Semi-arid habitats are vulnerable to changes in the vegetative community. Exotic grasses which are adapted to semi-arid conditions have greatly altered the composition of the ground level flora resulting in a decrease of native grasses and forbs that are important to the resident animal species. Past studies of the threatened Texas tortoise (Gopherus berlandieri) suggest exotic grasses limit resource availability and reduce habitat quality. Exotic grasses may constrain Texas tortoise by altering microclimate, restricting tortoise movement, and reducing availability of preferred forage. The purpose of this experiment was to determine the effect of altered habitat composition from exotic grasses on tortoise growth, home range size. In 2014-2015, 203 Texas tortoises were located, marked and measured for carapace length and body mass in Tamaulipan thornscrub habitat in South Texas. Of these, 30 tortoises were radio-tagged and located weekly to determine home ranges. Ground cover was estimated using the point - intercept method at each location and throughout the available thornscrub habitat. Home range size and ground cover parameters were compared using regression analysis and compositional analysis. Temperature loggers were placed under patches of native and exotic grass to assess differences in suitability of exotic grasses and native grasses for thermal shelter.
Results/Conclusions
Texas Tortoise home range sizes did not vary based on ground cover parameters. Bare ground was the strongest parameter in selection of Texas tortoise home ranges, with forbs being selected for over both exotic and native grass. Within their home range, inactive tortoises exhibited a strong selection sites with rat middens and cactus, likely as thermal or hiding shelter. Forbs were associated with these inactive sites more often than exotic and native grasses. Mean adult female carapace length was positively correlated with forb cover within the home range. Native grasses provided a more suitable thermal microclimate than exotic grasses, with lower mean maximum temperatures and fewer days above stress levels. While our results do not contradict the idea that exotic grasses may limit Texas tortoises, other factors interact in determining home range size and tortoise biology. Forbs were the potential dietary resource that had the most significant impact on growth and site selection. Exotic grasses have been shown to reduce forb availability and could have great impacts on Texas tortoise populations. However, in habitats where bare ground is plentiful, it is unlikely that exotic grass cover will exclude other cover types enough to affect overall Texas tortoise functioning.