Gopherus agassizii



INTRODUCTORY


 

Jeff Servoss, USFWS.


AUTHORSHIP AND CITATION:
Meyer, Rachelle. 2008. Gopherus agassizii. In: Fire Effects Information System, [Online]. U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station, Fire Sciences Laboratory (Producer). Available: http://www.fs.fed.us/database/feis/ [].

FEIS ABBREVIATION:
GOAG

COMMON NAMES:
desert tortoise

TAXONOMY:
Gopherus agassizii (Cooper) is the scientific name of the desert tortoise, a member of the Testudinidae family [33]. According to a review, populations north and west of the Colorado River are generally accepted as a taxonomic unit separate from those south and east [20]. Two reviews [40,56] include information on desert tortoise taxonomy and genetics.

SYNONYMS:
Xerobates agassizii (Cooper) [8,98]
Scaptochelys agassizii (Cooper) [103]

ORDER:
Testudines

CLASS:
Chelonia

FEDERAL LEGAL STATUS:
Populations north and west of the Colorado River are listed as Threatened [104].

OTHER STATUS:
Information on state-level protection status of desert tortoises in the United States is available at NatureServe, although recent changes in status may not be included.

WILDLIFE DISTRIBUTION AND OCCURRENCE

SPECIES: Gopherus agassizii
GENERAL DISTRIBUTION:
Desert tortoises occur from extreme southwestern Utah and southern Nevada south through southeastern California, western and southern Arizona, and most of Sonora to northern Sinaloa, Mexico [12,38,43,46,49,55,73,95]. They may occur in extreme southwestern Chihuahua, Mexico, but are unlikely to occur in Baja California [28,46]. Due to several differences including variation in life history, habitat selection, and population status across their range, the desert tortoise's distribution is often split into subregions. Subregions typically include the Sonoran Desert, the eastern and western portions of the Mojave Desert, and the Sinaloa region ([27,49], reviews by [98,104]). In this review, populations from these regions are referred to as "Mohave desert tortoises", "Sonoran desert tortoises", and "Sinaloan desert tortoises". The term "Sonoran desert tortoise" includes tortoises from the Colorado subdivision of the Sonoran Desert (Colorado Desert) in southeastern California. A map of the desert tortoise's distribution can be found at the website for the conference Impacts of Climate Change and Land Use in the Southwest. Despite declines in population densities in many areas (see Status), information available as of 2008 suggests that desert tortoise's distributional area has not declined.

PLANT COMMUNITIES:
Desert tortoises occupy a wide range of desert shrub communities of the southwestern United States, including creosotebush (Larrea tridentata) scrub, Joshua tree (Yucca brevifolia) woodland, shadscale (Atriplex confertifolia) scrub, saltbush (Atriplex spp.) scrub, alkali sink, cactus scrub, desert washes, and paloverde (Parkinsonia spp.)-mixed cactus scrub associations. In the southernmost reaches of their range, desert tortoises occupy Sinaloan thornscrub and Sinaloan deciduous woodland [27,43,49,55]. Ecotones between these communities are also used [9,38,55,72] and may be preferred in some areas [9] (see Plant species composition). A single desert tortoise was observed in a ponderosa pine (Pinus ponderosa) woodland in Saguaro National Park, Arizona [3].

The habitats used in Mojave and Sonoran deserts differ, with vegetation communities typical of valley bottoms used more often in the Mojave than in the Sonoran Desert [46,98,107]. Creosotebush associations are characteristic desert tortoise habitat in the Mojave Desert [9,49,55]. Some of the highest densities of desert tortoises occur in creosotebush and creosotebush-white bursage (Ambrosia dumosa) vegetation in the western Mojave Desert [55,73,99]. Paloverde- and saguaro (Carnegiea gigantea)-dominated associations are characteristic habitat in the Sonoran Desert [55,98,107]. For instance, in the Sonoran Desert of southeastern Arizona, desert tortoises occupied a paloverde-mixed cactus community but not adjacent, lower-elevation creosotebush habitat [107]. Near Tucson, Arizona, 82.8% of desert tortoise sign occurred in mixed paloverde-cactus habitat compared to 17.2% in creosote-white bursage habitat (Walchuk and Devos 1982, cited in [55]).

Plant communities occupied by Mojave desert tortoises include: Plant communities occupied by Sonoran desert tortoises include:

BIOLOGICAL DATA AND HABITAT REQUIREMENTS

SPECIES: Gopherus agassizii
Many of the citations used in this section are reviews [5,6,10,28,35,38,47,49,51,55,61,73,87,95,98,105,106]. The 1994 outline review of Grover and DeFalco [55] is probably the most extensive in terms of literature incorporated. It is cited frequently in the biological and habitat sections of this review. Ernst and others [38] is a 1994 review of desert tortoise morphology, life history, and habitat requirements and is part of a larger collection of information on turtles in the United States. In addition to information from surveys performed in southern California, Luckenback [73] discusses data and summarizes topics such as life history and diet based on the literature as of 1982. Spang [98] authored a Bureau of Land Management desert tortoise habitat management plan that included a review of desert tortoise biology and habitat with a management focus. The book edited by VanDevender [105] contains several articles [5,6,28,35,51,61,87,106] that address various aspects of desert tortoise ecology in the Sonoran Desert, including distribution, life history, diet, and cover and nutritional requirements.

LIFE HISTORY: The desert tortoise is a long-lived species that is slow growing and generally has low reproductive rates. Desert tortoises are inactive during most of the year. Aestivation assists in reducing water loss and exposure to extremely high temperatures during hot, dry periods, and winter hibernation facilitates survival during freezing temperatures and low food availability. Desert tortoises are most active following seasonal rains, when they move through 10- to 100-acre (4-40 ha) home ranges. Individual home ranges often overlap [5,8,10,37,38,44,75].

Desert tortoises tolerate water, salt, and energy imbalances on a daily basis. This ability apparently allows them to use unpredictable and ephemeral resources to meet nutritional requirements over the course of a year [89]. For detailed information on desert tortoise physiology and energetics see these sources: [55,58,59,83,87,89,95,113].

Longevity and survivorship: Desert tortoises can live over 30 years and may be able to live more than 50 years [6,38,47,55]. Conservative estimates of maximum life spans were 32 years for individuals in the western Mojave and Sinaloan regions, 35 years for individuals in the Sonoran Desert, and 48 to 53 years for individuals in the eastern Mojave Desert (Germano 1992, cited in [38,47]). There are several reports of wild desert tortoises living more than 50 years [55]. Causes of mortality include predation, disease, human-related factors (see Threats), and environmental factors such as drought, flooding, and fire [11,50,55].

Survivorship of adult desert tortoises is typically over 90%, with lower values generally occurring in western populations [6,70,73,95,103]. In the Ivanpah Valley of southeastern California, annual adult survivorship was 95.6% from 1980 to 1981 [103]. Estimates of annual adult survivorship in 3 Sonoran desert tortoise populations in Arizona were 94% or higher [6]. However, annual adult survivorship as low as 75% has been reported (Turner and Berry, unpublished data cited in [47]), and rates from 80% to 90% are not uncommon [11,47,50,103]. Adult survivorship is generally lower in the western Mojave than in the eastern Mojave or Sonoran deserts [47]. For instance, an estimated 29% of Sonoran, 11% of eastern Mojave, and about 5% of western Mojave desert tortoises live more than 25 years (Germano 1992, cited in [38]). Desert tortoise annual mortality rates in California ranged from 0% to 17.3%, with sites in the western Mojave exhibiting higher mortality rates than those in the eastern Mojave or Sonoran deserts [11].

Young desert tortoises have much lower survivorship than adults. Only 2% to 5% of hatchlings are estimated to reach maturity (Holing 1986, cited in [55]). According to reviews, adult and subadult desert tortoises typically comprise the majority of populations [55,73]. Estimates of survival from hatching to 1 year of age for Mojave desert tortoises range from 47% [51] to 51%. Survivorship of Mojave desert tortoises from 1 to 4 years of age ranges from 71% to 89% [47]. Estimates of survivorship for Sonoran desert tortoises less than 7.1 inches (18 cm) long ranges from 84% to 93%, but these estimates are biased to large individuals. The survival of desert tortoises less than 2 years old is likely lower [6]. In a laboratory experiment, hatchling survival was significantly (P<0.0005) associated with incubation temperature, with over 80% of hatchlings from clutches incubated at 82 F (28 C) or 84 F (29 C) surviving 277 days, and less than 35% of hatchlings from clutches incubated at 77 F (25 C) or 81 F (27 C) surviving 277 days [69].

Drought may increase mortality rates of juveniles and adults. Low rainfall and the associated lack of annual plant production were suggested as likely reasons for reduced adult survival on the less productive of 2 sites in the eastern Mojave Desert of southern Nevada [70]. Below-average rainfall and the related reduction in dry standing crop and biomass of grasses in 1981 were probably factors in reduced survival of adult desert tortoises in the Ivanpah Valley in 1981 compared to 1980 [103]. Juvenile desert tortoises are likely more vulnerable to drought than adults [84,113]. See Food Habits for a discussion of impacts of rainfall on forage availability, and Predators for citations that suggest a possible interaction between rainfall, forage production, and rates of predation on desert tortoises.

Maturation: Although the desert tortoise is slow-growing, often taking 16 years or longer to reach about 8 inches (20 cm) in length [48], growth rate varies with age, location, gender, and precipitation. Desert tortoises grow comparatively fast when young [6,38,48]. Of growth rates observed in desert tortoises less than 20 years old, the fastest (12.3 mm/year) occurred in 4- to 8-year-old desert tortoises from the western Mojave Desert, and the slowest (6.0 mm/yr) occurred in 16- to 20-year old desert tortoises from the Sinaloan region. Desert tortoises in the western Mojave and Sinaloan deserts generally grow faster than those in the eastern Mojave and Sonoran deserts [48]. Males and females grow at similar rates, although females can grow slightly faster when young [51,102]. Desert tortoise males may grow larger than females (Laderle 1999, cited in [91]). Growth rate of desert tortoises may be related to precipitation. Growth of desert tortoises in Nevada was greatest following winters with high precipitation [102], (Medica and others 1975, cited in [73]). However, Germano [48] found a significant (P<0.001) inverse relationship between the average width of growth rings on the scales of desert tortoise shells and average annual precipitation.

Female desert tortoises in the western Mojave and Sinaloan regions reach reproductive maturity at smaller sizes and younger ages than females in the eastern Mojave and Sonoran deserts. Desert tortoises generally reach reproductive maturity from 15 to 20 years of age and when more than 7 inches (18 cm) in length. Reproductive females as small as 7 inches [28,47] and as young as 10 years old have been observed in the western Mojave Desert. However, reproductive maturity at 13 to 14 years is more common in this area. Estimates of age of reproductive maturity for the Sinaloan region of Mexico are also 13 to 14 years. Average age of reproductive maturity was 15.4 years for females from the eastern Mojave Desert and 15.7 years for females from the Sonoran Desert [48]. The smallest reproductive female in a study on Yucca Mountain, southern Nevada, was 8.2 inches (20.9 cm) [80], and a 7.4-inch (18.9 cm) reproductive female was observed in the eastern Mojave Desert [47]. The smallest reproductive female observed in paloverde-mixed cactus habitat in the Sonoran Desert was 8.7 inches (22.0 cm) long [4].

Reproduction: Each year female desert tortoises in the Mojave Desert lay 0 to 3 clutches, and those in the Sonoran Desert lay 0 or 1 clutch, with clutches typically comprised of 4 to 6 eggs [4,6,78,80,81,101]. Both Mojave and Sonoran desert tortoise females may increase egg production in years of above-average rainfall [6,101]. Trends associated with Mojave desert tortoise egg production include more eggs in clutches when fewer clutches are laid [80,101,110] and large Mojave desert tortoise females laying more eggs per year, more clutches, and more eggs per clutch than small females [78,80,101,110]. Available information suggests that less than 75% of desert tortoise eggs typically hatch [47,73,78,101]. The sex ratio of desert tortoise populations is usually around 1:1 [6,11,55,103,107], but instances of female-biased and male-biased sex ratios have been reported [6,50].

Desert tortoises in the Mojave lay more clutches per year than those in the Sonoran Desert. Sonoran desert tortoises lay a maximum of 1 clutch each year [4,6,81]. In paloverde-mixed cactus habitat in the northeastern Sonoran Desert of Arizona, from 36% to 80% of females reproduced each year during a 4-year period [4]. In contrast, Mojave desert tortoises lay up to 3 clutches a year and typically average between 1 and 2 clutches per reproductive female per year [78,80,101,103]. Near Goffs, California, desert tortoises most often laid 1 or 2 clutches per year, although females laying 0 and 3 clutches were observed [101]. Reported average annual number of clutches per female in the Mojave Desert range from 1.1 [103] to 1.89 [101]. Females that laid 2 clutches on a Mojave Desert site in Nevada laid their first clutch significantly (P<0.001) earlier than females that laid only 1 clutch [80]. On 2 southern California sites greater annual egg production was associated (P<0.05) with early first clutches [110].

Desert tortoises's clutches typically average 4 to 6 eggs [4,6,78,81,101], although clutches of 1 to 15 eggs have been reported [6,38,47,55,73]. Average clutch sizes of desert tortoises in paloverde-mixed cactus habitat in the northeastern Sonoran Desert ranged from 3.8 to 5.7 eggs. The author suggests that this is smaller than typical clutches from the Mojave Desert [4]. Average clutch size of desert tortoises was 5.2 eggs on a site in extreme southwestern Utah [78] and 4.5 eggs near Goffs, California [101]. On 2 sites in southern California, larger desert tortoise clutches were comprised of smaller eggs [110].

Productivity may be greater in years with above-average rainfall than in years with little rainfall. In southeastern California, the annual average number of clutches per reproductive female was higher in years with above-average winter rainfall [101], and the percentage of reproductive desert tortoises was low following nearly 10 years of drought in the Maricopa Mountains in southern Arizona (Wirt and Holm 1997, cited in [6]). Averill-Murray and others [6] discuss the influence of rainfall on desert tortoise productivity and summarize preliminary data that suggest a greater proportion of smaller desert tortoises breed and clutches are larger in years with high rainfall.

Females in the Mojave Desert that lay fewer clutches tend to lay more eggs per clutch [80,101,110]. On 2 sites in southern California, females that laid one clutch had significantly (P<0.05) larger first clutches than females that laid more than one clutch [110]. On a site at Nevada's Yucca Mountain, single-clutch females laid an average of 0.9 egg more than 2-clutch females [80].

The size of female Mojave desert tortoises may influence the number of clutches laid, the size of clutches, and the number of eggs laid per year [78,80,101,110]. Large female body size was associated with increased number of eggs produced per year in southwestern Utah [78], Yucca Mountain [80], and southern California populations [110]. Larger females laid larger clutches at Yucca Mountain [80] and at 2 sites in southern California [101,110]. In contrast, the clutch size of Sonoran desert tortoises does not appear related to female body size [4,6,81]. Although large females on 2 sites in southern California tended to lay more clutches than small females [101,110], there was no relationship between female size and number of clutches laid by females on a Yucca Mountain site [80]. Egg size was positively associated with female size in paloverde-mixed cactus habitat in the Sonoran Desert of Arizona [4] and in the Desert Tortoise Natural Area of southern California [110].

Desert tortoise hatching rates of less than 75% are common [47,73,78,101]. Of 71 eggs protected from predators after being laid by wild desert tortoises on a southwestern Utah site, 52 hatched [78]. Of 57 eggs collected from southern California nests and moved to predator-proof locations, 26 hatched by 31 October and 17 "seemingly viable" eggs did not hatch by 3 May of the following year [101]. According to a review, estimates of the percentage of eggs that produce hatchlings in the eastern Mojave Desert ranged from 46% to 67% [47]. Luckenback [73] notes that hatching rates in captivity are often near 60% but can be over 80%. In a laboratory experiment, temperature influenced hatching rates and hatchling gender [69]. Incubation temperatures from 81 to 88 F (27-31C) resulted in hatching rates of 83% or more, while incubation at 77 F (25 C) resulted in a 53% hatching rate. Incubation temperatures less than 88 F (31 C) resulted in all male clutches [69].

For discussion of desert tortoise social behavior, including courtship, mating, dominance hierarchies, nest defense, and defensive behaviors of juveniles, see Vaughn [107] and the following reviews: [10,38,55].

Annual cycle: The activity peak of Mojave desert tortoises occurs in spring [73], while the activity of Sonoran desert tortoises peaks in late summer to fall [5,74]. Desert tortoises in creosotebush vegetation of the western Mojave Desert in California started eating daily from 24 March to 2 April and were aestivating by 21 June [64]. Mojave desert tortoises are most active in April and May, and the level of summer activity increases in eastern populations [5]. In contrast, food consumption by Sonoran desert tortoises in southern Arizona peaked from July to October [74]. Several populations exhibit 2 activity peaks, 1 in spring and another in late summer or fall [5,73,78,83,106]. A review discusses instances of increased winter precipitation preceding greater spring activity of Sonoran desert tortoises [106]. In Mojave Desert populations, summer precipitation is likely to influence the amount of activity in the summer and fall [73,83]. Desert tortoises in the western portion of the range likely spend more time aestivating during summer than those in eastern populations due to the lack of summer monsoon rains in the western regions [106]. See Activity and movement for more information on the impact of rainfall on desert tortoise activity.

Desert tortoises hibernate during the winter. In creosotebush scrub of the western Mojave in California, desert tortoises emerged from 24 March to 2 April in a year with greater than average rainfall [64]. On a site on the northern edge of the Mojave Desert in southwestern Nevada, 98% of individuals hibernated from 15 November to 15 February. Results of this study suggest that the timing of the start of hibernation may be more variable than emergence from hibernation in the Mojave Desert. Desert tortoises on the site emerged from hibernation from 15 February to 28 April, while the first individual to begin hibernating started on 18 August, and the last began on 7 December [90]. Desert tortoises in the Sonoran region generally begin hibernating from October to December and emerge from March to May [5,7,75,107]. Emergence dates are more variable than starting dates [5,75], with desert tortoises emerging from hibernation as early as 21 February in the Picacho Mountains of south-central Arizona [107] and as late as 16 August on another southern Arizona site. Average length of hibernation on the former site was 188.5 days but ranged from 88 to 315 days [75]. In the Picacho Mountains the average hibernation length was 125 days [107]. Hibernation length ranged from 118 to 154 days on a site dominated by yellow paloverde, saguaro, and creosotebush in the San Padro Valley of southeastern Arizona [7]. Female desert tortoises tend to emerge from hibernation earlier than males [5,7,75,90] and may begin hibernating later than males on some sites. In addition, juveniles may emerge earlier than adults [90]. In southern California, desert tortoises less than 4 years old fed mostly from 23 January to 18 February (22 of 30 feeding observations). The study period was 22 October to18 February [112].

Desert tortoises in the Mojave nest in May and June [5,101,103,110], while those in the Sonoran Desert nest from June or early July to August [4,5,81,107]. In the Ivanpah Valley, desert tortoises nested from 15 to 28 May, with some laying again from 12 to 25 June in 1980 [103]. In the eastern Mojave near Goffs, California, and the western Mojave Desert near California City, California, desert tortoises laid their first eggs in April or May, with 70% nesting again in May or June. Timing of the 1st nest did not differ between the 2 sites, and timing of the 2nd nest was later (P<0.01) in the western population [110]. Eggs hatched from September to October in the eastern Mojave Desert and August to September in the western Mojave Desert [5]. Eggs laid in southwestern Utah hatched from 21 August to 12 September, following an estimated incubation period of 89.7 days [78]. In paloverde-mixed cactus habitat series of the northeastern Sonoran Desert of Arizona, desert tortoises laid eggs from 27 June to 25 July over a 4-year period [4]. Averill-Murray and others [5] observed laying as late as 30 August in the Sonoran Desert. Sonoran desert tortoise eggs typically hatch from the end of summer to fall [4,5]. During a 4-year study on a paloverde-mixed cactus site, the latest a hatchling was observed emerging was 29 October [4]. The incubation period of desert tortoise eggs generally ranges from 90 to 120 days; some eggs may overwinter and hatch the following spring [4,38,55,73]. In a laboratory experiment, average incubation time decreased with increasing temperature, ranging from 124.7 days when incubated at 77F to 78.2 days when incubated at 88 F (31 C) [69].

Activity and movement: Temperature strongly influences desert tortoise activity level. Although summaries note that desert tortoises can survive body temperatures from below freezing [7,107] to over 104 F (40 C) [5,38], most activity occurs at temperatures from 79 to 93 F (26-34 C) [95,107]. The influence of temperature is reflected in daily activity patterns, with desert tortoises often active late in the morning during spring and fall, early in the morning and late in the evening during the summer, and occasionally becoming active during relatively warm winter afternoons [5,55,60,73,112]. In the Picacho Mountains, most desert tortoises were active when temperatures were 79 to 86 F (30 C), and activity levels were higher at these temperatures than at temperatures from 97 to 104 F (36-40 C). Active desert tortoises were not observed when temperatures were below 52 F (11 C) or above 104 F [107]. From 27 November to 23 January in the Mojave Desert of California, significantly (P≤0.004) higher percentages of juvenile desert tortoises were active on days with higher minimum temperatures than on days with lower minimum air temperatures [112]. Desert tortoises less than 2.4 inches (6.0 cm) in length were observed at significantly (P<0.05) lower temperatures than larger individuals at several California sites [13]. See Cover Requirements for details of shelter sites used to regulate body temperature and [55] for citations addressing the physiology of desert tortoise thermoregulation.

Desert tortoise activity generally coincides with rainfall. July thundershowers triggered desert tortoises in the eastern Mojave of Nevada to emerge from aestivation [83]. In the south-central Mojave Desert the proportion of locations outside burrows and distance traveled per day was significantly (P<0.001) lower in spring and summer of a dry year than in a year with above-average precipitation [37]. Greater spring foraging activity in the Sonoran Desert of Arizona occurred in years with high winter and spring rainfall than in years with low winter and spring rainfall [106]. However, in southern California the percent of juvenile Mojave desert tortoises active during winter was not correlated with amount of rainfall [112].

Although desert tortoises spend the majority of their time in shelter, movements of up to 660 feet (200 m) per day are common and long-distance movements do occur. The common, comparatively short-distance movements presumably represent foraging activity, traveling between burrows, and possibly mate-seeking or other social behaviors. Long-distance movements could potentially represent dispersal into new areas and/or use of peripheral portions of the home range. Desert tortoises in a southern Arizona plant community with several grass species, catclaw acacia, and velvet mesquite spent an average of 95.6% of the days they were observed in shelter sites [75]. In the eastern Mojave Desert of Nevada, movements less than 660 feet (200 m) were most common [86]. According to a technical report on the status of a relocation project (cited in [10]), desert tortoises commonly traveled 1,540 to 2,700 feet (470-823 m) per day. Berry [10] did not include information on the desert tortoises these estimates were based on, such as resident or reintroduced status or time since reintroduction. Long distance movements of 2 or more miles (≥ 3 km), including movements through atypical habitat, were noted by Averill-Murray and others [6] and Berry [10]. In at least one study area in southwestern Utah, desert tortoises migrate short distances between winter hibernation dens in washes and adjacent summer feeding grounds [116]. Other citations addressing desert tortoise movement are included in the locomotion section of Grover and DeFalco [55].

Density and home range: Estimates of desert tortoise densities vary from less than 8 individuals/km on sites in southern California [11] to over 500 individuals/km in the western Mojave Desert (Marlow, personal communication cited in [73]), although most estimates are less than 150 individuals/km [6,11,28,50,55,95,114]. Of 29 sites in California, 8 sites had densities less than 8 individuals/km, 6 sites had densities from 8 to 39 individuals/km, and 13 had densities from 42 to 184 individuals/km [11]. Densities of desert tortoises at several locations are provided in the following reviews: [38,55]. For information on changes to population densities, see Status.

The often overlapping home ranges of desert tortoises generally average from 10 to 100 acres (4-40 ha) [5,8,10,37,38,44,75,86], although average home ranges as small as 2.2 acres (0.9 ha) [37] and as large as 131 acres (53 ha) (Berry 1974a, cited in [10]) have been observed. Variations in home range sizes are likely due to differences in gender, season, and the availability of resources.

Males may have larger home ranges than females in some areas. For example, during the second breeding season following translocation to a site in southern Nevada, the home range of male desert tortoises averaged 63 acres (25.5 ha), which was significantly (P=0.0315) larger than the female desert tortoise average home range size of 22 acres (8.9 ha) [44]. In a creosotebush-white bursage community of the south-central Mojave Desert, male home ranges were significantly (P<0.001) larger than those of females on 2 sites surveyed in a drought year and on 1 of the 2 sites surveyed in a year of above-average precipitation [37]. On 2 of 3 sites in the Mojave Desert, the home range size of male desert tortoises was significantly (P≤0.05) larger than the size of female home ranges [45]. When home range data from Nevada were combined with data collected from 2 other studies, one in the Mojave and the other in the Sonoran Desert, male home range size was significantly (P≥0.003) larger than female home range size [86]. Studies with small sample sizes found no differences in the size of male and female home range size in the Sonoran Desert [8,75]. In extreme southwestern Utah, reproductive female desert tortoises had home ranges that averaged 124 acres (50 ha), while home ranges of nonreproductive female desert tortoises averaged 33 acres (13.5 ha). However, this difference was not significant [78]. Home range size was not related to desert tortoise size in either the Picacho Mountains [8] or the Desert Tortoise Conservation Center in Nevada [86].

Home range size may vary in relation to rainfall and season, with home range size increasing with increasing resources. Desert tortoises on 2 creosotebush-white bursage sites in the south-central Mojave had significantly (P<0.001) smaller home ranges in a year with 25% of average precipitation compared to a year with 225% of average precipitation [37]. Females on a Mojave Desert site in California that received 4.8 inches (122 mm) of rain had significantly (P≤0.05) larger home ranges than those on the site that received 1.1 inch (29 mm) of rain. However, this relationship was not observed the following year when the sites received 7.6 inches (192 mm) and 4.2 inches (108 mm) of rain, respectively. Results suggest that home range size may be larger when resources are most abundant [45]. Areas used from summer to fall were larger than those used from winter to spring on a Sonoran Desert site (P=0.0002) [75]. A review notes similar trends on several other sites in Arizona [5].

PREFERRED HABITAT: Desert tortoise tend to occur on low- to midelevation sites with soil suitable for den construction and adequate rainfall, including hillsides, bajadas, washes, and valleys. They may also be associated with high plant diversity and/or ecotones [8,9,38,46,98,117].

Elevation: Desert tortoises are generally most common from around 1,000 to 3,500 feet (300-1,070 m), with higher elevations occupied primarily in the Mojave Desert and lower elevations used throughout the desert tortoise's range. Desert tortoises tend to occur below 2,600 feet (800 m) in the northern and eastern Sonoran Desert and around and below 1,000 feet (300 m) in southern Sonora and northern Sinaloa, Mexico. In the Sonoran Desert, elevations from 1,000 to 1,640 feet (300-500 m) may support more uniform and abundant populations than lower or higher elevations [46]. In the eastern Mojave the desert tortoise's distribution consistently extends up to the 3,940-foot (1,220 m) elevation contour [27]. Desert tortoises are uncommon above 3,000 feet (915 m) around the base of the Sierra Nevada and above 3,300 feet (1,000 m) in the western Mojave Desert [73]. High-elevation mountains also limit desert tortoise distribution in Utah. Desert tortoise apparently prefer elevations from about 980 to 3,500 feet (300-1,070 m) in California, 1,320 (400 m) to 3,500 feet in Nevada, and 2,500 (760 m) to 3,500 feet in Utah [55]. However, it has been suggested that increased search effort at higher elevations in the eastern Mojave Desert may uncover greater use of these areas than previously reported [27]. Reasons for less use of high-elevation areas in the southern than in northern portions of their range are unknown but may be related to aspect, soil [46], and/or the potential detrimental impacts of excessive moisture [28].

Despite typically being found at low- to midelevations, reviews note desert tortoises occurring from below sea level in Death Valley [55] and near sea level in Mexico [28] to elevations over 7,200 feet (2,200 m) in Death Valley National Monument ([55] and Sanchez, personal communication cited in [73]). In the eastern Mojave, desert tortoises were located at maximum elevations from 4,100 to 5,250 feet (1,250-1,600 m) [27]. Most desert tortoises on a Yucca Mountain study site were found at elevations from 3,300 to 4,270 feet (1,000-1,300 m) [91]. According to unpublished Arizona Game and Fish data (cited in [3]), desert tortoises occurred on sites as high as 5,300 feet (1,615 m) in Arizona. A single desert tortoise was observed at 7,640 feet (2,328 m) in ponderosa pine-dominated habitat of Saguaro National Park. It is possible, though unlikely, that this individual was transported to the remote location [3].

Topography: Desert tortoises in the Mojave occur on valley bottoms much more frequently than desert tortoises in the Sonoran and Sinaloan regions ([7,8,46], reviews by [49,55,98]). Sites in the Mojave near Goffs, California and Las Vegas, Nevada, had slopes of 4% or less, while Sonoran Desert sites had slopes of over 40% [55]. In winter desert tortoises used 41% to 80% slopes of the Picacho Mountains more than expected and 0% to 20% slopes less than expected based on availability [8].

Desert tortoises tend to use south-facing slopes, although they also use other aspects. In a community of mixed grasses, catclaw acacia, and velvet mesquite in southern Arizona, aspect at Sonoran Desert tortoise burrows averaged 182 S. Desert tortoises showed a significant (P<0.0005) preference for south-facing burrow entrances [75]. In paloverde-creosotebush-saguaro habitat in southeastern Arizona, most desert tortoises hibernated in burrows on south-facing slopes [7]. A model developed to predict important features of their habitat suggests that desert tortoises in the north-central Mojave Desert tend to use southwest-facing slopes and avoid north-facing slopes [1]. Although not significantly different from random locations, most desert tortoise burrows faced south on a site that transitioned from Mojave to Sonoran Desert vegetation [72]. Desert tortoises also used south-facing bajadas in southern California. However, use of northern and northwestern aspects in Pima County, Arizona, was also reported [55]. In the Picacho Mountains, desert tortoise used different aspects throughout the year and avoided (P<0.001) south-facing slopes in winter [107]. Most desert tortoise burrows on a site in Nevada occurred on north-, northeast-, and east-facing slopes (Burge 1978, cited in [72]).

Soil: Due to the importance of burrows for shelter, reduction of water loss, and regulation of body temperature, soil characteristics may have a strong influence on desert tortoise density and distribution [9,114]. Burrow construction requires soil that crumbles easily during digging and is firm enough to resist collapse. Desert tortoises commonly use sites with sandy loam soils with varying amounts of gravel and clay, and tend to avoid sands [55,73,95]. One explanation for fewer desert tortoise burrows than expected in a big galleta-white bursage community in the southwestern Mojave Desert was that sandier soils (90% sand) in these areas may have inhibited burrow construction [9]. However, sands are used by desert tortoises in stabilized dunes in the Pinto Basin of Joshua Tree National Park [73]. A model based on data from the north-central Mojave suggests that desert tortoises avoid stony soils and tend to use sites with loamy soils [1]. Although hardpans (i.e., caliche layers) can limit desert tortoise burrowing [114], dens are sometimes constructed under exposed caliche layers in wash banks [55]. For more information on desert tortoise shelter sites, see Cover Requirements.

A comparison of soil maps and desert tortoise distribution and density in southern Nevada suggested that the following soil characteristics were negatively related to desert tortoise abundance: low available water-holding capacity, shallow (<40 inches (100 cm)) depth to a limiting layer, fragments larger than 3 inches (8 cm) on the surface, excess salts, soil temperatures below 59 F (15 C) at 20-inch (51 cm) depths, and soils prone to flooding. These factors may directly interfere with desert tortoise den construction and/or reduce cover and forage availability [114]. Because desert tortoises may consume soil to maintain adequate calcium levels (see Food Habits), they may prefer sites with high soil calcium content [9,55].

Water availability: The annual precipitation in desert tortoise habitat is extremely variable and averages 4.9 inches (125 mm). The creosotebush communities that provide typical Mojave desert tortoise habitat generally receive from 2 to 8 inches (50-200 mm) of rain each year. Most precipitation falls in winter in the western Mojave; summer precipitation is more common in the eastern Mojave and Sonoran deserts [55]. In the eastern Mojave Desert of southwestern Utah and northwestern Arizona, plant moisture content was highest in spring [76]. A review notes that summer rainfall is often localized in the eastern Mojave and Sonoran deserts [87].

Desert tortoises may be associated with washes on some sites. In the southwestern Mojave Desert near Twentynine Palms, California, a high level of desert tortoise activity was observed near washes. Burrows were generally associated with washes, although the relationship was not statistically significant. Use of washes as travel corridors and availability of diverse plant species (see Plant species composition) may explain this trend [9]. Winter burrows of desert tortoises in southwestern Utah tended to occur in the banks of washes [116]. Burge (1978, cited in [9]) found a high occurrence of burrows associated with desert washes in southern Nevada. Although only a few eggs and hatchlings were observed in the Picacho Mountains, they were all found in washes [107].

Plant species composition: The plant species important to desert tortoises likely vary with location. Near Twentynine Palms, California, 71% of desert tortoise burrows were associated with creosotebush, and desert tortoises avoided the only community without creosotebush. Desert tortoises rarely used the interior portion of the creosotebush-white bursage community, possibly due to the lack of big galleta, a potentially important component of their diet [9]. Burrows on a site that transitioned from Mojave to Sonoran Desert vegetation were not significantly closer to creosotebush than random sites. Burrows were significantly (P=0.04) farther from yucca (Yucca spp.) than random sites [72]. In southern Nevada, however, burrows were frequently associated with Mojave yucca (Y. schidigera) and catclaw acacia despite the relative rarity of these species (Burge 1978, cited in [72]).

In the southwestern Mojave, desert tortoises selected communities with high plant species diversity and ecotones between communities [9]. Desert tortoises were captured in a diverse wash community significantly (P<0.005) more than expected based on a random distribution. Their burrows were also significantly (P<0.0005) closer to ecotones than a set of random points. Use of high-diversity areas and ecotones may be related to increased food availability and/or the importance of plant species that cooccur in ecotones between communities [9].

The nonnative grasses invading desert tortoise habitat, such as red brome (Bromus rubens), common Mediterranean grass (Schismus barbatus), and buffelgrass (Pennisetum ciliare), have the potential to reduce habitat availability. These species can alter fuel characteristics, primarily increasing the continuity of fuels ([19], reviews by [23,40,41,92]). Greater fuel continuity can increase fire frequency ([53,96], reviews by [22,23,92]) and, in the case of buffelgrass, the fuels have the potential to increase fire severity [40]. Since these grasses are more tolerant of fire than native plant species in desert tortoise habitats, they can increase in dominance following fire ([31,63,94], reviews by [22,40,41,92]). According to reviews, these grass/fire cycles [20] can lead to the eventual conversion of desert tortoise habitat to nonnative grassland [22,33,40,41,92]. For more detail on the effects of nonnative grasses on fire regimes in desert tortoise habitat and the impact of these changes on desert tortoises, see Altered fire regimes.

COVER REQUIREMENTS:
Burrows and other shelter: Desert tortoises take shelter in burrows, rock shelter sites, and pallets to regulate body temperature and reduce water loss [38,55]. Burrows are tunnels dug into soil by desert tortoises or other animals [28], rock shelter sites are spaces or tunnels protected by rocks and/or boulders [75], and pallets are depressions in the soil [107]. Site characteristics, season, and region likely influence the type of shelter used. The number of burrows used, the extent of repetitive use, and the occurrence of burrow sharing are variable. Males tend to occupy deeper burrows than females. Seasonal trends in burrow use are influenced by desert tortoise gender and regional variation. Desert tortoise shelter sites are often associated with plant or rock cover. Desert tortoises often lay their eggs in nests located at the entrance of burrows. Information on burrow structure is available in these sources: [55,116].

Burrows are important for desert tortoise temperature and water regulation. Burrows and other shelter sites allow desert tortoises to slow their rate of heating on hot summer days [5] and provide protection from cold during the winter [7,73]. The humidity within burrows prevents desert tortoise dehydration [38,55]. In fenced enclosures on a site in southern California, long burrows were more humid than short burrows, and higher humidity contributed to reduced water loss in juvenile desert tortoises [113]. Burrows may also provide protection from predators [9]. The apparent association between certain soil characteristics (see Soil) and desert tortoise density in southern Nevada suggests that the availability of adequate burrow sites can influence desert tortoise densities [114]. In addition, a review reports a significant (P=0.0154) positive correlation between the number of burrows and population density in the Sonoran Desert, and suggests that a lack of adequate shelter sites could limit population density [6].

The use of the various types of shelter may be related to the soil [7,27,38], the abundance of rocks and boulders [75], and the temperature extremes that occur on a given site [38]. In the Mojave Desert of southern Nevada, burrows were used most in the hottest and coldest months, and pallets and sites with no cover were used most in April and May, months with moderate temperatures [91]. Deep burrows are used frequently in the northern portions of the desert tortoise's range, and burrows are used less often in the southernmost portions of the desert tortoise's range [27,49,55,73].

The number of burrows used by desert tortoises varies spatially and temporally. During an approximately 18-month study in paloverde-cactus mixed scrub of the Picacho Mountains, desert tortoises used an average of 7.6 burrows or pallets [8]. In the Mojave Desert of southern Nevada, desert tortoises used an average of 11.7 burrows per year [91]. Desert tortoises used 12 to 25 shelter sites yearly on another Nevada site (Burge 1978, cited in [73]). Desert tortoises at Joshua Tree National Park used a mean of 11.6 to 13.8 burrows in a year with above-average precipitation, which was significantly (P<0.001) more than the average of 6.2 to 6.9 burrows used by desert tortoises at another location, the Marine Corps Air Ground Combat Center in California, during the same year. These values were significantly (P<0.001) greater than the number of burrows used on the same sites in a year with below-average precipitation [37]. In a community comprised of several grasses, catclaw acacia, and velvet mesquite in the Sonoran Desert of southern Arizona, significantly (P<0.0005) more shelter sites were used from winter to spring than from summer to fall [75].

Desert tortoises use some burrows repeatedly, although reuse of burrows is variable [6,55]. Fidelity to burrows was high in an area of the Sonoran Desert with little soil development and few shelter sites, with one burrow used for 6 consecutive years [6]. In southern Nevada 83% of burrows used in 1974 were reused in 1975 (Burge 1978, cited in [55]). In the Mojave Desert of southern Nevada, an average of 5 new burrows were used per year, which represented 39% to 52% of the total yearly burrow use. Approximately 80% of the new burrows were less than 3 feet (1 m) deep [91]. In contrast, on a site in extreme southwestern Utah, only 4 of 56 burrows used in 1973 were used again in 1974 (Coombs 1974, cited in [55]). Reuse of winter burrows in a southeastern Arizona was not observed during a 2-year study [7]. Reviews report that burrow collapse can be common [55,95], which may necessitate increased use of new burrows.

Desert tortoises share burrows with other desert tortoises and with several other animals including mammal, reptile, bird, and invertebrate species. According to a review, 23 desert tortoises were observed in one burrow in southwestern Utah [38], while in the Picacho Mountains, no winter shelter sites were shared [107]. Sharing of burrows is more common for desert tortoises of opposite sexes than for desert tortoises of the same sex [6,75]. Other animals that occur in desert tortoise burrows include white-tailed antelope squirrels (Ammospermophilus leucurus), woodrats (Neotoma spp), collared peccaries (Pecari tajacu), burrowing owls (Athene cunicularia), Gambel's quail (Callipepla gambelii), rattlesnakes (Crotalus spp.), Gila monsters (Heloderma suspectum), beetles (Coleoptera), spiders, and scorpions (Arachnida) [52,55,73,107].

Male desert tortoises tend to use deeper winter burrows (hibernacula) than females in the Sonoran and Mojave deserts [5]. In Sonoran desertscrub of southeastern Arizona, hibernacula used by females were shorter (P<0.02) and had significantly (P<0.01) larger differences between maximum and minimum temperatures than hibernacula used by males [7]. In the Mojave Desert of southern Nevada, male desert tortoises tended to use deep burrows more, and were generally located deeper in burrows, than females, although the average minimum depth of hibernacula used by male and female desert tortoises was similar (35 and 33 inches (89 and 85 cm), respectively) [91]. In the Picacho Mountains, time spent within winter burrows was positively associated with burrow depth [107], suggesting that short burrow depth may promote early emergence of females from hibernation [7] (also see Annual Cycle). The average length of juvenile desert tortoise burrows on a Mojave Desert site in southern California was 20.8 inches (52.7 cm), with active juveniles in significantly (P=0.017) shorter burrows than inactive juveniles [112].

Seasonal trends in burrow use may vary by gender and region. The use of more burrows by females in spring and by males in summer and fall on a site in southern Nevada reflects increased activity of females earlier in the year [91]. In the Sonoran Desert of southern Arizona, the average length of hibernacula was 43 inches (108 cm), significantly (P<0.05) shorter than the average length of summer burrows (64 inches (162 cm)). The author stated that this was opposite the trend observed in the Mojave Desert [107]. Burrows occurred on steep (>45%) slopes on Sonoran Desert sites in Arizona [7]. It has been suggested that desert tortoise use of steep slopes in the Sonoran Desert prevents exposure to thermal sinks, periodic flooding, and damp soil [7,107]. Winter burrows used by desert tortoises tend to occur on south-facing slopes in both the Mojave [73,107] and Sonoran deserts (see [7] and Topography).

Shelter sites are often associated with creosotebush, other shrubs, or rocks. A model based on data from a site in the north-central Mojave in California suggests that desert tortoises tend to avoid areas with very little plant cover [1]. On a site in the southwestern Mojave Desert, 97% of burrows were associated with shrubs, and 71% were partially or completely under a creosotebush. Most of the remaining burrows were associated with white bursage or big galleta [9]. In the Mojave of southern Nevada, most desert tortoise burrows greater than 3 feet (1 m) deep were under boulders (50.8%). Deep burrows were also under caliche (25.7%) or shrubs (13.3%). Most burrows less than 3 feet deep were under shrubs (51.4%) or boulders (26.3%) [91]. Throughout the desert tortoise's range, shrubs protect the majority of burrows and pallets, especially those of juvenile desert tortoises [13,95]. Juveniles in a southern California study site were only rarely observed in the open [112].

Nests: Desert tortoises often lay their eggs in nests dug in sufficiently deep soil at the entrance of burrows [6,6,38,55,81,107] or under shrubs [38,55]. In paloverde-mixed cactus vegetation in Arizona, nest burrows faced north and were on 20 to 30 slopes [81], and in the Picacho Mountains the few eggshells and hatchlings observed were in burrows in washes at the base of the mountains [107]. Nests are typically 3 to 10 inches (8-25 cm) deep [38]. For more information regarding nest construction and other egg laying behaviors, see Grover and DeFalco [55].

FOOD HABITS:
The desert tortoise diet is variable. Reviews cited in this section provide helpful compilations of available information. Van Devender and others [106] review the diet of desert tortoises in the Sonoran Desert and northern Sinaloa, Mexico, including a list of plant species eaten by Sonoran desert tortoises with the site and year of observation. Oftedal [87] provides a list of major forage species including their seasonality, growth form, and regions of importance. Brief summaries of several diet studies are included in Grover and DeFalco [55]. Desert tortoise foraging behaviors are described by Grover and DeFalco [55] and Vaughn [107].

Desert tortoises are herbivores with a varied diet of primarily grasses and forbs. Although nonnative species generally do not comprise a major portion of the desert tortoise's diet, some can be important components. Plant species composition of the desert tortoise's diet is likely influenced by plant community composition, plant phenology, rainfall, and nutritional quality.

Desert tortoises forage on the leaves, stems, flowers, fruits, and seeds of a variety of grass and forb species. Although the 5 most abundant species comprised over 60% of the diet in all cases, desert tortoises have been observed foraging on 32 plant species on a site in northwestern Arizona [39], 44 species on a site in the western Mojave Desert of California [64], 50 or more species on sites in the Sonoran Desert of Arizona [74,107], and 79 plant species in blackbrush-dominated habitats in Utah [39]. Most of these species are annuals and herbaceous perennials [39,59,64,74,107]. Several reviews note the importance of annuals in the diet of desert tortoises [38,55,73,106]. At the Desert Tortoise Research Natural Area in California, Mojave desert tortoises most often ate the leaves, stems, and flowers of annuals and herbaceous perennials, but occasionally selected seeds. Selection of plant parts varied with species [64]. The only vegetative portion of plants Luckenback [73] observed desert tortoises eating were the pads of beavertail prickly-pear (Opuntia basilaris). In a community comprised of several grasses, catclaw acacia, and velvet mesquite in southern Arizona, desert tortoises ate the fruits and seeds of a variety of plant species and browsed the leaves of a few species [74]. Cutleaf filaree (Erodium cicutarium) [39,64,88,107], plantains (Plantago spp.) [39,74,88], and milkvetches (Astragalus spp.) [64,73,74] comprised a substantial proportion of desert tortoise diets at sites in both the Sonoran and Mojave deserts. In the Sonoran Desert, lupines (Lupinus spp.) [74,107], threeawns (Aristida spp.), gramas (Bouteloua spp) [74,106], slender janusia (Janusia gracilis), and mallows (Hibiscus spp., Abutilon spp., and Sida spp.) [106] were important components of the diets of desert tortoises on more than one site. Evening primroses (Camissonia and Oenothera spp.) [64,73,88] and red brome [39,74] comprised a substantial portion of Mojave desert tortoise diets. Other species consumed on Mojave Desert sites included phacelia (Phacelia spp.) [64,73], desert dandelions (Malacothrix spp.) [73,88], and big galleta [9,73]. Desert tortoises also forage on spurges (Euphorbiaceae), including narrowleaf silverbush (Argythamnia lanceolata) [106,107] and Euphorbia spp. [74] in the Sonoran Desert and white-margin sandmat (Chamaesyce albomarginata) in the Mojave Desert [64]. Shrubs [107] and the pads and fruits of prickly-pears (Opuntia spp.) [55,73,74,103,106,117] are occasionally important components of the desert tortoise's diet.

Some nonnative species may be important components of desert tortoise diets. As noted above, cutleaf filaree was commonly used in both the Mojave and Sonoran deserts [39,88,107], and red brome commonly occurred in the diets of Mojave desert tortoises [39,56,73]. Common Mediterranean grass was an important species in the diet of desert tortoises near Goffs, California [59], and in extreme northwestern Arizona [39]. In contrast, in the central Mojave, desert tortoises consumed only 0.02% of the Mediterranean grass plants encountered [88]. As the diversity of annual species increased in the northeastern Mojave Desert, the prevalence of nonnative plants in the diet declined [39]. In a western Mojave Desert study area, 78% of the diet was comprised of 9 native species that occurred at very low densities. On this site cutleaf filaree was the only nonnative species eaten, and it accounted for 3.3% of observed bites taken by desert tortoises [64]. A nutrition experiment comparing native smooth desert dandelion (Malacothrix glabrata) and Indian ricegrass (Achnatherum hymenoides) with nonnative cutleaf filaree and common Mediterranean grass found that life form (forb or grass) and phenological stage were, in most cases, more strongly related to plant nutritional value than geographic origin, with the fresh forbs being more nutritious than dried grasses in several respects [82].

Phenology influences desert tortoise forage availability, through its influence on plant species availability and productivity. While annual plants are available during spring, they comprise a large portion of the desert tortoise's diet [39,64,107]. Annual forbs were a larger component of the diet in the early spring than late spring and summer in the northeastern Mojave Desert [39], and annual plants comprised more of the desert tortoise's diet in spring and summer than in fall in Sonoran desert scrub of southern Arizona [107]. Grasses were a larger component of the diet in late spring than in early spring in the northeastern Mojave Desert [39] and comprised a larger portion of the diet in the summer and fall than in spring in a Sonoran Desert community comprised of several grasses, catclaw acacia, and velvet mesquite [74]. In Sonoran desert scrub in southern Arizona, the proportion of forbs decreased during the course of the year and the use of shrubs increased, peaking in the fall at 78% of total dry weight [107].

Rainfall may also influence desert tortoise diet. In the Mojave Desert of California, fewer annuals were eaten in spring and more prickly-pear were eaten in the summer of a year when less than 1.6 inches (400 mm) of precipitation fell from September to March than in spring and summer of the previous year, when between 2.6 and 3.0 inches (670 and 750 mm) of precipitation fell in the same timeframe. Biomass production was much greater in the year with higher rainfall [103]. In the eastern Mojave Desert of southern Nevada, annual biomass production was positively correlated with rainfall on 2 sites [70]. On 2 sites in the eastern and western Mojave Desert, energy acquisition by desert tortoises was constrained by rainfall through its impact on food availability and the apparent requirement for free-standing water [89]. The number of plant species available was inversely related with consumption of nonnative plants in the northwestern Mojave Desert [39]. The impacts of the amount and timing of rainfall on species composition of forage plants are summarized by Oftedal [87]. Reduced plant biomass during drought years may at least partially explain instances of lower desert tortoise reproduction and survival in drought years.

The relative water, protein, and potassium content of plants may influence forage species selection due to desert tortoise's need to retain water and protein and excrete potassium [88]. Food items selected by desert tortoises in the western Mojave [64] had combinations of these nutrients that were more nutritious than those of the most abundant species [87]. Examples of species that have high nutritional value in this regard are primroses (Onagraceae), cutleaf filaree, legumes (Fabaceae), mustards (Brassicaceae), and spurges (Euphorbaceae) [87]. For mineral and nutrient content of desert tortoise forage species from the northeast Mojave Desert and a discussion of the importance of moisture content on these values, see McArthur and others [76]. Consumption of bones, feces, and soil by desert tortoises may provide supplemental calcium, water, and/or other nutrients [39,55,108]. Nagy and others [85] suggest that desert tortoises with a body mass to shell volume of 0.64 g/cm are in good condition.

PREDATORS:
Desert tortoise eggs and juveniles are more susceptible to predation than adults. Five of 8 desert tortoise nests were preyed upon in paloverde-mixed cactus vegetation of the northeastern Sonoran Desert in Arizona [4]. According to reviews, the smaller size and softer shells of desert tortoises less than about 5 years old make them more vulnerable to predation [55,73]. Known predators of desert tortoise eggs and juveniles include Gila monsters [52,55,73,107], coachwhip snakes (Mastiocophis flagellum) [55,73], and common ravens (Corvus corax) [11,15,38,55,73]. Reviews note predation on eggs and juveniles by kit foxes (Vulpes macrotis), bobcats (Lynx rufus), western spotted skunks (Spilogale gracilis) [55,73], coyotes (Canis latrans), American badgers (Taxidea taxus), ringtails (Bassariscus astutus), and domestic dogs (Canis familiaris) [55]. Potential predators include red-tailed hawks (Buteo jamaicensis), other hawks (Accipitrinae) [6,15,38,73], and greater roadrunners (Geococcyx californianus) [6,15,55]. Coyotes and kit foxes [6,38,55,73] are known predators of adult desert tortoises throughout the desert tortoise's range. Golden eagles (Aquila chrysaetos) may prey on juvenile and adult desert tortoises [11,15,38,55,73]. In the Sonoran Desert, grey foxes (Urocyon cinereoargenteus) and mountain lions (Puma concolor) prey on desert tortoises [6], and in some regions of Mexico jaguars (Panthera onca) prey on desert tortoises [28]. Bobcat, American badger [6,38,55,73], and feral dogs [6,10,38,55] are potential predators of adult desert tortoises.

Higher levels of predation may occur in years with poor plant production and in areas with heavy human use. In years with low rainfall, declines in forage production and associated small mammal populations may result in greater predation pressure on desert tortoises [55,95,103]. Predation rates by common ravens increase with increasing common raven abundance, such as near human development [65]. Desert tortoises close to developed areas are also more likely to encounter domestic dogs [46]. Other threats to desert tortoises near human populations are discussed in Threats.

MANAGEMENT CONSIDERATIONS:
Status: Population declines of 10% or more were reported during the 1980s in the western Mojave and in the Colorado Desert [12,98]. Some of the largest declines occurred at the Desert Tortoise Natural Area in the western Mojave, where densities dropped from an estimated 150 individuals/km in 1979 to less than 25 individuals/km 1992. At Chuckwalla Bench in the eastern Colorado Desert, density declined from about 225 individuals/km in 1979 to about 75 individuals/km in 1992 [12]. A review of desert tortoise status is provided in Grover and DeFalco [55].

Threats: Desert tortoises are threatened by many, primarily human-caused, factors. These include direct human-caused mortality; increased predation; disease; and habitat degradation from the accumulation of trash [109]; overgrazing; off-road vehicle use; and nonnative grass invasion. Loss and fragmentation of habitat due to urbanization and other development also threaten desert tortoises [55,61,79,98,117]. In 1995, populations in areas of the Mojave and Colorado deserts with comparatively little human activity were generally stable or declining more slowly than populations in areas with high levels of human activity [12]. More detailed reviews of these threats are provided by Howland and others [61] and Grover and DeFalco [55].

Desert tortoise mortalities due directly to humans can be high near human populations. The high percentage of desert tortoise carcasses that had been shot in the western Mojave Desert (20.7%) was likely due to accessibility and proximity to cities [11]. Capture and/or human predation of desert tortoises occur in some regions of Mexico. Although human predation is typically infrequent and impeded by low desert tortoise densities and rough terrain, in a few areas human capture or consumption of desert tortoise may negatively affect desert tortoise populations [46].

Release of captive desert tortoises with upper respiratory disease syndrome, which is typically fatal, into the wild may have resulted in the infection of wild desert tortoises [95,100]. Sonoran desert tortoises have not been as greatly impacted by upper respiratory and other diseases as Mojave populations, possibly due to their relatively lower densities and because they are generally less water stressed [35]. However, disease has been suggested as a cause for drastic declines in some populations in the Colorado Desert [12].

Habitat degradation from overgrazing, vehicle use, and invasion by nonnative grasses may have substantial negative impacts on desert tortoises. Although data on the effects of livestock grazing on desert tortoises are lacking [18,27], the impacts of severe grazing on their habitat suggest that high levels of grazing likely have negative impacts on desert tortoises. Potential effects of intense grazing that could negatively impact desert tortoises include soil compaction, decreased cover of annual plants, introduction of nonnative species, competition for forage, and the potential for trampling desert tortoises and their burrows [55,111,117]. Instances of livestock trampling desert tortoises and/or their burrows are noted in these sources: [30,95,97].

Vehicles also have direct and indirect impacts on desert tortoises. In the western Mojave, desert tortoise sign occurred significantly (P<0.05) less often within 1,300 feet (400 m) of roads than farther from roads. Vehicle-caused mortality was suggested as a likely explanation for the trend [17]. According to reviews, use of off-road vehicles damages habitat [55,97], and the increase in access provided by roads and use of off-road vehicles can exacerbate other human-caused threats such as desert tortoise collection, introduction of weeds and disease, and shooting [55].

Invasion of nonnative grasses and the potential for a nonnative grass/fire cycle pose major threats to desert tortoises (reviews by [12,22,24]). This is further discussed in Fire Ecology.

Climate change may negatively affect the desert tortoise if droughts become more frequent or severe (reviews by [55,59]) or if precipitation increases and results in the spread of nonnative plant species [14].

Population Management: Relocating desert tortoises may augment populations and mitigate negative impacts of development, fragmentation, or other disturbances when implemented appropriately [44,61]. Consideration of the social structure of the resident population and the movements of relocated individuals may increase the likelihood of success [10]. Other considerations when relocating desert tortoises include disease transmission and the genetic relationships of the relocated and resident desert tortoises [61]. Bury and others [27] cite several sources that discuss the instances when relocation is appropriate. Raising young in captivity for release, known as head-starting, may assist in augmenting wild populations. Limitations of head-starting and methods for release are addressed in Germano and others [51]. For information on care of captive desert tortoises, see these reviews: [51,55,105].

Methods used to study desert tortoise populations include radio-tracking [70,75,107], passive intergraded transponder tags [16], and specially trained dogs [29]. Duda and others [37] recommend that monitoring efforts should address variability in desert tortoise activity levels due to season, precipitation level, site, and desert tortoise gender. Temporal and spatial variability in burrow use suggests that the use of burrows as a population index is limited [91].

It has been suggested that management actions that may disturb desert tortoises are best performed in the winter, as desert tortoises are least likely affected when inactive [37]. However, management actions during winter should minimize disturbance of individuals to prevent rousing them from hibernation [90].


FIRE EFFECTS AND USE

SPECIES: Gopherus agassizii
Discussion on the effect of fire on desert tortoises is based almost entirely on anecdotes, presumed desert tortoise responses to likely changes in habitat, and potential interactions of fire with desert tortoise life history. The only study to investigate the effects of fire on desert tortoises involved surveying for desert tortoise carcasses following wildfires in the Sonoran and Mojave deserts [42]. Lovich [71,72] provides observations made after a wildfire occurred in desert tortoise habitat in southern California. Two of the most frequently cited articles in the discussion below are reviews, including one that focuses on the fire ecology of Sonoran desert tortoises [40] and another that discusses the impacts of nonnative grasses and fire on desert tortoise habitat [22]. Brooks and others [21], Brooks [20], and Rice and others [92] review the interaction between nonnative grass invasion and changes to fire regimes. Reviews addressing the fire ecology and effects of fire in communities occupied by desert tortoises are also cited in this section [24,41].

Despite the anecdotal nature of most of the literature, it seems clear that the desert tortoise is vulnerable to fire. It has few adaptations to avoid or escape fire, especially during active periods. Fires also threaten desert tortoise habitat, since few native desert shrubs are fire-adapted and fire can promote the spread of nonnative grasses.

DIRECT FIRE EFFECTS ON ANIMALS:
Desert tortoises are vulnerable to fire-caused mortality, especially during active periods [42,72,116]. Juveniles are likely more vulnerable than adults because their higher surface area to volume ratio makes them more sensitive to heating [22]. Estimates of mortality due to a moderate-severity fire in Saguaro National Park range from over 4% to nearly 13% [40,42]. This degree of fire-caused mortality could have dramatic negative impacts on desert tortoise populations due to desert tortoise's slow growth, low reproductive rates, and already threatened status [40]. Higher rates of mortality would be expected in more severe fires [40,42].

Jeff Lovich, USGS. Desert tortoise mortality due to the
1995 Verbenia Fire in southern California.

The timing of fire likely has strong impacts on the extent of fire-caused mortality and the age and sex classes most affected. Generally, late spring and summer are the times of highest risk due to the increased activity of desert tortoises and the dry conditions (see Fire ecology). Sonoran desert tortoises are likely more vulnerable to late summer and fall fires than Mojave desert tortoises, due to their greater activity levels in the fall (see Annual cycle). The fire in Saguaro National Park that resulted in mortality rates of at least 4% started in mid-May. Many Sonoran desert tortoise carcasses were found following a September fire in Arizona [42]. In the Sonoran Desert, females may be more impacted than males due to nesting activity from June to August, when fires are mostly likely to occur [40,42]. In the Mojave Desert, the apparent tendency for increased juvenile activity during late winter and early spring compared to adults [60,90,112] suggests that winter and early spring fires could have greater impacts on juveniles than adults.

Desert tortoises on the surface or in shallow or exposed shelter sites are the most vulnerable during fire [22,42,72]. For example, many of the carcasses of desert tortoises killed in 4 fires in the Sonoran and 2 fires in the Mojave Desert were found on the surface, under overhangs, or in shallow burrows [42]. Given the reduced use of deep burrows in the Sonoran Desert [27,49,55,73], a greater proportion of Sonoran desert tortoise shelter sites may provide inadequate protection from fire compared to Mojave Desert shelter sites. Desert tortoises in shelter sites under or near heavy fuel loads may also be more susceptible to fire-caused mortality [42,72].

Use of burned sites by desert tortoises has been observed [42,71,115]. Density of desert tortoises in a Sonoran desertscrub community in a Saguaro National Park site 2 years after a May wildfire was estimated at 33 adults/km. One of the captured desert tortoises was 1.8 inches (4.5 cm) long, suggesting that reproduction was occurring on the site [115]. A review suggests desert tortoise movement could be restricted after fires due to reduced cover and increased surface temperatures [40].

Some desert tortoises are injured by fire but survive [40,72]. Fire-scarred tortoises were observed following fire in Arizona [40] and after the 1995 Verbenia Fire in the Colorado Desert [72].

HABITAT-RELATED FIRE EFFECTS:
Since most native shrubs in desert tortoise habitat are not fire-adapted, fire in desert tortoise habitat would likely result in reduction of plant cover and the diversity of forage species. Alterations to the fire regime, such as increased ignitions and the potential for conversion of habitat into nonnative grasslands, are major threats to desert tortoises.

Most shrubs that protect desert tortoises from temperature extremes and predators (see Cover Requirements) are killed or top-killed by fire [22,24,36,40,42]. The year following a fire in the Mojave Desert of southern California, cover in burned areas was about 3% of cover in unburned areas [68]. Fire-caused mortality rates of creosotebush are often over 60% [26,34,77]. White bursage experienced 89% mortality after a fire in the Coachella Valley in California [26]. Several other plant species in desert tortoise habitat are sensitive to fire, including saguaro, yellow paloverde, and blue paloverde [31,93,94]. High mortality of several desert shrub species was observed after prescribed fires in a buffelgrass community of central Sonora, Mexico [63]. For a review of this study, see its Research Project Summary. Many other plant species in desert tortoise habitat are at least moderately sensitive to fire, including Joshua tree, Mojave yucca, ocotillo, blackbrush, white burrobrush, catclaw acacia, Anderson wolfberry, barrel cactus, and big galleta. A review summarizes the effects of fire on plant species in the low-elevation desert shrubland zone of southern California [24].

Potential long-term effects of reduced cover include increased nonnative plant species (see Fire ecology), increased erosion, changes in species composition, and altered habitat structure [42].

Fire could also result in short-term declines in the availability of some forage species, potentially reducing diversity of high quality food species and/or the availability of forage species throughout the desert tortoise's active period [22]. Although increases in nonnative grasses following fire (See Fire ecology) could increase forage availability, the associated decline in forage species diversity is likely to detrimentally affect desert tortoises in the long term [40].

High-severity fires, repeated fires, and/or fires during the desert tortoise's active period will likely have greater indirect impacts on desert tortoises than low-severity, infrequent, and/or dormant-season fires. Similarly to direct fire effects, fires occurring while desert tortoises are active would probably have greater indirect impacts due to decreased cover and food availability. Fire severity is likely to influence the impacts on desert tortoise habitat, with greater shrub mortality and lower sprouting rates following more severe fires. Repeated burning would have greater negative impacts on plant species used by desert tortoises for cover and would likely increase nonnative species abundance [22] (see Fire ecology).

Fire ecology: Although data on historic fire regimes in the Mojave and Sonoran deserts are lacking [41,62], the light patchy fuels of the Mojave and Sonoran Deserts suggest long fire-return intervals (see the Fire regime table). Brooks and Minnich [24] summarize information on presettlement fire regimes in habitats where desert tortoises occur. The seasonality of historic fires in this area is largely unknown.

Contemporary fires in the Mojave and Sonoran deserts generally burn during the summer [22,40,54,96]. Based on southern California fire occurrence data from 1980 to 1995, the peak fire season is from May to August, with July and August having the highest occurrence of fire in the Mojave and May and June having the highest occurrence of fire in the Colorado Desert [22]. Most fires in south-central Arizona occurred in summer, with incidence of smaller (x= 37 acres (14.9 ha)) human-ignited fires peaking in May and larger (x= 111 acres (44.9 ha)) lightning-ignited fires peaking in July [96]. A review of the fire ecology of Sonoran desert tortoises [40] notes that fires generally occur in summer, and Gottfried and others [54] state that fires on their southeastern Arizona study area generally occur before the late summer monsoon rains.

Biomass accumulation due to high rainfall [31,62] increases the fire hazard. A review notes that perennials such as Rothrock's grama (Bouteloua rothrockii) could potentially support fire in desert shrubland after periods of high precipitation [40]. Red brome density appeared to increase following above-average precipitation in southern California from 1992 to 1994 [42]. High precipitation may result in peaks in desert tortoise reproduction. Given the possible importance of periods of high recruitment to long-term desert tortoise population persistence and the vulnerability of young individuals, a review suggests that these fires may have large impacts on desert tortoise populations [22].

Altered fire regimes: Although reviews note that lightning was the predominant ignition source historically [40,41], in recent decades human-caused ignitions account for the majority of fires in the desert Southwest. Human-ignited fires accounted for almost 66% of fires on the Tonto National Forest of Arizona from 1955 to 1983, with a significant (P<0.05) increase in number of human-started fires during the study period [96]. From 1980 to 1995, 75% of fires on Bureau of Land Management lands in southern California were started by humans [22]. Reviews note that the increase in human-ignited fires, such as those started by campfires, fireworks, and vehicle use, is a threat to desert tortoises [24,40,41] and likely increases the rate of conversion of desert tortoise habitat into nonnative grassland.

Establishment of nonnative grasses, primarily red brome, common Mediterranean grass, and buffelgrass, is altering fire regimes and habitats in the Mojave and Sonoran deserts. Nonnative grasses provide abundant and continuous fuels, resulting in increased fire frequency. Since these species can increase in dominance following fire, repeated burning can convert native-dominated desert tortoise habitat into nonnative grassland. These grasslands are, in turn, likely to burn repeatedly. Thus, a grass/fire cycle is established. Reviews of the impacts of nonnative grasses on fire regimes and community composition, including descriptions of the nonnative grass/fire cycle, are available in these sources: [20,21,33,41,92].

 
 
Unburned desert tortoise habitat with high continuity of nonnative grass. Nearby site burned in the 2005 Goodsprings Fire in southwestern Nevada.

Photos by Greg Carttar, 3rd St. R & D Production Services.

Nonnative grasses provide continuous fuel with characteristics that often differ from those of native species. Dried nonnative annual grasses were the only species in the Mojave Desert of southern California to accumulate continuous fuels that persisted into the fire season [19] (also see the Research Project Summary of that study), a trend noted in several reviews [23,40,41,92].

More abundant and continuous fuels have contributed to increased fire frequencies in the desert Southwest. Fires in Joshua Tree National Park have become more frequent and larger since the introduction of nonnative grasses [53]. On the Tonto National Forest of Arizona the estimated amount of time for the entire desert area to burn based on data from 1970 to 1983 was 226 years, substantially lower than the 340-year estimate based on 1955 to 1969 data [96]. The number of fires per year below 4,200 feet (1,280) in the Mojave and Colorado deserts increased significantly from 1980 to 1995 [22]. On some sites in the Mojave Desert, average fire-return intervals have declined from over 30 years to 5 years [23]. However, increased precipitation toward the end of Schmid and Rogers' 1955 to 1983 study period [96], and more human-caused ignitions likely account for at least some of the increase in fire frequency [22,96].

Fire severity may also be altered. Nonnative grasses vary in their contribution to fire severity. Buffelgrass often fuels high-severity fires. A review describes a buffelgrass fire that scorched the soil and cracked bedrock [40]. Brooks [19] found that fire in predominantly Arabian schismus (Schismus arabicus) and common Mediterranean grass burned more slowly, had smaller flame lengths, and resulted in patchier burns than fire fueled by red brome, cheatgrass (Bromus tectorum), and/or Chilean chess (B. berteroanus).

Nonnative grasses often increase in dominance following fires [22,40,41,63,92,94]. Red brome "appeared dominant" after fire on a site in the Sonoran Desert of south-central Arizona [94], and buffelgrass increased after prescribed fires in Sonora, Mexico [63]. See the Research Project Summary of this study for further information. Mediterranean grass (Schimus spp.) increased in biomass and cover following fire on Sonoran Desert sites [31]. See the FEIS reviews of red brome and buffelgrass for information on their responses to fire.

Given the effects of nonnative grasses on fuel loads and the impact fires have on the native and nonnative species in these communities, multiple fires can lead to conversion of desert tortoise habitat into nonnative grassland [22,33,40,41,92]. Ibarra documents the increase of buffelgrass and the decline in native species following prescribed fires on a site in Sonora, Mexico [63]. Although there is little research on the effect of increased cover of nonnative grasses on desert tortoises, a review states that conversion of thornscrub habitat to buffelgrass will "eliminate most desert tortoises" from a site in central Sonora, Mexico. Several factors may contribute to potential desert tortoise avoidance of or exclusion from these areas, including inhibited movement, increased fire-induced mortality, and reduced availability of shelter sites and diverse forage [40].

FIRE MANAGEMENT CONSIDERATIONS:
Given the sensitivity of desert tortoises to the direct and indirect impacts of fire (see Habitat-related Fire Effects), exclusion of fire from desert tortoise habitat is widely recommended [22,23,40,117]. To mitigate the potential negative impacts of fire suppression on desert tortoises and their habitat, the following practices have been recommended [36,53]:

The occurrence of fire in desert tortoise habitat can be minimized by reducing human ignitions [22,23] and preventing nonnative grasses from establishing and/or becoming dominant [21,23]. A review notes that public education and regulations help limit human-caused ignitions, but acknowledges that increasing human populations will make reductions difficult. Actions that can be taken to prevent the establishment and spread of nonnative species include reducing disturbance, monitoring areas where nonnative plant species are likely to establish, and controlling newly established populations while small. On sites where nonnative grasses have already established, options are more limited due to the likelihood that fuel reduction techniques, such as prescribed fire and grazing, will have negative effects on desert tortoises [22].

Tips for mitigating the impact of nonnative plants on the fire regime include restricting land uses that increase dominance of nonnative species and creating firebreaks to prevent the spread of fire into desert shrub vegetation [21]. Early season prescribed fire in conjunction with planting native species may help restore habitat and reduce fuel loads in areas already converted to nonnative grassland with a high probability of recurrent fire. However, Brooks and others [22] do not include details of implementing such fires, and these practices are not recommended for sites where desert tortoises still occur.

APPENDIX: FIRE REGIME TABLE

SPECIES: Gopherus agassizii
The following table provides fire regime information that may be relevant to the desert tortoise. Follow the links in the table to documents that provide more detailed information on these fire regimes.

Fire regime information on vegetation communities in which desert tortoises may occur. For each community, fire regime characteristics are taken from the LANDFIRE Rapid Assessment Vegetation Models [67]. These vegetation models were developed by local experts using available literature, local data, and/or expert opinion as documented in the PDF file linked from the name of each Potential Natural Vegetation Group listed below. Cells are blank where information is not available in the Rapid Assessment Vegetation Model.
California Southwest Great Basin
California
Vegetation Community (Potential Natural Vegetation Group) Fire severity* Fire regime characteristics
Percent of fires Mean interval
(years)
Minimum interval
(years)
Maximum interval
(years)
California Shrubland
Saltbush Replacement 70% 100 60 200
Mixed 30% 235 10  
Southwest
Vegetation Community (Potential Natural Vegetation Group) Fire severity* Fire regime characteristics
Percent of fires Mean interval
(years)
Minimum interval
(years)
Maximum interval
(years)
Southwest Shrubland
Salt desert scrubland Replacement 13% 200 100 300
Mixed 87% 31 20 100
Desert shrubland without grass Replacement 52% 150    
Mixed 48% 165    
Southwestern shrub steppe Replacement 72% 14 8 15
Mixed 13% 75 70 80
Surface or low 15% 69 60 100
Southwest Woodland
Mesquite bosques Replacement 32% 135    
Mixed 67% 65    
Great Basin
Vegetation Community (Potential Natural Vegetation Group) Fire severity* Fire regime characteristics
Percent of fires Mean interval
(years)
Minimum interval
(years)
Maximum interval
(years)
Great Basin Shrubland
Creosotebush shrublands with grasses Replacement 57% 588 300 >1,000
Mixed 43% 769 300 >1,000
Blackbrush Replacement 100% 833 100 >1,000
Salt desert scrubland Replacement 13% 200 100 300
Mixed 87% 31 20 100
Salt desert shrub Replacement 50% >1,000 500 >1,000
Mixed 50% >1,000 500 >1,000
*Fire Severities:
Replacement=Any fire that causes greater than 75% top removal of a vegetation-fuel type, resulting in general replacement of existing vegetation; may or may not cause a lethal effect on the plants.
Mixed=Any fire burning more than 5% of an area that does not qualify as a replacement, surface, or low-severity fire; includes mosaic and other fires that are intermediate in effects.
Surface or low=Any fire that causes less than 25% upper layer replacement and/or removal in a vegetation-fuel class but burns 5% or more of the area [57,66].

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