INVASIVE PLANT SPECIES ALTER DIETS AND REDUCE FITNESS AND SURVIVAL IN JUVENILE MOJAVE DESERT TORTOISES

Kristina Drake; UC Davis, SDSU, & US Geological Survey; kkdrake@ucdavis.edu; Kenneth E. Nussear, Todd C. Esque, Andrew J. Berger, Lizabeth Bowen, Shannon Waters, Nathan Custer, Rebecca Lewison, A. Keith Miles

Habitat disturbance in the Mojave Desert alter the native plant composition used for food by Mojave desert tortoises. Food species such as native grasses and native forbs are often intermixed or replaced with non-native grasses such as red brome. We hypothesized that changes in available forage species and the nutrition they provide would negatively influence tortoises. To test this hypothesis, we conducted an experiment with juvenile tortoises to monitor the effects of altered diets on their overall body condition, immunological responses (gene transcription), growth, and survival. Tortoises were fed either a mixture of four native forbs, a native grass (Vulpia octoflora), a non-native grass (B. rubens), and native forbs combined with either a native or a non-native grass. Tortoises fed forbs had improved body condition and immunological responses, grew more, and had higher survival rates than those consuming other diets. Health and body condition quickly declined for tortoises foraging only on the native grass or non-native grass. Within four months, 32% of individuals fed only V. octoflora and 37% fed only B. rubens were found dead or removed from the experiment due to poor condition. In contrast, tortoises fed forbs had zero mortality and none were removed from the experiment.

Ecology and Conservation of Desert Tortoise     Student Paper