26th Annual Meeting and Symposium of the
Desert Tortoise Council, March 16-18, 2001
Abstracts

Results of Line Distance Transects Conducted at Two Marine Corps Ranges: Chocolate Mountains Aerial Gunnery Range and Marine Corps Air Ground Task Force Training Command, California

Peter Woodman
Kiva Biological Consulting, PO Box 1210, Inyokern, CA 93527

In 2000, line distance transects were conducted at two U.S. Marine Corps Ranges in the California Deserts: Chocolate Mountains Aerial Gunnery Range (CMAGR) in Riverside and Imperial Counties and the Marine Corps Air Ground Task Force Training Command (MCAGTFTC), Twentynine Palms, San Bernardino County, California. All of the transects were walked with three personnel. One individual (team leader) remained on the centerline at all times and was responsible for searching a belt one meter on either side of the centerline, thus they walked on or within 1 m of the transect line at all times. The other two individuals flanked the team leader, one on each side of the line. The two flankers generally walked a straight line, 3 to 5 m from the transect centerline. Approximately half of their search time was spent searching from their position and looking towards the centerline and the other half looking out from their position. If an object of interest was seen then the field worker would walk out too investigate, then walk back to the position they left before resuming their coverage of the transect. A transect took approximately two hours to walk. Approximately half of the transects were read using a Trimble Pro XRS GPS system with real time accuracy of one to five meters. The other half of the transects were read by using a 10 foot length of PVC pipe placed over each rebar for visibility so that the transect centerline could be determined at all times. In those areas where a minimum of two PVC poles could not be seen, surveyors flagging was placed on shrubs on the transect centerline so that the centerline was known at all times. The PVC poles were placed on the rebar the afternoon prior to their being read, generally the following morning. The PVC poles were collected as the transect was being searched A set of 12 adult tortoises (6 male, 6 female) were fitted with AVM transmitters at both sites in order to calculate g0 (the percentage of tortoises available to be observed). Generally, a set of ten tortoises (five male and five females) were located as many times as possible during the period of time that transects were walked, typically between 0700 and 1200 hours. Each tortoise could generally be found twice during the morning. The objective was to revisit each tortoise as many times as possible while transects were being walked.

Chocolate Mountains Aerial Gunnery Range

The approximately 387,200 acre Chocolate Mountains Aerial Gunnery Range is located in the central portion of the Colorado Desert, in Riverside County, California. It is a live-fire air to ground and air to air range. Most of the base is a buffer for approximately 30 target sites that are on the CMAGR. The target sites range from 200 to 1000 acres, are heavily impacted, and portions of many are virtually denuded. The surrounding habitat is impacted very little, if at all. Only two ground-based activities occur on the base: a Navy SEAL training area is on the west edge, and EOD clears the target areas of unexploded ordnance. Two utility corridors traverse the CMAGR: a Gas Company natural gas pipeline and an electrical transmission line. That portion of the CMAGR north and east of the Chocolate Mountains is within the Chuckwalla Bench Desert Wildlife Management Area (DWMA) and within Critical Habitat (the boundaries for both the DWMA and Critical Habitat are the same). Two strata were determined to be on the CMAGR, the Chuckwalla Bench Strata on the north end and the Milpitas Wash strata in the eastern portion. It was estimated that approximately half of the Chuckwalla Bench strata is on the CMAGR and that approximately 15% of the Milpitas Wash strata is on the CMAGR. In 1997, forty-one line distance transects were established within Critical Habitat on the CMAGR. The transects are four kilometers long, one kilometer on a side, and in the shape of a square or diamond. Thirty transects are in the Chuckwalla Bench strata and 11 transects are in the Milpitas habitat. The locations were not randomly selected but were spaced throughout the two strata. The transects at the CMAGR have now been read twice: 1999 and 2000. In 2000 the transects were read between 26 April to 4 May 2000. A total of 40 live desert tortoises were detected on 20 of the 41 transects (164 km walked) that were read in 2000 (Table 1). Thirty-four (85.0%) of the tortoises were in the Chuckwalla Bench and six (15.0%) were in the Milpitas Wash stratum. The encounter rate was 0.25 on the Bench and 0.09 on the Milpitas stratum. When the data are pooled for both strata, and all tortoises are included, the encounter rate was 0.24 tortoises per km. The encounter rate was slightly higher in 2000 on the Bench (from 0.21 to 0.25) and slightly lower on the Milpitas (0.12 to .09). The pooled encounter rate increased from .21 in 1999 to .24 in 2000. The density, when data for both strata are pooled, was 0.23 tortoises per hectare, a CV of 25.63%, with a 95% confidence interval from 0.14 to 0.37. Approximately 72,843 hectares of Critical Habitat were surveyed on the CMAGR. Thus these data indicate an estimate of 16,754 tortoises with a range of 10,198 to 26952 at 95% confidence. We found 178 tortoise carcasses on the 41 transects. Several tortoises were found that had died in 2000, thus it appears that the high mortality rates that began in the mid-1980's still continue. Two hundred twenty-five carcasses were found in 1999, many more than were found in 2000. The reason for the fewer carcasses may be that we were concentrating our search more at the centerline and not searching as wide a transect as we were in 1999, thus we did not find as many carcasses. This project was funded by Marine Corps Air Station, Yuma via Contract No. N68711-97-M-8808. Ron Pearce and Bob Riley (MCAS, Yuma) and Patricia Cutler (Southwest Division, Naval Facilities Engineering Command) provided invaluable assistance.

Marine Corp Air Ground Task Force Training Command

Marine Corp Air Ground Task Force Training Command (MCAGTFTC) is a 600,000 acre facility located within the Mojave Desert near the City of Twentynine Palms, San Bernardino County, California. MCAGCC is broken down into 22 Training Areas, functional units that allow different types of training to occur without jeopardizing safety. The primary mission of MCAGTFTC is to develop, conduct, administer, and evaluate the Marine Corp's Combined Armed Exercise (CAX) Training Program. It allows infantry troops, artillery and armored battalions, fixed-wing aircraft, and attack helicopters to work closely together in various maneuvers and exercises in a live-fire environment, allowing commanders to practice combat command control in a realistic setting. The MCAGTFTC provides training for approximately 1/3 of the Marine Corps on an annual basis. The MCAGTFTC is in the southeastern portion of the Western Mojave Recovery Unit as delineated in the Recovery Plan. The MCAGCC is not within a DWMA, the northwest corner of the MCAGCC (Sunshine Peak Training Area) is adjacent to the east edge of the Ord-Rodman DWMA. The north edge of the Joshua Tree DWMA is 15 to 20 miles south of the MCAGTFTC. The 62 transects were read between 14 to 20 April and 7 to 18 May 2000. A total of 29 live desert tortoises were detected on 19 of the 62 transects. Eight (27.6%) of the tortoises were on the surface when they were detected and 21 (72.4%) were visible in burrows. The encounter rate was 0.12 tortoises per kilometer. The estimate of density was 0.048 tortoises per hectare with a CV 28.33%. At 95% confidence the range was 0.027 to 0.083 tortoises per hectare. Approximately 114,330 hectares were sampled, thus the estimate of density is 5488 tortoises, with a range of 3087 to 9489 tortoises, at 95% confidence. The high CV and broad estimate indicate that a much greater sample size needs to be obtained. This can be accomplished by walking more transects, finding more tortoises on the established transects, or by pooling the data from several consecutive years and calculating an estimate of density with the pooled data. We found 66 tortoise carcasses on 36 of the 62 transects. We did not collect data for the carcasses due to our time constraint of walking transects during the morning or evening activity period. Thus, nothing can be said regarding the time since death, size classes, signs of disease, sex ratio, or cause of death. Several tortoises were found that had died in within the past year. This survey was funded by Marine Corps Air Ground Combat Center, Twentynine Palms through Contract No. N68711-99-M-6629. Rhys Evans and Carolyn Martus of MCAGCC provided logistical support. Rhys Evans (MCAGCC) and Patricia Cutler of Southwest Division provided technical support.