25th Annual Meeting and Symposium of the
Desert Tortoise Council, April 21-24, 2000
Abstracts

Distribution and Relative Abundance of Desert Tortoises on the Marine Corps Air Ground Combat Center, Twentynine Palms, California

Peter Woodman¹, Gilbert O. Goodlett², and John Westermeier^3
1Kiva Biological Consulting, P.O. Box 1210, Inyokern, CA 93527
²EnviroPlus Consulting, 1660 W. Franklin, Ridgecrest, CA 93555
³Jones & Stokes, Inc., 17310 Red Hill Ave., Suite 320, Irvine, CA 92614-5600

In 1997 and 1999, surveys for the desert tortoise (Gopherus agassizii) were conducted on the Marine Corps Air Ground Combat Center (MCAGCC), Twentynine Palms, San Bernardino County, California. The survey encompassed approximately 595,860 acres and was conducted in the summer and fall of 1997 and the summer and winter of 1999.

A two-pronged approach was used to estimate the distribution and relative abundance of desert tortoises on the training areas. Two previously established 1 km² plots, Emerson and Sand Hill 2, were surveyed, for 28 days each, to provide reliable data on abundance, demographic parameters, and health of tortoises at these two locations. A total of 920 relative abundance transects were conducted on the 22 training areas in order to estimate distribution and relative abundance of desert tortoises throughout the MCAGCC. Data were also collected on human impacts on the training areas, including the number of vehicle tracks, roads, expended ordnance, and other range residue.

The Emerson Lake plot is located approximately 1 km east of the western boundary of MCAGCC. Fourteen live tortoises were found on the plot. Three tortoises were immature and the remainder were adults. Five of the adults were male and six were female. Tortoise abundance was estimated at 39 tortoises/mi2. Carcasses of 16 tortoises were found. Nine tortoises were estimated to have died within four years, but of the nine only three carcasses were of adults. Tortoise densities appeared to be stable. No tortoises had signs of Upper Respiratory Tract Disease (URTD) but three tortoises had minor amounts of peeling scute laminae. However, six tortoises (43%) had various amounts of trauma to the shell and forelimbs. We believe that this trauma was due to dogs (Canis familiaris).

The Sand Hill 2 plot is located in the southwestern portion of the MCAGCC. Thirteen live desert tortoises were found on the plot. Except for one subadult male, all of the tortoises were adults. Six tortoises were male and seven were female. Tortoise abundance was estimated at 77 tortoises/mi². Carcasses of 16 tortoises were found. Eleven tortoises were estimated to have died within four years, but of the 11, seven were juvenile or immature and four were adults. No tortoises had signs of URTD but the plastrons of two adult males were affected by cutaneous dyskeratosis. However, 11 (85%) tortoises had varying amounts of trauma to the shell and forelimbs. As at the Emerson plot, we believe this trauma to be due to dogs.

The relative abundance transects indicated that tortoises were present on all 22 training areas. Only three areas were estimated to have an abundance greater than 50 tortoise/mi². These areas were the south end of the Bullion Training Area, the western edge of the Emerson Lake Training Area, and the southern portion of the Sand Hill Training Area. These areas account for approximately 10,899 acres of habitat. Approximately 56,345 acres of habitat had an estimated abundance of 21-50 tortoise/mi² and approximately 111,398 acres had an estimated abundance of 6-20 tortoises per mi². Tortoise sign was not found in the northeastern portion of the MCAGCC in the Black Top, Lead Mountain and Lava Training Areas. This may be due to low elevations in the area as well as the large substrate that is typical of the area.

Forty-six live desert tortoises were observed on the 920 transects. Of the 46 live tortoises, 29 were visually inspected for signs of URTD and cutaneous dyskeratosis. One subadult male tortoise had conjunctivitis, a sign of URTD. This tortoise also had severe cutaneous dyskeratosis, peeling scute laminae, and was missing his right rear leg. No other tortoises had signs of URTD. Three other tortoises, all adult males, had cutaneous dyskeratosis, none of which were severe.

The carcasses of 125 tortoises were found on the transects. The size class structure of the carcasses was 92 adults, 25 immatures, and 8 juveniles. Sixty-seven (53.6%) of the carcasses were of tortoises that had died more than four years previously. The cause of death for 95 (76%) tortoises is unknown. The cause of death for 17 (13.6%) was believed to be predation, 16 of which were juvenile or immature. All six juveniles and one small immature were probably killed by common ravens (Corvus corax) and the remainder were believed killed by canids, probably coyotes (Canis latrans). Vehicles were thought to be the cause of death for 23 (18.4%) of the dead tortoises. Several other carcasses were found that had been crushed, but due to the type of bone fractures, they were probably dead when crushed.

Because of its mission as a training facility, the MCAGCC is impacted by a variety of human uses including off-road vehicle travel, target practice with various sizes and types of ordnance, camping, and trash. The most prevalent impacts are associated with vehicular travel, both on and off roads, and ordnance. Impacts are heaviest in the corridors used for travel to and from mainside to the training areas. These corridors are generally in the valley bottoms and gently sloping bajadas. The hills and mountainous areas tend to be less impacted and many of these areas have not been impacted by military training activities.

More than 700 vehicle tracks were found on ten (1.1%) of the 920 transects. Three hundred fifteen (34.2%) transects had fewer than 25 vehicle tracks counted, 118 of which had no vehicle tracks. Expended ordnance was found on 824 (89.6%) of the transects. More than 700 pieces were found on four transects, all of which were in target areas.

These surveys were funded by MCAGCC via contracts DACW05-95-d-003 with Jones & Stokes, Inc. and N68711-99-M-6629. Sharon Jones and Rhys Evans from MCAGCC and Bill Fisher from Southwest Division, Naval Facilities Engineering Command provided invaluable assistance. John Westermeir, Jones & Stokes, was the Project Manager and Richard Rust was the GIS specialist.