West Mojave Plan FEIR/S - Desert Managers Group

• Since 1975, federal land managers (BLM and the military bases) have used “tortoise signcount” surveys to determine relative tortoise abundance and distribution over regionallandscapes.Desert Tortoise Presence-Absence Surveys (1990 - 2002): Since the tortoise was listedas threatened in 1990, city and county planning departments have required focused tortoisesurveys on undeveloped lands as per USFWS (1992) protocol. Such sites have been surveyedalong transects spaced at 30-foot intervals, which is intended to result in 100 percent coverage ofthe site and a determination of presence or absence of tortoises.In 1998, about 250 consultant’s reports for presence-absence tortoise surveys wereobtained from San Bernardino County. Of these, 234 reported either presence or absence oftortoise sign. An additional 595 presence-absence surveys associated with specific projects inSan Bernardino County and elsewhere were obtained in 2002 by the planning team, have beendigitized for GIS analysis, and are used in this analysis.There have been at least five other, recent programmatic surveys or summaries ofprevious surveys for tortoise occurrence within urban areas: (1) 225 square miles in Lancaster(Tierra Madre Consultants, Inc. 1991); (2) 200 square miles encompassing portions of Adelanto,Apple Valley, Hesperia, and Victorville (Tierra Madre Consultants, Inc. 1992); (3) 100 squaremiles in Palmdale (Feldmuth and Clements 1990); (4) 38 square miles in Ridgecrest andInyokern (Circle Mountain Biological Consultants 1997); and (5) 38 square miles in YuccaValley (Tierra Madre Consultants, Inc. 1993).Distance Sampling Surveys in the West Mojave Planning Area (2001 – 2002): Unlike thepresence-absence surveys discussed above and sign count surveys discussed below, distancesampling surveys are intended to look only for animals. Carcass information is also collected,but is not used to determine tortoise densities. These data are collected in the spring when adulttortoises are most likely to be active, depending on climatic factors, particularly rainfall. Theyprovide an independent look at tortoises during the spring (which can be compared to thesummer-fall observations of tortoises that were incidentally observed during sign count surveys).Using information collected during the 2001 survey, “encounter rates” were determinedfor each of the four proposed DWMAs. An encounter rate of 0.15 tortoises/kilometer indicatesthat the surveyors, on average, had to walk about 10 kilometers (about 6 miles) to see a tortoise.Encounter rates for the four DWMAs follow: 0.178 in the Ord-Rodman, 0.156 in PintoMountain, 0.145 in the Fremont-Kramer, and 0.115 in the Superior-Cronese. Higher encounterrates indicate that tortoises are seen relatively more often for the same unit area surveyed.Encounter rates are only a small part of determining tortoise densities. Althoughdensities were calculated for the 2001 distance sampling effort, they have wide-ranging“coefficients of variation,” which indicates that the estimates are very rough. For example, thenumber of tortoises per square kilometer was determined to be 11.66 for the Ord-Rodman, 10.31for the Pinto Mountain, and 9.58 for the Fremont-Kramer and 7.59 for the Superior-CroneseDWMAs (source: Memo distributed at MOG TAC meeting in about June 2001). Thecorresponding coefficients of variation were 14.46, 26.69, 15.38, and 18.21, respectively. ForChapter 3 3-79