Population dynamics, survival, and movements of Texas tortoises in a national park in southern Texas

Abstract The Texas tortoise ( Gopherus berlandieri ) is considered a threatened species in Texas, USA, and is the only North American tortoise species without a conservation plan. The species is a prominent associate of the Tamaulipan thornscrub vegetation community, which experienced dramatic losses due to land conversion over the twentieth century. During 2008–2024, we monitored a population of Texas tortoises on a federally protected site in Cameron County, Texas, using a combination of capture‐mark‐recapture and either biannual (2008–2018) or annual (2019–present) visual encounter surveys. Using 11 years of the dataset (2014–2024), we modeled abundance, population trends, annual survival, and inter‐loma (i.e., habitat islands) movement using an open‐population spatial capture‐recapture model. We encountered 357 unique tortoises (252 during the 11 modeled years); the model estimated the population to be as high as 273 individuals in 2014 but only 204 individuals by 2024, with annual apparent survival estimates of 87% and 85% for males and females, respectively. The decline in park‐wide estimated abundance was mirrored in half of the individual survey units, with the steepest declines occurring in the units most heavily invaded by the invasive Guinea grass ( Megathyrsus maximus ). Although annual probability of inter‐patch movement was low, tortoises were more likely to move between lomas that were closer in proximity and did not require crossing open expanses of cordgrass prairie. Texas tortoises can be difficult to monitor owing to their low detectability and because they are more transient than previously thought. Large‐scale loss of thornscrub and increased vehicle traffic surrounding the park isolate the population and limit exchange of tortoises between the park and nearby off‐site lomas. We identified intra‐park management actions for improving the population's viability despite opportunities for connecting the park with a larger protected landscape becoming more limited.