Desert tortoises (Gopherus agassizii) are selective herbivores that track the flowering phenology of their preferred food plants

PLoS ONE
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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.

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Additional publication details

Publication type Article
Publication Subtype Journal Article
Title Desert tortoises (Gopherus agassizii) are selective herbivores that track the flowering phenology of their preferred food plants
Series title PLoS ONE
DOI 10.1371/journal.pone.0116716
Volume 10
Issue 1
Year Published 2015
Language English
Publisher Public Library of Science
Contributing office(s) Western Ecological Research Center
Description e0116716; 32 p.
Country United States
State California
County Kern County
Other Geospatial Desert Tortoise Research Natural Area
Online Only (Y/N) N
Additional Online Files (Y/N) N