title: Desert tortoises (Gopherus agassizii) are selective herbivores that track the flowering phenology of their preferred food plants.
abstract: Previous studies of desert tortoise foraging ecology in the western Mojave Desert suggest that these animals are selective herbivores, which alter their diet according to the temporal availability of preferred food plants. These studies, however, did not estimate availability of potential food plants by taking into account the spatial and temporal variability in ephemeral plant abundance that occurs within the spring season. In this study, we observed 18 free-ranging adult tortoises take 35,388 bites during the spring foraging season. We also estimated the relative abundance of potential food plants by stratifying our sampling across different phenological periods of the 3-month long spring season and by different habitats and microhabitats. This methodology allowed us to conduct statistical tests comparing tortoise diet against plant abundance. Our results show that tortoises choose food plants non-randomly throughout the foraging season, a finding that corroborates the hypothesis that desert tortoises rely on key plants during different phenological periods of spring. Moreover, tortoises only consumed plants in a succulent state until the last few weeks of spring, at which time most annuals and herbaceous perennials had dried and most tortoises had ceased foraging. Many species of food plants--including several frequently eaten species--were not detected in our plant surveys, yet tortoises located these rare plants in their home ranges. Over 50% of bites consumed were in the group of undetected species. Interestingly, tortoises focused heavily on several leguminous species, which could be nutritious foods owing to their presumably high nitrogen contents. We suggest that herbaceous perennials, which were rare on our study area but represented ~30% of tortoise diet, may be important in sustaining tortoise populations during droughts when native annuals are absent. These findings highlight the vulnerability of desert tortoises to climate change if such changes alter the availability of their preferred food plants.
authors: Departamento de Vertebrados, Museu Nacional, Universidade Federal do Rio de Janeiro, Rio de Janeiro, RJ, 20940-040, Brazil.
authors: W Bryan
authors: WB
authors: Jennings
authors: U.S. Geological Survey, Western Ecological Research Center, Falcon Business Park, 21803 Cactus Avenue, Suite F, Riverside, California, 92518, United States of America.
authors: Kristin H
authors: KH
authors: Berry
mesh_terms: D000818:Animals; D044822:Biodiversity; D002140:California; D002755:Choice Behavior; D003889:Desert Climate; D004032:Diet; D035264:Flowers / Q000502:physiology*; D005843:Geography; D060434:Herbivory / Q000502:physiology*; D010945:Plants, Edible; D012621:Seasons; D014426:Turtles / Q000502:physiology*
references: Comp Biochem Physiol A Comp Physiol. 1972 Mar;41(3):535-49
references: 4401729
references: J Wildl Dis. 1999 Apr;35(2):212-38
references: 10231748
references: Oecologia. 1983 Jan;56(1):84-92
references: 28310773
references: Science. 2007 May 25;316(5828):1181-4
references: 17412920
references: Nature. 2014 Feb 6;506(7486):47-51
references: 24499916
references: Proc Biol Sci. 1999 Jun 7;266(1424):1091-100
references: 10406128
references: Oecologia. 1976 Mar;24(1):21-42
references: 28308851
references: Oecologia. 1998 Dec;117(3):365-373
references: 28307915
references: J Wildl Dis. 1991 Apr;27(2):296-316
references: 2067052