Star Cactus Herbivory

This project was a collaborative effort involving several botanists from Texas State University-San Marcos including Dr. Paula S. Williamson and Anna Strong as well as two Texas Parks and Wildlife Department botanists, Jackie M. Poole and Sandy Birnbaum and star private consultant Gena Janssen. Dr. Richard Strauss of Texas Tech University also contributed significantly through his statistical expertise and advice.

The focus of my portion of this project was on the effects mammalian herbivores were having on populations of the endangered star cactus (Astrophytum asterias) in Starr County, Texas. It is simply an ironic twist that the only remaining populations of star cactus are found within Starr County and in no way does the name of the cactus reflect the name of the county (which was named after James Harper Starr). The star cactus received its common name from the star like shape of the plant.

Individual star cactus from Starr County, Texas

Through the employment of Trailmaster motion sensor cameras we documented the two most common predators of star cactus: desert cottontails and Mexican ground squirrels.

Desert cottontail eating a star cactusPhoto of Mexican ground squirrel taken by the Trailmaster system in Starr County, Texas

We used transects and quadrat sampling for five populations of star cactus to assess the level of herbivore induced mortality among sites. We found that reductions in population sizes attributable to mammalian and insect herbivory varied among sites, ranging from 16 to 54% . However, these sites were located relatively close to one another, indicating that fines-scale differences in mortality risks and local forces influencing demographics of endangered aridland plants such as A. asterias could be critical to conserving such species. Our research demonstrated that even in a range-restricted species, threats may differ among local sites, requiring local, adaptive management strategies to effectively conserve the species.

Reticulate collared lizard captured in Starr County, Texas