Desert Tortoise Population Densities and Distribution, Piute Valley, Nevada Randal W

RANG ELANDS 14(4), August 1992 239

Desert Tortoise Population Densities and Distribution, Piute Valley, Nevada Randal W. Wilson and Robert D. Stager

A soils and vegetation survey was completed during the Piedmontslopes and the basinfloor. The three major April 1988 in PiuteValley, Clark County,Nev. The survey landform typesof the Piedmontslopes are erosional fan covered approximately 47,000 acres of Public Lands remnants, insetfans, and fan skirts. The vegetationcom- administered by theBureau of Land Management located munity surveyed were creosote/whitebursage with the south of Searchlight,Nev., and 60 miles southeast of Las herbaceous understory ranging from predominantlya Vegas. Desert tortoise (Gopherus agassizii) have been mixtureof introducedand native annual forbs and grasses studied in Piute Valley since 1979. The objectives of the in the southern portion of the area to perennial grasses surveywere to comparesoils and vegetative characteris- with annuals in the north. tics to tortoise density and distribution and attempt to The soils and vegetationwere classifiedand analyzed identify soils characteristics that may be used to predict usingstandard USDA-SCS and USDI-BLM field methods. tortoise distribution and densities. The vegetationwas categorized to range site (ecological site). Soil properties examined included rock fragment content, texture, consistence, pH, color, effervescence, data consisted air KWY l5 and electroconductivity.Vegetation of dry production by plant species, determination of range condition (ecologicalstatus) and the correlationof range sites to soils. The presence or absence of tortoise sign was documented during the soils and vegetationmapping to sup- plementexisting tortoise data. Soil transects were care- fully located to represent the dominantsoil types, determine soil map unit percent compositionand compareto thetortoise distributionand density data and field obser- p—v vations to determine if an association may exist. Results Eight soil map units were developed in the study area from the soil survey. Map units 1 through 7 will be dis- cussed in this paper. The climatic conditions were ideal for studying spring vegetative growth responses on the different soils. The winter and spring storms were documented on site as gentle and uniform with a goodS to 10 inches percolation into the soil profile throughout the surveyarea. Any vegetative differences are interpretedas being morea function of soil propertiesand characteris- El: tics than climate for this study. The firstarea reviewed, soil mapunit 1, is locatedalong the old railroad grade southwest of Searchlight. The component landforms in this soil mapping unit are erosional fan remnants with an alternating pattern of inset FIg. 1. Location ofthe study area, Piute Valley, Nevada. fans. The dominantsoil in this unit occurs on erosional The climate is typical Mojave Desert with cool winters fan remnants and is classified as a fine-loamy, mixed and hot summers. The area receives 5 to 8 inches of thermic Ustalfic Haplargidsrepresenting 60 percent of precipitation annually from low intensity uniformly dis- the unit. There is a limiting argillic horizon from 11 to 43 tributed winter storms and scattered, high intensitysum- inches. Tortoise burrowswere lacking in this soil though mer storms. The elevation varies from about 2,300feet to sign and tortoiseactivity were documented. 3,500feet. The major physiographicparts of thevalley are The second majorsoil in this unit is located in the inset fans, and is classified as a coarse-loamy, mixed, calcare- The authors are Soil Conservationist,U.S. Departmentof Agriculture, Soil conservation Service, Las vegas, Nev.; RangeConservationist, U.S. Depart- ous, thermicUstic Torriorthentsrepresenting 35 percent ment of Interior, Bureauof Land Management, Las vegas, Nev., respectively. of the unit. This profile lacksany restrictivesoil feature 240 RANGELANDS14(4), August 1992 and has stablestructural strengththroughout the profile. mixed, thermic Typic Camborthidsrepresenting 25 per- This map unit has a low to medium density of desert cent of the map unit. tortoise with about 50 to 100 per square mile. Tortoise This map unit has the highestdensity of deserttortoise burrowsoccurred predominantlyin the insetfans. Creo- with 150 or greater per square mile. Tortoise burrows sate bush, white bursage, and big galletta grass are the appeared to be uniformlydistributed on both soils. Creo- major perennial plant species present. Twelve native sote bush, and white bursage are the major perennial annuals and three introducedannuals were identified. plant species present. Seven native annuals and two Soil map unit 2 is located approximately two miles introducedannuals were identified. southwestof the Searchlight landing strip. The compo- Map unit4is located south ofTen MileWell inthe south nentlandforms in this unit are fanskirts with intermingled central part of the survey area. This unit has three major streamterraces. The dominantsoil in this unit occurs on componentsthat occur in a complex association. They fan skirts, and is classifed as a coarse-loamy, mixed, are fan skirts, fan remnants, and inset fans. thermic Typic Camborthids. It represents 50 percent of The dominantsoil inthis unit occurson fan skirts and is the map unit. classified as a coarse-loamy, mixed, thermicTypic Cam- The secondmajor componentoccurs on stream terra- borthidsrepresenting 55 percent of the unit. ces and is classifiedas a coarse-loamy, mixed, thermic The secondcomponent in this unit occurs on fan rem- Typic Calciorthids representing 35 percent of the map nants and is classifiedas a loamy-skeletal, mixed, thermic unit. Typic Calciorthids representing20 percent of the map This map unit has a low to mediumdensity of desert unit. tortoise with about 50 to 100 per square mile. Tortoise The third componentoccurs in inset fans and active distribution appeared to bethe same in both major soils. channels and is classifiedas a sandy or sandy-skeletal, Creosote bush, white bursage, and big galletta grass are mixed, thermicTypic Torriorthentsrepresenting 16 per- the major perennial plant species present. Ten native cent of the map unit. This soil is occasionally flooded, annuals and three introducedannuals were identified. with flooding generally occurring during prolonged high- intensitystorms in the mid and late summer months. This area has a medium to high density with 100 to 150 and 150 or greater animals per acre. Component1 had most of the tortoise burrows. Fewer tortoise sign was noted in component2 with very or extremely cobbly sur- faces and in component 3. Creosote bush, and white bursage are the major perennial plant species present. Nine native annuals and two introduced annuals were identifed. Map unit 5 is located in the southern portion of the surveyarea. The two major components are eolian sand sheets and inset fans occurring in an association. The dominant soil occurs on the eolian sand sheets classified as a mixed, thermic, Typic Torripsamments representing 70 percent of the unit. This soil lacks any structural strength and has loose soil consistence throughoutthe profile. The secondmajor componentoccurs on fan skirts that are intermingled along the periphery of the unit and is classifiedas coarse-loamy, mixed, thermic Typic Cam- borthids. No tortoiseburrows were noted in the eolian sand sheet soil with most burrowing activity identified on the fan skirts. Tortoisedensity is low to medium with about 50 to Map Unit 3 Showing greater vegetativeproduction and canopy 100 per square mile in this map unit. Creosote bush, and cover associatedwith a goodAvailable WaterCapacity. white bursage are the major perennial shrub species Soil map unit 3 is locatedeast of the town CalNevAri. presentwith 3 to40 percent, by weight, big gallettagrass. This unit covers most of the valley bottom. The compo- Five native annuals and one introduced annual were nent landformsin this unit are fan skirts and stream terra- identified. ces. The dominantsoil in this unit occurson fan skirtsand Map unit 6 is divided by U.S. Highway 95 south of is classified as a coarse-loamy, mixed, thermic Typic Searchlight,Nev. The componentlandforms in this unit Camborthidsrepresenting 65 percent of the map unit. are erosional fan remnants with alternating patterns of The next major component in this unit occurs on insetfans and active channels. The dominantsoil in this stream terraces and is classified as a coarse-loamy, unit occurs on fan remnants and is classified as a Fine- RANGELANDS 14(4), August 1992 241

100 per square mile in this map unit. Creosote bush, andwhite bursage are the major perennial plant species present with trace to 3 percent, by weight, big galleta. Spanish daggeris commonon thefan remnants. There were over four nativeannuals identified and two introducedannuals. Conclusions The objective of the survey was to compare soils and vegetative habitat characteristics to tortoise density and distribution and identify possible soils and geomorphic relationships that may beassociated and later used in the field to predicttortoise distribution and densities for similar soils. Dr. Hardy, as quoted in E. Jaegers Desert Wildlife (1961) concludedthat soils largelydetermine habitatand distribution of the deserttortoise. Hardy determined that the soil must be sufficiently free from rocks to permit digging and compact enough to maintain a strong arch- way over the burrow. Woodburyand Hardy (1948) and Miller and Stebbins (1964) found that tortoise habitat types are restrictedto suitablesoils for den construction. Luckenbach (1976) noted that preferred habitattypes in the Providence Mountainsregion were areas with good Map Unit2 Showing lessvegetative production andcanopy cover denning potential having soil characteristics of sandy associatedwith a lower Available WaterCapacity. loam to lightgravel clay. loamy, mixed, ThermicTypic Haplargids representing 50 The observations and data collected for this paper in percentof the map unit. Tortoise burrowing activity was PiuteValley corroborate earlier findings and go further to not observed on this soil. suggest an association between certain soil characteris- The next major componentin this unit occurs on inset tics and geomorphic relationshipsand tortoise density fans and is classifiedas a coarse-loamy, mixed, thermic and distribution. Typic Camborthidsrepresenting 35 percent of the map The soil characteristicsidentified were available water unit. Tortoise burrowingactivity was prevalent in thissoil. capacity (AWC), soil consistence, depth to a limiting Tortoise density is mediumto high with about 100 to 150 layer, rock fragment content, soil salinity, soil tempera- per squaremile. ture, and frequencyof flooding. Creosote bush, andwhite bursage are themajor peren- The greaterAWC, thegreater the vegetative productive nial plant species present. There were over seven native capacity under the same climatic regime. In soils with annuals and one introducedannual identified. similar denning characteristics, forage productivity may Map unit 7 is locateddue west ofthe power linecorridor be moreimportant for determiningtortoise densities due running in the middleof the survey area. The component to available forage and canopy cover as indicatedin the landformsin the unit are deeply dissected erosionalfan photosfor map units 2 and 3. Soil consistenceconsiders remnants with narrow alternating pattern of inset fans the structuralstrength and limits to digging ina soil. Soils and active channels. The dominatesoil in this unitoccurs with good structural stability and little to no digging lim- on summits of fan remnants and is classified as a fine, itations as in map unit 3 appear to providebetter tortoise montmorrillonitic,thermic Petrocalicic Paleargids repre- burrow locations. senting 55 percentof the unit. Depthfrom the surfaceto a limiting layer is defined as No burrowingactivity was noted on this soil and popu- the depth to lithic or paralithiccontact and/or to a hard- lationstudies show this area to be a lowerdensity tortoise pan. A limiting layerwithin Oto 20 inchesof the surface as area. The insetfans and washes cutting through this soil occurs in the dominant soil in map unit 1 may restrict often expose the indurated petrocalicic or "caliche" tortoise burrowing activity, affecting their distribution horizon where burrowing activity was noted underneath and population density. Tortoise burrows usually fall the layer. within40 inches ofthe surface and are crucial tosurviving The next major componentsin this unit occurs on side the winter cold and summer heat. slopes of the fan remnants and is classifiedas a loamy, Rock or coarse fragment content is determined on a mixed, thermicshallow Typic Paleorthids representing 30 percent by weight basis. Soils with very cobbley and percent of the unit. Most of the tortoise burrows and extremely cobbley surface textures may limit tortoise activity noted wereon this soil, andthe insets and washes mobility on the surface. The more gravel or rock frag- which comprise 15 percent as inclusions in the mapping ments present in theprofile, as in map unit 2, the lowerthe unit. Tortoise density is low to mediumwith about 50 to 242 RANGELANDS 14(4), August 1992

Table 1. Soil propertiesthat may beassociated with deserttortoise While thereare numerous indicatedassociations between distribution and density. vegetationand tortoise density, a moredetailed study is needed to develop them. There may be a relationship between the intro- Elements/SoilProperty RestrictiveFeature highly productive and opportunistic duced annuals such as filaree and Schismus barbatus 1. soil consistenceat Structural limited Dry strength, and tortoise These are known be their 0-40 inches digging density. to part of diets. Filaree is highly nutritious and has a high water 2. Available water capacity Droughty content during its growth periodand Schismus is highly droughttolerant, growing even on the driestyears tooffer 3. to to rock or cemented Depth limiting layer Depth pan a staple, though lower quality, food supply. in inches limiting depth of digging The deserttortoise spends approximately90 percentof 4. Coarsefragments (pct Surfacemobility may be its life in a subterranean environmentwhere the burrow by wt >3 inchessize) on limited surface layer protectsit from thecold winters,hot summers, and preda- tors. Since soils and their properties comprise the desert 5. Salinity (MMHOS/CM)' Excess salts tortoise's subterranean environment and the vegetative 6. Soil temperature (at 20 in. Temperatureat burrows depth community its surface environment, mappingthe soil to in F) degrees soil series, determiningtheir properties, correlatingrange 7. Flooding Flooding sites and determiningthe existing vegetative community can assist the 8. Soil horizon properties Limited digging manager in understanding and managing (0 to 40 in.) thedesert tortoise more effectively. It may even be possible to their 'The soils in Piute Valley had low salinity levels at < 2 MMHOS/CM with accuratelypredict distribution and densities medium hightortoise densities.The EldoradoValley located NW of Piute Valleyhas and develop a computer model to simulate various envi- lowtortoise densitiesand it's soils have salinity levels > 8 mmhos/cm. ronmental conditionssuch and drought,grazing, or other structural factors. stability of a burrow and the harder it is to dig. A in How soil affect tortoisedistribution and den- soil survey can be useful providing preliminary salinity may for locations and is not certain.There be a between interpretations burrowing potential sity may relationship information condition thewater balanceof thetortoise and the saline contentof plant community relating to range the and the soil in or vegetative seral stages on theavailability of forageand forage the burrow itself. desert tortoise. Soil is defined asthe mean annual coverfor the Interpretations providedby a temperature temper- soil should be used as of an ature at a of 20 inches. This soil is divided survey only part overall depth property habitat evaluation for the deserttortoise. into greaterthan 59 degrees Fahrenheit (F) and less than 59 degrees F. This temperature break coincideswith the Literature Cited boundary between the Thermic (59 to 72 degrees F) and Luckenbach,R.A. 1976. Field Estimatesof California Populationsof Mesic (47 to 59 degrees F) soil temperature regimes. It Gospherusagassizii. I. Procedures. P. 22-37 In: N.J. Engberg,S. also coincides with the northernmost distri- Allan,and R.L. Young(eds.). DesertTortoise Council Proceedings geographic of 1976 bution the desert tortoise Symposium. of in the state of Nevada. Woodbury, A.M., and R. Hardy. 1948a.Studies of the Desert Tor- Soils that are frequentlyflooded could present obvious toise, GospherusAgassizii. Ecol. Monogr. 18:145-200. hazards to a burrowing animal. Woodbury,A.M., and R.Hardy. 1948. Studiesof the DesertTortoise, These soil characteristicswere to the soils in Gospherusagassizii. Biol. Abstr., 22:2016-201 7. applied United States Dept. of Agricuiture. 1975. Soil Taxonomy (Soil Sur- the seven map units in the survey area. The results are vey Staff, 1975). that map units with the highest ratings also have the United States Dept. of Agriculture. 1962. Soil Survey Manual (Soil densities of desert tortoise in the Puite SurveyStaff). HandbookNo. 18. highest Valley. R.C. Stebblns. 1964. The Lives of Desert in Table 1 summarizes these soil Miller, H.A., Animals properties. Joshua Tree National Monument.University of California Press, Berkeley. P. 452.