Desert Tortoise Surveys on the Precision Impact Range Area October 2006, Edwards Air Force Base, California Prepared by: M.E. Cablk Division of Earth and Ecosystem Sciences Desert Research Institute Nevada System of Higher Education Ken E. Nussear US Geological Survey Todd C. Esque US Geological Survey prepared for: U.S. Air Force Environmental Management Edwards Air Force Base, California September 2007
EXECUTIVE SUMMARY This report describes the results of surveys conducted for desert tortoises (Gopherus agassizii) over a two week time period in October 2006. The desert tortoise is a protected species that is listed as threatened federally (55 FR 12178) and by the state of California (www.dfg.ca.gove/bdb/pdfs/teaanimals.pdf). Approximately 65,000 acres of Edwards Air Force Base fall within the Fremont-Kramer critical habitat as defined by the U.S. Fish and Wildlife Service (USFWS 1994a) and for this reason the installation must comply with FR 2519 to protect desert tortoise critical habitat as well as meet the requirements of the Sikes Act and Endangered Species Act. Previous surveys have indicated that desert tortoises occupy portions of Edwards AFB, and therefore they are currently managed for by the base. Previously conducted tortoise surveys on EAFB have had variable results. The survey methods used in 1991-1994 to estimate tortoise densities using the total corrected sign method are no longer employed due to significant variability in the relative density estimation. Tortoise transect surveys were conducted in 2002 2005 in the Precision Impact Range Area (PIRA; which is entirely within critical habitat) as part of the USFWS range wide monitoring program. Rangewide density estimates for tortoises in the Mojave have had high variation among years due to low encounter rates on transects, especially when calculated at the level of the Recovery Unit, or DWMA (USFWS, 2006). Therefore desert tortoise population density estimates for the PIRA specifically that have been estimated to date are unlikely to be precise. In an effort to obtain more accurate population density estimates for desert tortoises specifically in the PIRA, surveys were conducted using dogs trained to find desert tortoises. The use of tortoise detector dogs to conduct surveys has been shown to be a viable method under natural field conditions on the NTC Ft. Irwin in fall 2005 with results comparable to human surveys (Nussear et al., 2008). The results of the tortoise surveys conducted in 2006 by tortoise dogs on the PIRA lead to the following conclusions: Desert tortoise counts on the PIRA remain low to the extent that population estimates are not calculable Tortoises located in 2006 were found in the vicinity where tortoises had been found in earlier years Observations of tortoise sign were few, therefore it is not expected that tortoises are more abundant than were observed or have been observed in past surveys It is unclear what activities or if activities might be affecting the population(s) on the PIRA, if populations are at natural levels, or if populations are in decline Additional surveys may be warranted 1
INTRODUCTION Edwards Air Force Base (EAFB) lies in the southwest corner of the Mojave Desert of California. It crosses Los Angeles, Kern, and San Bernardino counties and is approximately 121,810 ha in area (Figure 1). Historically the land that is now the installation was homesteaded and the common settlement activities of the early 20 th century were conducted on the property (including livestock grazing and mining). By 1950 the Air Force had a well established installation on the western shore of Rogers Dry Lake. Today, EAFB employs about 11,000 military and civilian personnel. It is home to the U.S. Air Force Flight Test Center (AFFTC), the National Aeronautics and Space Administration s (NASA) Dryden Flight Research Center, and hosts the Air Force Research Laboratory (AFRL). The AFFTC mission is to conduct and support research, development, test and evaluate manned and unmanned aerospace systems. Figure 1. Location of Edwards Air Force Base. Edwards AFB lies in the western Mojave Desert 60 miles north of Los Angeles. This graphic is from the 2002 Integrated Natural Resources Management Plan for Edwards AFB (2002). The climate at EAFB is continental desert and is semi-arid to arid. Winter temperatures may dip as low as 3 F while summer maximum temperatures may exceed 110 F. Mean annual precipitation in the Antelope Valley and EAFB is 5 inches, 90 percent of which falls between November and April. Regional precipitation patterns for the area originate from the southwest. Antelope Valley is a broad alluvial plain 2,400 square miles in size that is a large and closed basin. Soils on EAFB are typically alkaline and lakebed soil ph typically exceeds 8. There are no 2
natural perennial water bodies but there are a number of artificial water sources including ponds and wastewater effluent. There are seven management areas delineated for EAFB. Of interest for this report is the Precision Impact Range Area (PIRA), which covers a large area of the eastern part of the base. The PIRA is used for flight tests, disposal of ordnance, and supports communication equipment. As its name implies, its primary use is for testing the precision of air-ground bombing and to evaluate aircraft targeting capabilities. The 2002 Integrated Natural Resources Management Plan (INRMP) states that the PIRA supports high desert tortoise densities and has some of the highest quality of habitat on the base (Figure 2). Figure 2. Desert Tortoise Critical Habitat Units as designated by the U.S. Fish and Wildlife Service. EAFB falls within the Fremont-Cramer Critical Habitat Unit which covers a large part of the eastern area of the base. Eight habitat types have been identified and mapped for the base based on elevation, slope, aspect, and natural plant and wildlife associations (EAFB, 2002). The PIRA includes three of 3
these, namely creosote bush scrub, Joshua tree woodland, and xerophytic saltbush (Figure 3). The INRMP (2002) defines each of these vegetation types as follows. Creosote bush scrub is dominated by Larrea tridentata (Figure 4), which is also the dominant understory for Joshua Tree woodland at this location. The visually dominant vegetation in Joshua Tree woodland is the Joshua Tree, Yucca brevifolia (Figure 5). Xerophytic saltbush scrub is dominated by Atriplex polycarpa, also called allscale (Figure 6). Of these three habitat types on the PIRA, desert tortoises are associated primarily with creosote bush scrub and Joshua tree woodland although they may be found in saltbush scrub. There are three management zones (USFWS 1994b) for desert tortoise with the heaviest military use being within the PIRA in a 4,480 acre zone. Figure 3. Within the PIRA and corresponding to Desert Tortoise Critical Habitat Units, there are three vegetation zones. Tortoises are most likely to occur in creosote bush scrub and Joshua Tree woodland but can occur in any of the vegetation zones. 4
Figure 4. Creosote bush scrub covers the majority of the Fremont/Kramer Desert Tortoise Critical Habitat Unit that covers the PIRA on EAFB. Figure 5. Joshua Tree woodland is one of three vegetation zones in the PIRA and Desert Tortoise Critical Habitat Unit. 5
Figure 6. Xerophytic saltbush scrub is the third of three vegetation types mapped for the PIRA in the Desert Tortoise Critical Habitat Unit of EAFB. The Mojave population of desert tortoises (Figure 7) is federally and California State listed as Threatened. They are a long lived terrestrial reptile adapted to the severe conditions in the Mojave and Sonoran Deserts of California, Arizona, Nevada, and Utah and Mexico. Desert tortoises spend the majority of their time underground in burrows (Woodbury and Hardy 1948, Nussear and Tracy 2007). They are herbivorous and feed primarily on native grasses and forbs. Typical reproductive age is reached around approximately 20 years of age (Medica pers comm) and mating occurs throughout the activity season, but heavily in the fall. There are two time periods over the course of the year when desert tortoises are most active. In the spring and early summer, desert tortoises are active to feed and lay eggs. In the fall they are also active breeding and seeking burrows in which to spend the winter. An INRMP was completed in 2002 in accordance with AFI 32-7064 Integrated Natural Resources Management. This plan is required by the Sikes Improvement Act of 1997 (16 United States Code [USC] 670a-670o) and identified management goals, methods, activity schedules, responsibilities, monitoring, and a host of other activities and concepts that support ecosystem management for the installation. The INRMP formally recognized four practical steps identified by the U.S. Government Accounting Office (U.S. GAO, 1994) as the basis for ecosystem management. These are 1) ecosystem delineation; 2) use of the best available science; 3) clearly defined, desired future conditions; and 4) the use of science-based adaptive management. Five general measures of success were subsequently identified in the INRMP and the first one listed is stabilize or increase the desert tortoise population. To support this measure of success, a better understanding of desert tortoise populations on EAFB is needed. Ideally EAFB needs information on: desert tortoise distribution base-wide, demographic distribution(s), assessment of 6
critical habitat and other habitat on base for supporting desert tortoises, genetic profile of the population(s), disease assessment, and threats assessment. Figure 7. A Mojave Desert Tortoise. The Mojave population of desert tortoises is a federally and California state listed as threatened. They are found on portions of EAFB and are actively managed. Four desert tortoise surveys were conducted on EAFB between 1991 and 1994 to quantify relative density estimates across the base (AFFTC 1996). The method used was Total Corrected Sign (TCS) on triangular transects. This method estimates desert tortoise abundance based on all sign of desert tortoises including scat, burrows, tracks and carcasses, and attempts to relate these counts to sites of known density. There are many drawbacks to TCS as a method of density estimation due to the inherent variability in the amount of sign produced by individual desert tortoises, the relative difficulty in quantifying the age of sign encountered, and the paucity of plots where true density and it relationship to sign counts is known such that sign counts can be calibrated. As a result of the TCS surveys, desert tortoise densities were estimated to be as high as 26.5/km 2 although a total of only four live desert tortoises and 99 carcasses were actually observed. In the original report estimates of 105 square miles sampled, in fact, 7 desert tortoises were estimated per square mile where no sign was observed. EAFB resource managers recognized the limitations of the TCS method and sought to improve estimates of desert tortoise densities. In addition they established long-term monitoring plots for monitoring habitat quality for the desert tortoise. Our approach was intended to collect desert tortoise data that could be analyzed using statistical 7
methods to provide population density estimates. We did not survey the long-term monitoring plots because they were not of suitable size to generalize results over larger areas of the base. We also wanted to incorporate the results of previous surveys to provide a more comprehensive data set for EAFB as a whole. OVERVIEW OF THE APPROACH The objectives of this work were threefold: 1. provide proof of concept as to whether or not DTK9 teams can be used to collect data that will be useful for resource management of EAFB; 2. to conduct desert tortoise surveys throughout the east range of Edwards Air Force Base on the PIRA including areas not previously surveyed; 3. provide data that may be used to estimate desert tortoise densities in conjunction with existing desert tortoise data for EAFB; Background Prior desert tortoise Surveys As discussed above, desert tortoise surveys have been conducted on EAFB in the past. Desert tortoise sign was recorded during surveys conducted across the base between 1991 and 1994, and the spatial patterns in desert tortoise sign were used to provide a general sense of where desert tortoises may have occurred on base just prior to our surveys. A spatial depiction of observed desert tortoise sign from earlier surveys may provide insight into recent desert tortoise distribution across the base, however density, abundance, health, stress, or other specific information about the desert tortoise population pre-survey dates cannot be quantified or described based on these data. Mapping the carcass locations recorded during the prior surveys may also provide insight about where die-offs occurred, although cause and time since death remain unknown. The USFWS chose Line Distance Sampling (LDS) in 1998 as the method they would use for range-wide sampling and estimation of desert tortoise population densities (Burroughs and Williams, 2000). LDS was implemented range-wide in 2001, has continued through 2007 and the specifics of the survey methods have changed during this time in efforts to improve the precision of the density estimates. Initially actual straight line transects were surveyed with three different passes down each transect line. The first two passes involved two people walking the line exactly while the third pass involved walking on and off the line in a zig-zag pattern. In 2002 and 2003 the line transect changed to what is referred to as a bow-tie, which consisted of two squares connected diagonally at a common vertex. The zig-zag search pattern carried through from prior years of surveying. In 2004-2005, the transect lines were lengthened to a 12 km square and the zig-zag search pattern was no longer used. Instead, two people walked the line with one person following behind another. LDS transects were sampled within EAFB in each of the survey years. Figure 8 and Figure 9 show the location of LDS transects in the PIRA. The transects cover a good proportion of the PIRA although there are some areas that had little coverage. The bow-tie transects were selected 8
for resurvey over the larger 12 km box transects because they cover greater area within EAFB and are more widely distributed across the EAFB landscape. To meet the objectives of this project the data we collected were intended to complement existing desert tortoise survey data by focusing efforts on both existing bow-tie transects and surveying new bow-tie transects in areas not previously covered by surveys. Figure 8. LDS transects surveyed within the PIRA in years 2002 and 2003. The transects occur in all three vegetation zones. These transects were the basis for desert tortoise surveys in 2006 using desert tortoise dogs. 9
Figure 9. LDS transects in 2004 and 2005 changed from the bow-tie to large squares. Their location on the PIRA also changed. Dog Teams Four experienced desert tortoise detector dog teams participated in the surveys. We defined a desert tortoise detector dog team as one dog and its handler. These teams were previously trained 10
and shown to be effective and safe when finding desert tortoises. Each of the four dog teams had between one and two seasons of desert tortoise survey experience before arriving at the Desert Tortoise Conservation Center (DTCC) in Clark County, NV for pre-survey re-assessment in October 2006. Three days of training were conducted prior to beginning surveys on the PIRA between October 1 and 3, 2006. The training involved re-introducing the dogs to live adult desert tortoises, assessing their ability to behave properly in the presence of adult desert tortoises, and calibrating their alert process. All of the dogs were performing independent trained alerts safely on live desert tortoises on the surface, in shrubs and in burrows before they were fielded at EAFB. Surveys The area we surveyed was the Precision Impact Range (PB6) and the Precision Impact Range East. A total of 35 LDS bow-tie transects that were surveyed in 2002 and 2003 and 16 new bowtie transects were resurveyed by the DTK9 teams (Figure 8). Each desert tortoise dog team surveyed two transects per day. The 16 new transects were selected by generating spatially random points that became the center point of a new bow-tie transect that matched the existing LDS transects in size and orientation. Each of the 16 new transects was 4 km in length. The total distance surveyed was 204 km (Table 1). Surveys were conducted from October 6, 2006 through October 15, 2006 (Figure 10). The teams surveyed October 6-8 followed by a rest day. They then surveyed October 10-11 followed by a rest day, and finally surveyed October 13-15. Table 1. Total distance surveyed within each of the mapped habitat types for the PIRA and the total area mapped as each habitat type. Data Collection Habitat Type km surveyed Total hectares Creosote Bush Scrub 140.25 17,680.3 Joshua Tree Woodland 37.12 5,330.8 Xerophytic Saltbush Scrub 26.71 2,315.2 Total 204.08 25,326.3 Each detector dog team was accompanied by a desert tortoise biologist who also served to assist with navigation and confirmed desert tortoise presence or absence when dogs alerted. Global positioning system (GPS) data were collected at each corner of each transect. Start and stop time were recorded. Location and information on individual desert tortoises observed were recorded. Meteorological data were collected at 5 minute increments during surveys using a HOBO station and data logger placed central to each survey area. Tortoises at a nearby focal site for Line Distance Sampling that are used to calibrate the number of animals available to observers were monitored on 10/10/2006. Two to three observations were made on each of 11 animals, totaling 26 observations during the time in which dogs were surveying transects. Tortoises were classified as occupying on of four microhabitat positions: In 11
the open, under vegetation, in the mouth of a burrow, or in a burrow. A notation was made in each case as to whether the tortoise was visible to the observer. Figure 10. Between 6 and 8 transects were surveyed each day between October 6, 2006 and October 15, 2006. This figure shows the dates transects were surveyed. 12
RESULTS Dog training at the DTCC There were a total of 153 dog-tortoise interactions during the three days spent training at the DTCC. Each dog team worked desert tortoises in three different configurations (burrow, shrub, surface) equally. There was no difference in alert responses for the three configurations across the dog teams (χ 2 = 1.70, D.F. = 2, p = 0.43). Desert tortoises found on EAFB transect surveys Two desert tortoises were located during our transect surveys, one on October 6 and one on October 7. Both desert tortoises were located in burrows. One desert tortoise measured 294 mm midline carapace length (MCL) and the other was 244 mm MCL. Figure 11 shows the transect locations where desert tortoises were located in our survey and in prior years during LDS surveys. In past LDS surveys variable numbers of tortoises were observed. For example, in 2002 a total of three live desert tortoises were observed, 14 were observed in 2003 and only two were observed in 2006. No live desert tortoises were observed during the 2001 LDS survey on the EAFB. Table 2 shows the number of live desert tortoises observed in each of the habitat types on the PIRA. The majority of desert tortoises were observed in creosote bush scrub (17 total), followed by xerophytic saltbush scrub (2) and Joshua Tree woodland (1). Figure 11 also shows the locations where and carcasses were observed during surveys of these transects for the years 2001, 2002, 2003 and our survey in 2006. Table 2. Habitat type in which desert tortoises were observed during surveys on LDS bow-tie plots in 2002, 2003 and 2006. Survey Year Habitat Type 2002 2003 2006 total Creosote Bush Scrub 3 13 1 17 Joshua Tree Woodland 0 1 0 1 Xerophytic Saltbush Scrub 1 0 1 2 In addition to desert tortoises found on the surveys three desert tortoises were located outside of the survey area at the entrance to the AFRL. Two of these desert tortoises were on the helicopter pad adjacent to the gate, and one was in the center of the road next to the check point (Figure 12). These desert tortoises were observed by the guard staff who requested we assist them in moving the one out of the road. A storm event was imminent and the three desert tortoises had moved onto the paved surfaces where rainwater would collect in depressions in the pavement. These desert tortoises are not recorded as part of our survey results but their presence should be noted. The personnel on duty were alert and acted immediately to protect the desert tortoise until it could be moved. 13
Figure 101. All desert tortoise observations for the years 2001, 2002, 2003 and 2006 on the PIRA. Transects are shown in blue for LDS original transects we resurveyed and in green for newly generated transects we surveyed for 2006. There were three desert tortoises observed in 2002, 14 in 2003 and two in 2006. Desert tortoise locations not associated with a transect in this graphic correspond to transects that were not surveyed or were large square transects shown in Figure 9, above. 14
Figure 12. A desert tortoise was found at the security check point awaiting imminent precipitation from an autumn rainstorm. Depressions in the asphalt create small basins where water collects and desert tortoises can drink. This desert tortoise was safely re-located off of the roadway and out of harm s way. Focal Tortoise Activity Tortoises at the focal site were mostly located in burrows (16 of the 26 observations), where 11 of the 16 were visible to the human observer. Four observations were of tortoises found in the mouth of the burrow, while 6 were in vegetation. If observations are treated in a similar manner to that employed by the FWS in range wide monitoring, this would result in a sampling availability of 21 of 26 observations, or 81%. Survey times Three dog teams surveyed a total of 15 transects and one dog team surveyed 8 transects before suffering an injury that prevented further work. The average time to complete a 4 km transect was 2 h 4 min (1 SD = 33 min) including breaks and rewards. The minimum survey time was 1 h 26 min and the maximum time was 4 h 42 min. There was no significant correlation between the time of day that the dog teams began surveying and the amount of time to complete the transects 15
(p = 0.3485, r = -0.13). Statistics on survey times by dog team are presented in Table 3 and survey times are presented graphically in Figure 11. There was no statistically significant difference in mean time to complete a transect with the exception of Fin and Nandi (F = 0.06, DF = 5 / 5, p = 0.0076). 6:00 4:48 Time to Complete Survey 3:36 2:24 1:12 0:00 6:00 AM 7:40 AM 9:21 AM 11:02 AM 12:43 PM 2:24 PM Start time Figure 11. This graph plots survey start time in the day and time to complete surveys. Table 3. Statistics on the amount of time each dog team spent surveying a single bow-tie transect. Team is the dog name corresponding to its team. N is the number of transects surveyed. Mean is the mean time in hours. SD and SE are standard deviation and standard error, respectively. The last column is the 95% confidence interval about the mean time to complete a transect survey. Team N Mean SD SE 95% CI of Mean Fin 6 2.53 0.27 0.11 2.25 to 2.81 Camas 15 1.86 0.38 0.10 1.648 to 2.07 Nandi 15 2.06 0.77 0.20 1.70 to 2.20 Denali 15 2.14 0.43 0.11 1.90 to 2.23 16
Environmental Conditions During Surveys Meteorological measurements were taken every five minutes. Table 4 shows summaries of wind speed, temperature and relative humidity during surveys. On average wind speeds were relatively calm. Temperatures decreased over the survey dates but increased on the last day of the survey. Relative humidity increased over the survey time period but decreased on the last survey day. These conditions are well within comfortable working conditions for the dog teams based on prior experience fielding them in the Mojave Desert (Cablk and Heaton, 2006; Nussear et al., 2008). Table 4. Summary statistics for wind speed, temperature and relative humidity during surveys. Date Mean Wind Speed (m/s) Mean temperature ( C) RH (%) ± SD ± SD ± SD 10/7 0.21 ± 0.58 19.18 ± 2.90 29.29 ± 11.79 10/8 0.04 ± 0.20 19.53 ± 5.11 30.40 ±16.60 10/10 0.12 ± 0.40 17.58 ± 4.18 39.59 ± 19.48 10/11 0.19 ± 0.45 14.17 ± 5.80 40.51 ± 12.96 10/13 0.10 ± 0.31 15.92 ± 2.10 54.45 ± 11.10 10/14 0.10 ± 0.49 14.45 ± 2.10 68.00 ± 10.98 10/15 0.07 ± 0.19 18.68 ± 5.08 50.27 ± 19.54 All Data 0.13 ± 0.40 16.9 ± 4.92 42.95 ± 19.14 Desert tortoise Surveys on EAFB PIRA Relative to the Western Mojave and EAFB Vicinity Management of desert tortoise populations is a multi-jurisdictional effort because desert tortoises are found on federal, state, and private lands. The Bureau of Land Management (BLM) designated a 38-square mile desert tortoise preserve (the Desert Tortoise Natural Area) in 1973. This preserve is 15 miles north of EAFB. Just five miles to the south of EAFB is a 3,000 acre state park, Saddleback Butte. Desert tortoises and other sensitive species are managed here by the California Department of State Parks. Each entity that owns or is responsible for managing lands that harbor desert tortoise is responsible for surveying and maintaining their habitat and desert tortoises. However there is no single universally accepted method for monitoring across all management agencies on all lands. For this reason it is difficult to understand population trends and quantify current population status relative to historic distribution and densities. The USFWS LDS effort is an attempt to quantify desert tortoise population trends using a single method range-wide. DISCUSSION AND CONCLUSIONS Our surveys did not result in enough observations to calculate an estimate of desert tortoise densities, but the PIRA does support desert tortoises at present. Our observations coupled with prior observations recorded during LDS surveys do not support the density estimates reported in the early 1990 s surveys using TCS but since we only observed two desert tortoises during our 17
surveys there is little we can offer with respect to density estimates. We would have expected to see more desert tortoises both during our surveys and in the LDS surveys to support previously reported desert tortoise density estimates unless there was a precipitous die-off between the surveys. This was the first use of dog teams to survey for desert tortoises using line transects rather than area searches. We applied the dog teams in this manner to collect data consistent with the current range wide sampling efforts and to try to use Distance Sampling methods to estimate population density, however because we only found two desert tortoises during surveys, no statistical analysis could be completed. Tortoises at the focal site indicated a high sampling availability, but this may be due to the method of classifying animals as available if they are visible, even though they are in burrows. It may be more appropriate to classify only the numbers of animals on the surface as available, which would lower availability in this case to only 63% if animals at the mouth of a burrow are counted as being available. One primary factor to consider in the success of any desert tortoise survey is the season during which surveys are conducted. There is much debate in the tortoise management community about the importance of spring versus fall surveys and the climate conditions for any surveying period. For example, whenever poor results occur during surveys or there are aberrations in the data, one of the first questions is whether the season or climate affected the results. Although the year would not be considered a wet year, there was a wet winter storm that occurred in the middle of our sampling period. This simultaneously has both a beneficial and negative effect. Desert tortoises are widely known to emerge during precipitation events even during cool episodes however, cool episodes generally depress desert tortoise activity except during the precipitation. The two factors may have cancelled one another out. Right or wrong, many in the tortoise management community also currently believe that spring is a better time to survey than fall. However, at least one recently completed research project reports very favorable results for tortoise surveys during fall of 2005 (Nussear et al., 2008). It may be difficult to effectively use dogs on transects for this application because of the potential for one-sided detection based on wind direction. Line distance sampling that employs people to detect desert tortoises assumes that all desert tortoises on the line will be observed, and dog teams must also operate under this assumption. However it is also assumed that there is a distribution off the line for detection of desert tortoises. Dog detection is based on olfaction as the primary method and visual as the secondary. Therefore if the wind is coming from a dog s left shoulder, then the dog is not receiving odor from targets off its right side. Anything to the dog s right in this example is generally not detected by olfaction and may be detected visually, but only if the target is within the dog s sight and the dog recognizes the shape as potential desert tortoise. We have observed many instances when dogs approach what appear to be, or are, desert tortoises from the upwind direction based on visual cues. Dogs verify desert tortoises as targets via olfaction so a dog that sees a desert tortoise upwind of it will move to a downwind location to verify the target before performing its trained alert. The opportunity for this to occur given winds perpendicular to the transect line is not expected to be frequent. Still, if the detection curves for dogs were estimable with precision it is possible that distance sampling could be used to estimate density estimates. We had hoped to have enough desert tortoise encounters to see 18
whether these potential factors would cause irregular detection curves for dogs, but again too few desert tortoise were observed. Fielding dog teams to locate desert tortoises along a transect line may be more useful if the assumptions associated with distance sampling can be met with a better understanding of the difference between olfactory view of the landscape and a visual view. If the dog teams can adjust their search strategy about the transect line in such a manner that their olfactory detection distance on either side of the line matches the expected human visual distance then surveying transects may be more effective. In scat detection, for example, dog teams will survey transect lines over long distances. However the purpose of these types of surveys is to collect scat for presence/absence determination and not for statistical population estimates. Therefore the distance off the line and thus the variability in detection swath about the transect line is not of critical importance. Where standardization in detectability off of a linear transect is an absolute requirement, olfaction based detection surveys may not be the best deployment method because of the effect of wind speed and direction. Dog teams have shown to be effective at conducting area searches for desert tortoises elsewhere in the Mojave Desert (Nussear et al., in review). The dog teams did perform well during the surveys in that they continued to work despite the low verified encounter rate and they did locate two live desert tortoises, both in burrows. One desert tortoise was found by a dog performing its trained alert. The second desert tortoise was located by its handler who visually inspected a burrow based on her dog s change in behavior. Reading the dog s behavior is an important element in the dog-handler team and this find is an example of why the dog-handler relationship is critical in the work setting, for any detection discipline. It is unknown why the dog did not perform his trained alert, although if there were a low level of odor exiting the burrow the dog not being able to fully commit to indicating desert tortoise. Our dogs are trained not to alert on residual scent alone as we do not want them to tell us where desert tortoises used to be, rather only where desert tortoises are at present. It is possible that during cool temperature conditions or with other environmental conditions, desert tortoises in burrows do not produce enough odor to distinguish between residual and live desert tortoise scent. Regardless, the desert tortoise was located as a product of the dog-handler team. The desert tortoise population(s) on EAFB and in particular the PIRA remains unquantified due to low detectability over multiple years of recent surveys. The earlier TCS survey results showed a pattern that suggests a die-off occurred in the interior of the PIRA, although we cannot offer any insight as to why that might have occurred. The trend in die-off is similar to what is occurring Mojave-wide. EAFB should continue to monitor the population within their boundaries and communicate with other entities responsible for collecting and analyzing desert tortoise monitoring data. 19
REFERENCES CITED AFFTC. 1996. Relative Density Estimates of Desert Tortoise on Edwards Air Force Base, California, August. Document on file at Environmental Management (AFFTC/EM), Edwards AFB CA. Burroughs, M. and B. Williams. 2000. Implementation of long-term monitoring of desert tortoise populations using line distance sampling methods. 25 th Annual Meeting and Symposium of the Desert Tortoise Council, Las Vegas, Nevada, April 21-24, 2000. Cablk, M.E. and J.S. Heaton. 2006. Accuracy and reliability of dogs in surveying for Desert Tortoise (Gopherus agasizii). Ecological Applications. 16(5):1926-1935. EAFB, 2002. Integrated Natural Resources Management Plan for Edwards Air Force Base, California. Prepared by Environmental Management Office, Air Force Flight Test Center, October 2002. CH2-AFC-SPLN-02310-0003. Nussear, K. E. and C. R. Tracy. 2007. Can modeling improve estimation of desert tortoise population densities? Ecological Applications 17:579 586. Nussear, K.E., T.C. Esque, J.S. Heaton, M.E. Cablk, K.K. Drake, C. Valentin, J.L. Yee, and P.A. Medica2008. Are wildlife detector dogs or people better at finding tortoises?herpetological Conservation and Biology. 3(1):103-115. USFWS. 1994a. Endangered and Threatened Wildlife and Plants; Determination of Critical Habitat for the Mojave Population of the Desert Tortoise. Federal Register 59(26): 5820-5866. USFWS. 1994b. Biological Opinion for the Precision Impact Range Area, Edwards Air Force Base, California, (1-8-94-F-6). 10 March. Document on file at Environmental Management (AFFTC/EM), Edwards AFB CA. USFWS. 1994c. Desert Tortoise (Mojave Population) Recovery Plan. U.S. Fish and Wildlife Service, Portland, Oregon. 73 pages plus appendices. USFWS. 1996. Biological Opinion on Establishment and Continued Use of an Off-Road Vehicle Area at the Air Force Flight Test Center in Kern, Los Angeles and San Bernardino Counties, California, (1-8-96-F-10). U.S. General Accounting Office (U.S. GAO). 1994. Ecosystem Management: Additional Actions Needed to Adequately Test a Promising Approach. Washington, D.C. GAO/RCED-94-111. 87pp. Woodbury, A.M. and R. Hardy. 1948. Studies of the desert tortoise, Gopherus agassizii. Ecological Monographs. 18(2):145-200. 20