The Relative Abundance of Desert Tortoises on the Nevada Test Site Within Ecological Landform Units - September 1998

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The Relative Abundance of Desert Tortoises on the Nevada Test Site Within Ecological Landform Units WORK PERFORMED UNDER Prepared for the U.S. Department of Energy Nevada Operations Office Environmental Protection Division Las Vegas, Nevada 89193-8518 Roy Woodward, Bechtel National Kurt R. Rautenstrauch, Science Applications International Corporation Derek B. Hall, Bechtel Nevada W. Kent Ostler, Bechtel Nevada

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This Report was prepared as an account of work sponsored by an agency of the U.S. Government. Neither the U.S. Government nor any agency thereof, nor any of their employees, makes any warranty or representation, expressed or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufac turer, or otherwise does not necessarily constitute or imply its endorsement, recommendation, or favoring by the U.S. Govern ment or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the U.S. Government or any agency thereof. This report has been reproduced from the best available copy. Available to DOE and DOE contractors from the Office of Scientific and Technical Information, P.O. Box 62, Oak Ridge, TN 37831. (423) 576-8401. Available to the public from the National Technical Information Service, U.S. Department of Commerce, 5285 Port Royal Road, Springfield, VA 22161.

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3.2 Transect Selection and Survey Protocol ....................................................................11 4.2 Revised Map of Tortoise Abundance ........................................................................17 1 Major roads, topographic features, and administrative areas of the Nevada Test Site .......... 3 2 The relative abundance of desert tortoises on the Nevada Test Site prior to 1996 3 The relative abundance of desert tortoises on 206 ecological landform units on the Nevada Test Site surveyed during 1966 ....................... 15 4 The relative abundance of desert tortoises on the Nevada Test Site, based on 1996 field surveys and historic data............................................................................................... 19 1 Relationship of sign detected per kilometer of transect walked to categories of abundance of desert tortoises ................................................................................................. 5

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Sign-survey transects were sampled in 1996 to better determine the relative abundance of desert tortoises on the Nevada Test Site (NTS). These transects were sampled within ecological landform units (ELUs), which are small, ecologically homogeneous units of land. Two-hundred and six ELUs were sampled by walking 332 transects totaling 889 kilometers (km) (552 miles [mi]). average of 0.32 sign was found per km walked. Seventy percent of the area sampled had a very low abundance of tortoises, 29 percent had a low abundance, and 1 percent had a moderate A revised map of the relative abundance of desert tortoise on the NTS is presented. Within the 1,330 km2 (514 mi2) of desert tortoise habitat on the NTS, 49 percent is classified as having no tortoises or a very low abundance, 18 percent has a low or moderate abundance, 12 percent is unclassified land being used by the Yucca Mountain Site Characterization Project, and the remaining 21 percent still has an unknown abundance of desert tortoises. Based on the results of this work, the amount of tortoise habitat previously classified as having an unknown or lowmoderate abundance, and on which clearance surveys and on-site monitoring was required, has been reduced by 20 percent.

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[mi2]) installation managed by the U.S. Department of Energy, Nevada Operations Office (DOE/NV) for defense programs, environmental restoration, waste management activities, and nondefense research and development. Although DOE/NV and its predecessor agencies have used the NTS since 1950, a large portion of the area is undisturbed and supports diverse plant and animal communities. The desert tortoise (Gopherus agassizii) is the only species resident on the NTS that is classified under the Endangered Species Act as threatened. Desert tortoises are found only in the southern portions of the NTS, and the abundance of tortoises varies throughout that area (EG&G/EM, 1991). In some areas, such as the southern alluvial fans of the CP Hills and the lower hills surrounding Rock Valley, tortoises are relatively common. In other areas, such as the bottom of Frenchman Flat, they are rare (EG&G/EM, 1991). Because the desert tortoise is classified as a threatened species (U.S. Fish and Wildlife Service [FWS], 1990), DOE/NV must implement the terms and conditions of a biological opinion to minimize impacts of its activities on this species. Mitigation actions typically required on the NTS include clearance surveys to find and remove tortoises from construction sites, monitoring of construction activities, restrictions on off-road driving, and fencing or covering of trenches. These methods may be effective, reasonable, and prudent in areas where tortoises are relatively common, but some are not useful in areas where tortoises are rare or absent. Therefore, the FWS has concurred that DOE/NV does not need to implement some mitigation actions for desert tortoises (e.g., clearance surveys, on-site monitoring) in those areas of the NTS where desert tortoises are not found or are found only in very low abundance (FWS, 1996). Numerous studies have been conducted to gather information on the range, distribution, and relative abundance of desert tortoises on the NTS. Transect surveys designed to determine the distribution and relative abundance of desert tortoises were initiated in 1981. During 1981-1984, transects were walked on and around Yucca Mountain, the possible site of a high-level radio active waste repository. In 1984, transects were walked in Frenchman Flat and the surrounding mountains to determine the potential effects of testing at the Hazardous Materials (HAZMAT) Spill Center on tortoises. In 1985, northern Frenchman and Jackass Flats were examined to better determine the northern boundary of the tortoise distribution on the NTS. Areas within the known range of the tortoise in Jackass Flats, Rock Valley, Mercury Valley, and Frenchman Flat were examined in 1986. The results of all these surveys are summarized in EG&G/EM (1991). During all surveys, 759 transects totaling 1,191 km (740 mi) were walked. Seventeen live tortoises and 363 other sign of tortoises were found, for an average of 0.32 sign observed per km walked. Using the criteria established by Karl (1981) for areas in southern Nevada (see Table 1), three areas were classified as having a low abundance of tortoises (CP Hills [0.79 sign/km], Rock Valley [0.46 sign/km], and Mercury Valley [0.41 sign/km]). Six other areas were classified as having no tortoises or a very low abundance of tortoises (hereafter called “very low”) (Yucca Sign of tortoises was found at elevations from 880 to 1,570 meters (m) (2,894 to 5,163 feet [ft]), and sign was more abundant above 1,220 m (4,003 ft) than has been reported previously for Nevada. Tortoises were more abundant on the NTS on upper piedmont slopes and mountain

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Contour Interval (100 m) /V Major Road Bechtenjggjfa Figure 1. Major roads, topographic features, and administrative areas of the Nevada Test Site.

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Table 1 Relationship of sign detected per kilometer of transect walked to categories of abundance of desert <0.4 Very Low or None* 0-10 0-3.9 a Referred to as very low throughout this report. slopes than in valley bottoms. They also were more common on or near limestone and dolomite mountains than on mountains of volcanic origin. Using the classification of vegetation associa tions developed by Beatley (1976), tortoises were rare or absent in the transition vegetation association on upper bajadas and mid-elevation valley bottoms dominated by blackbrush (Coleogyne ramosissimd) and in the transition association found in the bottom of enclosed basins that is dominated by spiny hopsage (Grayia spinosa), wolfberry (Lycium spp.), and saltbush (Atriplex spp.). Tortoises were most abundant in mountainous areas with unclassified vegetation associations and in areas dominated by creosotebush (Larrea tridentata). Additional studies were conducted at Yucca Mountain from 1989 through 1995 to evaluate the impacts of the Yucca Mountain Site Characterization Project on desert tortoises. During 1989, 55 km (34 mi) of transects were surveyed, primarily in Midway Valley and the slopes and ridges west of that valley, and an average of 0.85 sign was found per km walked (Karl, 1989). During ecological studies conducted from 1989 through 1995, 340 tortoises were marked in a 117-km2 (45.2-mi2) area (Lederle et al, 1997). The minimum density of tortoises in two intensively studied areas (13 and 5 km2 [5.0 and 1.9 mi2]) was 11 to 13 adult tortoises per km2 (28 to 34 per mi2). These results suggest that tortoises are more abundant in Midway Valley and adjacent areas where these surveys and ecological studies were conducted, compared to other areas at Yucca Mountain that were surveyed during 1981-1984 (EG&G/EM, 1991). Additional work also was conducted in Frenchman Flat during 1989-1995 to evaluate whether tortoises were present near the Radioactive Waste Management Site (RWMS) and the HAZMAT Spill Center. Twenty-nine preactivity surveys and eight zone-of-influence surveys, covering a total of 6.6 km2 (2.5 mi2), were conducted near the RWMS. Three preactivity surveys and two zone-of-influence surveys, covering 1.2 km2 (0.5 mi2), were conducted near the HAZMAT Spill Center. No tortoises or tortoise sign were found during any of these surveys (DOE/NV, unpub A study was conducted in 1993 to more accurately define the northern boundary of the range of totaling 338.2 km (210.1 mi) were walked along this boundary and 53 tortoise sign were recorded. Tortoise sign was found along the northern edge of Jackass and Frenchman Flats.

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Sign was found north of those valleys only in the Calico Hills at the south end of Topopah Valley and in the CP Hills at the extreme southern end of Yucca Flat. A revised map of the range of desert tortoises on the NTS was developed based on that study. Frenchman Flat where the likelihood of finding a tortoise was so low that they were excluded from the terms and conditions of the biological opinion. One of those zones is around the sufficient number of surveys had been conducted to determine that tortoises were absent or occurred at a very low abundance. In those areas, clearance surveys for desert tortoises are not required prior to ground-clearing activities, and an on-site monitor is not required during the activities (FWS, 1996, Appendix A). Those areas, labeled “none to very low” in Figure 2, include much of Mercury Valley, the Cane Springs Wash and Wahmonie Flats areas north of Skull Mountain, and the lower elevation areas in the middle of Jackass Flats. Also, 11 km2 (44 mi2) were identified where the abundance of tortoises was high enough to require all mitigation actions as specified in the terms and conditions of the biological opinion. These areas, labeled “low” in Figure 2, include the southern and eastern foothills and piedmont slopes of the CP Hills; the southern portion of Rock Valley along the edge of the Specter Range; an area in northeastern Jackass Flats, west of Kiwi Mesa; and the northern slopes and foothills of Mercury Ridge and Red Mountain, along the southern edge of Frenchman Flat. Insufficient data on tortoise abundance was available for an additional 533 km2 (206 mi2) (labeled “unknown” in Figure 2), within which DOE/NV is required to conduct all mitigation measures until sufficient information is collected. During 1996, a new design was used in determining the relative abundance of desert tortoises on the NTS. That design involved conducting transect surveys for tortoises within ecological landform units (ELUs), which are small, ecologically homogeneous units of land (i.e., having similar elevation, slope, aspect, soil, geologic parent material, and vegetation throughout). ELUs are not to be confused with habitat types or vegetation types. They are simply small, uniform sampling units that could be defended as areas that are truly unoccupied or of very low tortoise abundance. Most previous efforts to determine the relative abundance of desert tortoises on the NTS involved walking numerous transects systematically placed within large, heterogeneous regions of the NTS. While the results of those surveys had application to large areas, they failed to identify the variability in habitat and tortoise abundance that exits in those large areas. For example, based on historical transects, the entire Mercury Valley received one classification, none to very low {Figure 2). It is known that there are some areas within Mercury Valley where desert tortoise abundance is low or even moderate. Utilizing smaller sample sizes allow this variability in tortoise abundance to be identified, if it exists. During 1996, 206 ELUs, totaling 528 km2 (204 mi2), were surveyed. Most of these ELUs were within the 533 km2 (206 mi2) previously classified as “unknown,” having insufficient surveys to determine an abundance value. This report summarizes the results of those surveys and presents a new map of the relative abundance of desert tortoises on the NTS.

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Figure 2. The relative abundance of desert tortoises on the Nevada Test Site prior to 1996 field surveys (adapted from FWS,

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The natural environment within the range of the desert tortoise on the NTS has been described in detail by Beatley (1976), O’Farrell and Emery (1976), and EG&G/EM (1991). In this area, the landscape is dominated by two large valleys, Frenchman and Jackass Flats. During years of high precipitation, surface water collects and forms a shallow lake in the closed basin of Frenchman Flat (945 m elevation [3,100 ft]). Jackass Flats (915 m [3,000 ft]) and all smaller valleys in this region have drainage outlets. Elevations at the base of mountains in the southern third of the NTS range from 1,000 to 1,250 m (about 3,300 to 4,100 ft) and mountain peaks range from 1,400 to 1,800 m (about 4,600 to 5,900 ft). Most mountains in the southeastern part of the NTS (e.g.. Ranger Mountains, Specter Range, Spotted Range, CP Hills) are primarily limestones, dolomites, or shales from the late Precambrian and Paleozoic eras. The mountains in the south central and western parts of the NTS (e.g., Massachusetts, Skull, Timber, and Yucca Mountains) were formed primarily from volcanic activity during the Tertiary era (Stewart and Carlson, 1978; Frizzell and Shulters, 1990). The NTS straddles the transition zone between the Mojave and Great Basin Deserts. Plant communities typical of the Mojave Desert are found in the southern part of the NTS in Jackass Flats, Rock Valley, Mercury Valley, and parts of Frenchman Flat. The visually dominant plant in these communities is creosotebush and, depending upon the area, white bursage (Ambrosia dumosa), shadscale saltbush (Atriplex confertifolia), spiny hopsage, and Anderson’s wolfberry (Lycium andersonii) are the most common co-dominants. Within the transition zone between the two deserts, blackbrush is the dominant species on upper bajadas and in the bottoms of mid elevation valleys with drainage outlets. Blackbrush occurs in mixed stands with creosotebush and other species at elevations between 1,200 and 1,370 m (about 3,937 to 4,495 ft), and in nearly pure stands between 1,370 and 1,500 m (about 4,495 to 4,921 ft). The mean minimum and maximum daily temperatures at Camp Desert Rock (southeast comer of respectively. The average annual precipitation was 19.3 centimeters (cm) (7.6 inches [in]) (unpublished data, U.S. National Weather Service, Nuclear Support Office, Camp Desert Rock, Nevada). Variations in climate throughout the site are described by Beatley (1975).

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The relative abundance of desert tortoises …