A plant invasion affects condition but not density or population structure of a vulnerable reptile

By altering ecosystem structure and function, invasions by nonnative plants have the potential to alter the quantity and quality of habitat for animals. We examined effects of buffelgrass (Pennisetum ciliare), a nonnative grass that is increasing in distribution markedly throughout the southwestern United States and northern Mexico, on demographic characteristics and condition of Sonoran desert tortoises (Gopherus morafkai). In 2010 and 2011, we established 50 4-ha plots that spanned the gradient of buffelgrass cover in areas with environmental features characteristic of high-quality habitat for tortoises in southern Arizona. We surveyed each plot four times per year to characterize density, population structure, and condition of tortoises. We detected tortoises on 45 of 50 plots (90 %) and ≥1 tortoise during 114 of 200 surveys (57 %). We used a hierarchical model to estimate density of tortoises while accounting for imperfect detection rates. Density of tortoises averaged 0.35 individuals/ha (SE = 0.04) and did not vary appreciably with the amount of buffelgrass cover; similarly, age and sex structure of tortoise populations did not vary with buffelgrass cover. Condition of adult tortoises, however, averaged 10 % lower in areas where cover of buffelgrass was high (>15 %) relative to areas where buffelgrass was absent or cover was low (<1 %). Although the demographic characteristics we measured on this long-lived species did not vary with current levels of buffelgrass cover, reduced condition of tortoises in areas invaded by this nonnative grass could manifest as population-level effects over longer time periods, especially because buffelgrass is predicted to expand its current distribution.