Date of Award:

5-1-2000

Document Type:

Thesis

Degree Name:

Master of Science (MS)

Department:

Biology

Committee Chair(s)

Edmund D. Brodie Jr.

Committee

Edmund D. Brodie Jr.

Committee

Gary E. Belovsky

Committee

James A. MacMahon

Abstract

Desert tortoises (Gopherus agassizii) were located on steep slopes at three sites in the western Mojave Desert. These sites were located at: 1) the rifle range of the Marine Corps Logistics Base, Barstow, California, 2) the southern boundary of the Fort Irwin National Training Center, California, immediately west of Manix Tank Trail, and 3) the Lava Range of the Marine Corps Air Ground Combat Center, Twentynine Palms, California. Additionally, a valley site at the Sand Hill range of the Marine Corps Air Ground Combat Center, Twentynine Palms, California, was included for comparison. Study tortoises were found on slopes up to 33 degrees at two of the sites. Substrate particle sizes were variable at the steep sites, but uniformly fine at the Sand Hill. Sites with steep slopes offered more cover-site options (burrows, pallets, caliche caves, rocks, rodent middens) than did Sand Hill (burrows, pallets). Home ranges were larger in the El Niño year of 1998 than in the drought year of 1997. Home range size was correlated with body condition (at a site with relatively little variation in body size and abundant forage) or body size (at a site with uniformly low body conditions and sparse forage) in 1997, but was not correlated with either factor in 1998. Desert tortoises were present on hillside sites at low to moderate densities relative to other populations in the western Mojave Desert. We found that desert tortoises occupy steep slopes as resident populations. A remodeling of potentially occupied habitat and increased surveys in areas with a high degree of topographic complexity may be in order for the entire Mojave Desert. It is likely that such surveys would increase not only the amount of known occupied habitat, but would also increase population size estimates across the Mojave Desert.

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