (Aspidoscelis marmorata and A. septemvittata) found in the quadrats to assess competition between the two species. New methods were introduced for assessing abundance of termites (Isoptera) and recording species specific vegetative cover. Lizard/acre numbers were compared to rainfall totals for all years of study using both pre-existing and newly acquired data.; Gross vegetation compositions were compared using before and after photograph sets, cover percentages, species lists and published quadrat descriptions. Results suggest an overall vegetative increase from quadrat to quadrat. Moreover, certain plant species have changed in their overall abundance. Of these it is important to note that Eragrostis lehmanniana is becoming a dominant grass in the Green Gulch 2 and Tornillo Flat quadrats. Jatropha dioica, Rhus trilobata, Florensia cernua, Atriplex canescens. Celtis palida and Glandularia wrightii have either disappeared from all or some quadrats where they were previously recorded. A rapid increase in Gymnosperma glutinosum. covering nearly 20% of the Burnham Flat quadrat, was also observed in 2005, the following year none of this species was recorded on this quadrat.; These data suggest that the notable increase in vegetation density in all quadrats has apparently caused a shift in lizard diversity. High vegetation densities (primarily shrub density) on the Green Gulch 1 quadrat appears to have excluded A. marmorata and replaced it with A. septemvittata, which were previously found at this elevation. Increased vegetation density is also the likely reason for the continued disappearance of Cophosaurus texanus from all quadrats where it was either the most common or second most common lizard found in the 1950s.