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Protection Benefits Desert Tortoise (Gopherus agassizii) Abundance: TheInfluence of Three Management Strategies on a Threatened SpeciesAuthor(s): Kristin H. Berry, Lisa M. Lyren, Julie L. Yee, and Tracy Y. BaileySource: Herpetological Monographs, 28(1):66-92. 2014.Published By: The Herpetologists' LeagueDOI: http://dx.doi.org/10.1655/HERPMONOGRAPHS-D-14-00002URL: http://www.bioone.org/doi/full/10.1655/HERPMONOGRAPHS-D-14-00002

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PROTECTION BENEFITS DESERT TORTOISE (GOPHERUS AGASSIZII)ABUNDANCE: THE INFLUENCE OF THREE MANAGEMENT

STRATEGIES ON A THREATENED SPECIES

KRISTIN H. BERRY1,5, LISA M. LYREN

2, JULIE L. YEE3, AND TRACY Y. BAILEY

4

1 US Geological Survey, Western Ecological Research Center, 21803 Cactus Avenue, Suite F, Riverside, CA 92518, USA2 US Geological Survey, Western Ecological Research Center, 2177 Salk Avenue, Suite 250, Carlsbad, CA 92011, USA3 US Geological Survey, Western Ecological Research Center, Modoc Hall, Suite 3006, 3020 State University Drive East,

Sacramento, CA 95819, USA4 619 Pinon Court, Ridgecrest, CA 93555, USA

ABSTRACT: We surveyed an area of ,260 km2 in the western Mojave Desert to evaluate relationshipsbetween condition of Agassiz’s Desert Tortoise populations (Gopherus agassizii) and habitat on lands thathave experienced three different levels of management and protection. We established 240 1-ha plots usingrandom sampling, with 80 plots on each of the three types of managed lands. We conducted surveys in spring2011 and collected data on live tortoises, shell-skeletal remains, other signs of tortoises, perennial vegetation,predators, and evidence of human use. Throughout the study area and regardless of management area,tortoise abundance was positively associated with one of the more diverse associations of perennialvegetation. The management area with the longest history of protection, a fence, and legal exclusion oflivestock and vehicles had significantly more live tortoises and lower death rates than the other two areas.Tortoise presence and abundance in this protected area had no significant positive or negative associationswith predators or human-related impacts. In contrast, the management area with a more recent exclusion oflivestock, limited vehicular traffic, and with a recent, partial fence had lower tortoise densities and high deathrates. Tortoise abundance here was negatively associated with vehicle tracks and positively associated withmammalian predators and debris from firearms. The management area with the least protection—unfenced,with uncontrolled vehicle use, sheep grazing, and high trash counts—also had low tortoise densities and highdeath rates. Tortoise abundance was negatively associated with sheep grazing and positively associated withtrash and mammalian predator scat.

Key words: Desert Tortoise Research Natural Area; Fence; Land use legacy; Mojave Desert; Protectedareas; Sheep grazing; Vehicles

AGASSIZ’S Desert Tortoise, Gopherus agas-sizii (hereafter called Desert Tortoise ortortoise), is both a federally and state-listedthreatened species with designated criticalhabitat and recovery plans (US Fish andWildlife Service [USFWS], 1990, 1994a,1994b, 2011; California Department of Fishand Wildlife, 2013). Despite recovery efforts,populations have continued to decline andavailable habitat has been reduced largelybecause of human-related uses (USFWS,2010). In 2011, G. agassizii was split intotwo species, G. agassizii and G. morafkai, andthe geographic range of G. agassizii wasreduced by ,66% (Murphy et al., 2011).

Few populations and places within thegeographic range of G. agassizii are morethreatened than the western Mojave Desert(USFWS, 1994a). Since the arrival of explor-ers and settlers in the mid-1800s, the desert

ecosystem has experienced many uses andchanges that have affected the tortoise. Thehistorical patterns of land use have left alegacy that is important for understanding notonly the decline of the tortoise and its habitatbut also the potential effects of this legacy oncurrent management strategies to recover thespecies (Foster et al., 2003; Leu et al., 2008).Briefly, in the mid- to late 1800s, earlyexplorers and settlers engaged in dry-landfarming and agriculture (Norris, 1982), live-stock grazing and ranching (Wentworth, 1948;Powers, 1988, 2000), and mining (Vredenburget al., 1981). The 1900s were characterized bygrowth and expansion of human populations,military bases and facilities, energy andtransportation corridors, energy developmentsand facilities, and off-highway vehicle-orient-ed recreation (US Bureau of Land Manage-ment [USBLM], 1973, 1980, 2006; Hunter etal., 2003). With the exception of dry-landfarming, these uses have continued and the5 CORRESPONDENCE: e-mail, [email protected]

Herpetological Monographs, 28 2014, 66–92

E 2014 by The Herpetologists’ League, Inc.

66

total amount of land with surface disturbanceshas grown.

Livestock grazing, ranching, and othersurface-disturbing activities have caused pro-found changes to the distribution and compo-sition of annual and perennial vegetation oncetypical of the Mojave Desert (Minnich, 2008).The invasion and establishment of alien annualgrasses, such as the fire-prone bromes (Bromusmadritensis subsp. rubens, B. tectorum) andArab grasses (Schismus barbatus, S. arabicus),have severely altered the biomass and compo-sition of the annual flora in the region (Brooksand Berry, 2006; Brooks et al., 2006) and led toa fire-prone regime in some areas (Brooks andMatchett, 2006). Introduced forbs, such asErodium cicutarium and members of themustard family, also are highly successful andform a substantial part of the biomass in manyareas (Brooks and Berry, 2006; Minnich, 2008).The alien annuals have altered the foodsavailable to the tortoise; the alien grasses, inparticular, are not preferred foods in thisregion (Jennings, 2002) and may be detrimen-tal in diets of juvenile and adult tortoises (Nagyet al., 1998; Hazard et al., 2009, 2010).

Paved and dirt roads and recreational use ofoff-highway vehicles not only have createdsurface disturbances but also have providedaccess to previously remote wild lands (Brooksand Lair, 2009). Roads, whether paved or dirt,vehicle trails and routes, and tracks contributeto invasion and establishment of alien annualplants (Brooks and Berry, 2006). They canalter surface flow of water and nutrients,fragment the ecosystem, and reduce thesurface available for food and shelter for thetortoises and other animals (Brooks and Lair,2009). The increased access and recreationalplay areas also have created numerous areasdenuded or partially denuded of vegetation(Busack and Bury, 1974; Bury and Lucken-bach, 2002).

Another legacy associated with growth andexpansion of human populations in theMojave Desert is the concomitant growth ofsubsidized predator populations. Subsidizedpredators are predators with population sizessupported by anthropogenic sources of food,water, shelter, and perch sites. Subsidizedpredators have the potential to increase innumbers far beyond levels provided by the

natural desert prey base by using food andwater sources in nearby desert towns andsettlements. They include the Common Ra-vens (Corvus corax), coyotes (Canis latrans),and domestic dogs (Canis lupus familiaris).These predators have used the subsidies toexpand their populations and have engaged inhyperpredation of tortoises in some areas(Boarman, 1993; Boarman and Berry, 1995;Fedriani et al., 2001; Boyer and Boyer, 2006;Esque et al., 2010).

We designed this research project toevaluate how contiguous areas with threedifferent land-use histories and types ofmanagement have affected Desert Tortoisepopulations and the habitats where they live.For comparisons, we selected the fencedDesert Tortoise Research Natural Area (Tor-toise Natural Area) as most protected. TheTortoise Natural Area is internationally rec-ognized as a protected area with an irreplace-ability rank for threatened species in the top6% of protected areas worldwide (Le Saout etal., 2013). We chose critical habitat for thetortoise in the Western Rand Area of CriticalEnvironmental Concern as moderately pro-tected. Critical habitat in this region iscontained within three designated manage-ment areas: the Western Rand Area of CriticalEnvironmental Concern, western parts of theRand Mountains Management Area, andFremont–Kramer Desert Wildlife Manage-ment Area and Area of Critical EnvironmentalConcern (USBLM, 1980, 2006; USFWS, 1994b).The Fremont–Kramer Area of Critical Envi-ronmental Concern is also on the list ofglobally protected areas and has an irreplace-ability rank for threatened species in the top2% of protected areas worldwide (Le Saoutet al., 2013). Private lands are least protect-ed. Henceforth, we use the terms TortoiseNatural Area, critical habitat, and privatelands to refer to the three management areas.These areas are interconnected and experi-enced similar land-use histories through the1950s and 1960s, when nearby CaliforniaCity was established and off-highway vehicle-oriented recreation became a change agentin the region (USBLM, 1973). Throughoutthe interconnected areas, densities of tortoisepopulations were high in the late 1970sand early 1980s, ranging from 110 to 147

2014] HERPETOLOGICAL MONOGRAPHS 67

tortoises/km2, depending on location (Ta-ble 1; Turner and Berry, 1984; Berry et al.,1986a; Berry and Medica, 1995). For thethree management areas, our objectives wereto (1) describe the historical legacy andidentify recent differences in management,(2) compare tortoise abundance and otherpopulation attributes in 2011, (3) identifynatural and anthropogenic factors positivelyor negatively associated with tortoise abun-dance in 2011, (4) evaluate differences inmammalian and avian predators in 2011, and(5) discuss factors relevant to future recoveryefforts for tortoises and their habitats.

STUDY AREA

General Description

The northern and northeast boundaries ofthe study area are the Red Rock–Randsburg,Garlock, and Goler paved roads; from theseroads, the ,260-km2 study area extends souththrough the Fremont Valley and Rand Moun-tains to the southern boundary of the TortoiseNatural Area (Figs. 1, 2). The only paved roadwithin the study area is a ,8-km stretch of theRed Rock–Randsburg Road, which traversesthe northern part of critical habitat. Thenearest paved roads to study area boundariesare 1.6 km distant in the west (NeuraliaRoad), 3.2 km distant in the south (withinCalifornia City), and 3.2 to 16.0 km in the east(US Highway 395). The settlements of Cantiland Goler Heights are within 4 km in thenorthwest and 0.8 km in the north, respec-tively; the towns of Randsburg and Johannes-burg (populations 69 and 172, respectively)and the settlement of Red Mountain are

within 2.9 km in the northeast; and urbanizedCalifornia City (population of 14,327; USCensus Bureau estimate for 2011) is 3.2 kmto the south. More important, four high-userecreation areas are within 5 to 14 km of studyarea boundaries (Fig. 1): Red Rock CanyonState Park and three recreation areas withunrestricted access for off-highway vehicles(Jawbone Canyon, Dove Springs, and Span-gler Hills).

Within the study area, elevations rangefrom 590 m near the edge of Koehn Dry Laketo 1240 m at the crest of the Rand Mountains.Perennial vegetation is predominantly com-posed of white bur-sage (Ambrosia dumosa)and creosote bush (Larrea tridentata) allianc-es (California Department of Fish and Game,2010), which change with elevation andsurface disturbance. At the edge of KoehnDry Lake in the north, allscale saltbush(Atriplex polycarpa) and other Atriplex spe-cies are common. On the floor of the FremontValley and the toeslope of the Rand Moun-tains, perennial vegetation grades into creo-sote bush scrub with white bur-sage and manydifferent species of shrubs: cheesebush (Am-brosia salsola), goldenhead (Acamptopappussphaerocephalus), spiny senna (Senna ar-mata), silver cholla (Cylindropuntia echino-carpa), Nevada ephedra (Ephedra nevaden-sis), winter fat (Krascheninnikovia lanata),hop-sage (Grayia spinosa), Mojave indigobush (Psorothamnus arborescens), Acton en-celia (Encelia actoni), Mojave Desert Califor-nia buckwheat (Eriogonum fasciculatum),Anderson box-thorn (Lycium andersonii),Cooper’s box-thorn (L. cooperi), and wish-bone bush (Mirabilis laevis var. retrorsa).

TABLE 1.—Historic density estimates of Desert Tortoise populations from long-term plots in the three managementareas between 1979 and 1981, eastern Kern County, California. The density estimates are for all sizes of tortoises and arebased on mark–recapture techniques and stratification of the tortoises into size classes for analysis.

Name of plot, sizeLocation in

management areaYear ofsurvey

Tortoise density/km2

(95% interval) Reference

Desert Tortoise Natural Area(interior), 1.1 km2

Tortoise Natural Area 1979 147 (113–192) Berry et al. (1986a), Berryand Medica (1995)

Desert Tortoise Natural AreaInterpretive Center (insidefence), 4.53 km2

Tortoise Natural Area 1979 131 (111–155) Berry et al. (1986a)

Fremont Valley, 2.59 km2 Critical habitat 1981 110 (83–144) Turner and Berry (1984)Desert Tortoise Natural Area

Interpretive Center (outsidefence), 3.24 km2

Private lands 1979 114 (90–146) Berry et al. (1986a)

68 HERPETOLOGICAL MONOGRAPHS [No. 28

Joshua trees (Yucca brevifolia) are occasionalon the slopes and crest of the Rand Moun-tains. Plant nomenclature follows Baldwinet al. (2012).

The climate is typical of the western MojaveDesert. The nearby Tehachapi Mountains tothe west act as a rain shadow, influencing theamount of precipitation, frequency and veloc-ity of winds, and temperatures. One long-termweather station (Randsburg station) has rele-vant precipitation data: average annual rainfallis estimated to be 174.24 mm (NationalOceanic and Atmospheric Administration[NOAA], 2010–2011). More than 80% ofprecipitation occurs in fall and winter, be-tween October and March. Overall, the studyarea is situated sufficiently close to theTehachapi Mountains to receive more rain

than many desert areas at similar elevationsfarther inland.

Management Areas, Land Uses,and Protections

The three management areas have haddifferent histories of human uses for agricul-ture, mining, grazing, human settlements, andrecreation during the last 40 yr, based in parton land ownership (Table 2; Fig. 2). In thelate 1960s and early 1970s, most land withinthe study area was public and managed by theBureau of Land Management; the rest wasprivate. The Bureau of Land Managementadministers public lands for sheep grazing,mining, rights-of-way for transportation andutility corridors, leases, and sales; sheepgrazing and mining were predominant uses

FIG. 1.—General location of the study area and the three interconnected management areas in the western MojaveDesert, eastern Kern County, California. In proximity to the study area are three areas designated for unrestrictedrecreational vehicle use (orange), a state park (pink), and wilderness (brown). Critical habitat in the western MojaveDesert is shown as green.


of public lands in the area. In the mid-1960s,off-highway vehicle-oriented recreation be-came a driving force and by the early 1970s, itwas a major use in the region (USBLM, 1973).Private lands, mostly unfenced, undeveloped,and with absentee owners, were often in 5- to10-acre parcels. Sheep also grazed on theseproperties under the open range provisions ofthe Kern County Estray Ordinance (estab-lished in 1942). Mining exploration occurred,as did uncontrolled, unauthorized off-roadvehicle use.

In 1973, the Bureau of Land Managementdesignated specific management areas onpublic lands as open or closed to recreationalvehicle use under a California desert-widerecreation plan for off-highway vehicles(USBLM, 1973). The land that was toofficially become the Tortoise Natural Areawas closed to recreational vehicles and signs

were erected along the boundaries. The landthat was later to become critical habitat wasdesignated as open to recreation vehicle usethroughout, with no requirement to stay ontrails or designated routes (Table 2).

Seven years later, in 1980, two of the threemanagement areas received another, signifi-cant change toward protection, with publica-tion of the Bureau of Land Management’sCalifornia Desert Conservation Area Plan,1980 (USBLM, 1980; Table 2). The TortoiseNatural Area was designated as an Area ofCritical Environmental Concern and Re-search Natural Area, and the US Congressformally withdrew these federal lands fromthe mining laws and livestock grazing. In thesame year the Bureau of Land Managementfinished fencing the boundaries with hog-wirefencing (raised ,25 cm off the ground topermit movements of wild animals) and

FIG. 2.—The locations of the 240 sampling points and the distribution of the four perennial vegetation associationsdetermined by k clustering of means within the study area in the western Mojave Desert, eastern Kern County,California.


signed the area as closed to livestock grazingand recreational vehicles. Thus, the firstResearch Natural Area was created in theCalifornia deserts. Also as part of the samemanagement plan, the land later to becomecritical habitat received two separate designa-tions: the West Rand Area of Critical Envi-ronmental Concern and the Rand Mountain–Fremont Valley Recreation Area (USBLM,1980). Under the 1980 plan, vehicles in boththe West Rand Area of Critical EnvironmentalConcern and Rand Mountains–Fremont Val-ley Recreation Area were restricted to existingroutes.

Between 1980 and 2011, additional majorchanges occurred at the Tortoise Natural Areaand on private lands. The Bureau of LandManagement and California Department ofFish and Game, in conjunction with theDesert Tortoise Preserve Committee, Inc., anonprofit corporation, developed a manage-ment plan for the Tortoise Natural Area(Bureau of Land Management and CaliforniaDepartment of Fish and Game, 1988; Ta-ble 2). All three entities purchased small andlarge private inholdings within the TortoiseNatural Area boundaries for conservationpurposes and for providing a wider connectingcorridor to the adjacent critical habitat. Fromthe 1990s to 2011, both the Desert TortoisePreserve Committee and California Depart-ment of Fish and Game also began acquisitionof private lands adjacent and to the west andeast of the Tortoise Natural Area with thepurpose of expanding the Tortoise NaturalArea. At the time of our study in 2011, theseprivate lands, recently acquired for conserva-tion purposes, were in small and large blocks,had not been fenced, and were unprotectedfrom sheep grazing, recreational vehicle use,shooting, and dumping of trash.

Between 1980 and 2011, the West RandArea of Critical Concern and Rand Moun-tains–Fremont Valley Recreation Area experi-enced three significant protective actions (Ta-ble 2). When the Desert Tortoise was federallylisted as threatened in 1990 (USFWS, 1990),the Bureau of Land Management closed thelands to livestock grazing to better protect thetortoise. In 1994, critical habitat was formallydesignated for the Desert Tortoise, and theWest Rand Area of Critical Environmental

Concern and Rand Mountains–Fremont ValleyRecreation Area became part of critical habitat(USFWS, 1994b). However, during these threedecades, unauthorized vehicle travel off exist-ing routes was a significant and continuing issue(e.g., Goodlett and Goodlett, 1992; USBLM,2002) and became the subject of legal action(US District Court, 2009, 2011). To protect thehabitat, the Bureau of Land Managementfenced parts of the West Rand Area of CriticalEnvironmental Concern and closed the fencedportion to off-highway vehicle use from 2002 to2008, and again in 2009 because of unautho-rized use (details in Table 2).

METHODS

Collection of Data on Vegetation, DesertTortoises, Predators, and Human Impacts

We confined surveys within the study areato public land and to private lands that we hadpermission to access (lands held by the DesertTortoise Preserve Committee, Inc.). Weassumed, on the basis of 40 yr of experiencewithin the region, that these private landswere representative of much larger areas tothe east and west of the Tortoise Natural Areaboth within and outside our study areaboundaries (Fig. 1). To randomly select thehectare plots for the study, we acquiredgeographic information system layers of landownership from the Bureau of Land Manage-ment (USBLM, 2012) and Kern County (KernCounty Engineering, Surveying and PermitServices Department, 2011). We established80 1-ha plots using random sampling in eachof the three types of managed lands for a totalof 240 plots across the study area (Fig. 2). The240 plots constituted a 0.92% sample of thestudy area. The 80 plots in the West RandArea of Critical Environmental Concern werewithin or on the edge of critical habitat.

Field teams surveyed the plots in spring of2011 (3 April–25 May) using methods similarto those described in Keith et al. (2008).Because detection of live and dead tortoisesand other sign is imperfect, we designed fieldsurveys to maximize detection of live tortoises,tortoise sign, predator sign, and anthropogen-ic impacts. We scheduled surveys to coincidewith high aboveground activity levels for allsizes of tortoises, as well as for counting sign


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(Zimmerman et al., 1994; Lance and Rostal,2002). We selected a field team with demon-strated expertise in finding all sizes of livetortoises, shell-skeletal remains (includingfragments), and other tortoise sign. Fieldteam members also were experienced infinding predator sign and counting anthropo-genic impacts. Each plot was surveyed twiceand on the same day, with rare exceptions, bywalking 10-m-wide transects: once in a N–Sdirection and then in an E–W direction tocollect data on perennial vegetation, livetortoises, tortoise sign, shell-skeletal remains,predators, and human-related impacts. If signwas difficult to see with rough terrain or densevegetation, the transect width was narrowed.

Vegetation.—Because tortoise distributionand abundance may differ among vegetationassociations, we prepared a list of all perennialplant species likely to occur within the studyarea (shrubs, bunch grasses, cacti). For eachplot, field-workers recorded data on thesespecies by relative abundance: (0) absent fromthe plot, (1) one or two individuals, (2) rare,(3) sparse, (4) common, or (5) dominant orubiquitous (for definitions, see Glossary inBaldwin et al., 2012). The surveyor finalizedratings for each plant species after coveringthe plot twice.

Live tortoises.—We processed live tortoisesencountered on and off plots using protocolsdescribed by Berry and Christopher (2001).We recorded tortoise location, activity, andsex, and took measurements on carapacelength at the midline, plastron length fromnotch to notch, and weight. We collected dataon clinical signs of infectious disease, shelldisease, and trauma. Observations for poten-tial infectious diseases included general con-dition and behavior (e.g., active, listless,unresponsive); appearance of the nares (e.g.,presence or absence of a nasal discharge;amount, color, and opacity of discharge;occlusion of nares); presence of a dischargefrom the chin glands during the nonbreedingseasons in adult males and at any time of yearin adult females and juveniles; appearance ofeyes (e.g., sunken, wet, or crusted); presenceof caked dirt in, on, or near the beak or on theforelegs; and ulcers, plaques, or other lesionsin the oral cavity. Of particular interest weresigns of infectious disease, e.g., upper respi-

ratory tract disease caused by mycoplasmosis(Jacobson et al., 1991; Brown et al., 1994,2004; Johnson et al., 2006), lesions in themouth typical of herpesvirus (Johnson et al.,2005; Jacobson, 2007), a shell disease de-scribed as cutaneous dyskeratosis (Jacobson etal., 1994; Homer et al., 1998), and traumafrom dog attacks (Boyer and Boyer, 2006). Weranked the distribution, severity, and chronic-ity of clinical signs as none, mild, moderate, orsevere on forms similar to those published inBerry and Christopher (2001).

We took digital photographs of the carapace,plastron, posterior left costal, right and left eyesand periocular areas, and a frontal view of thenares and beak. If the tortoise had ectopara-sites, or unusual anomalies or injuries (e.g.,missing limb), we took additional images.

Shell-skeletal remains.—Field-workers col-lected shell-skeletal remains encountered onand off plots. Before collecting the remains,they noted condition of the remains and signsof human activities or predators that may havecaused or contributed to the death (e.g., trail ortracks of vehicles, human footprints, or pred-ator scat). They took photographs of theremains in situ to document the setting wheredeath may have occurred. In addition, theytook at least three images of each shell-skeletalremains before handling: (1) a general pictureshowing the remains within the context of soils,vegetation, and land uses (if any); (2) a close-upimage of the remains; and (3) a close-up imageof the oldest and most deteriorated portion ofthe scutes and bones. Then they placedremains in a heavy-duty ziplock plastic bagfor transfer to the US Geological Survey FieldStation for further analysis.

Tortoise sign.—We primarily collected dataon two types of tortoise sign: cover sites andscat. We measured and assigned shelters orcover sites (defined as burrows, caves, pallets,and rock shelters used by tortoises afterBurge, 1978) to one of five classes on thebasis of recency of use and condition (Berry etal., 2008; Keith et al., 2008). Similarly, wemeasured, aged, and recorded the numberand sizes of tortoise scats using three ageclasses of recency and states of deterioration(Berry et al., 2008; Keith et al., 2008).Additionally, we identified and estimatedrecency of tortoise tracks by looking for


impressions of scales and marks from toenailsand tails. For courtship rings, we looked fordepressions with freshly churned soil, tortoisetracks, drag marks from plastrons, and smallareas of dried, clotted soil. At drinking sites,we looked for impressions of toenails in driedmud and scats.

Predators.—To evaluate geospatial relation-ships between predators and live and deadtortoises and other tortoise sign, the field teamsearched for perches, nests, roosts, burrows,dens, and scats of avian and mammalianpredators. They recorded numbers and loca-tions of these sites and also examined them forfragments of tortoise skin, scutes, and bones.

Human impacts.—To document historicaland ongoing land uses, field-workers collecteddata on types and amounts of human distur-bance.

Data Analysis

Vegetation.—We categorized plots by veg-etation association by performing k-meansclustering analysis on perennial plant data(Version 6.0; StatSoft, Inc., 2001) using the sixordinal categories of abundance. We specifiedk 5 4 clusters for the analysis, and thenverified that the four associations of perennialspecies were of biological significance byevaluating composition, relative abundance,and diversity within each of the vegetationassociations and comparing each cluster to theHierarchical List of Natural Communitieswith Holland Types (California Departmentof Fish and Game, 2010) to assign a vegetationcommunity name. We used ANOVA tocompare elevations among different vegeta-tion associations and Tukey honestly signifi-cant difference (Tukey HSD) tests to conductpair-wise comparisons. We considered thosespecies with mean relative abundance valuesgreater than the midpoint between theminimum and maximum relative abundancevalues (2.35) as ‘‘abundant species.’’ We alsoused exact binomial tests to perform pair-wisecomparisons among the numbers of plotsassociated with each vegetation association toidentify differences both within and amongmanagement areas (binom.test function; RDevelopment Core Team, 2013).

Underlying all statistical tests was theassumption that our plots were independently

random in their representation of eachmanagement area. This assumption was metby the random process in which the plots wereselected; however, we systematically excludedcertain private lands not held by the DesertTortoise Preserve Committee because ofprivacy considerations. We have no reason toexpect that our sample plots differed substan-tially from the excluded private lands. TheANOVA tests further assume that data arenormally distributed and homoscedastic. Wedid not need to check these assumptions,because our models are highly robust tononnormal distributions due to the well-established asymptotic properties of the mod-el test statistics (Arnold, 1981). For example,Jacqmin-Gadda et al. (2007) used sample sizesof 50 and 200 simulated nonnormal data todemonstrate that general linear mixed models,as a generalization of ANOVA models, hadaccuracy rates similar to those when per-formed on correctly specified normal data.We considered P , 0.05 as statisticallysignificant for the ANOVA, Tukey HSD, andbinomial exact tests.

Live and dead tortoises.—We assigned asize–age class to live tortoises by carapacelength at the midline: juvenile # 99 mm,immature 5 100–179 mm, small adult/sub-adult 5 180–207 mm, and adult $ 208 mm.To assess whether sex ratios of subadult andadult tortoises differed significantly from theexpected 1:1 ratio, we used exact binomialtests to compare numbers of females andmales. We used a 1000-simulation bootstrapto estimate densities (tortoises/km2) and 95%confidence intervals of live subadult and adulttortoises and all sizes of tortoises for each ofthe three management areas, as well as for theentire study area (Barreto and Howland,2006). To avoid cumbersome reporting, wepresented the estimated densities/km2 fol-lowed by the 95% interval in parentheses.We also estimated relative age of adults usingage classes developed by Berry and Woodman(1984). For shell-skeletal remains, we mea-sured or estimated carapace length, estimatedtime since death (dead # 4 yr, . 4 yr), andassigned causes of death (see details ofmethods in Berry et al., 2013). We calculatedcrude annual death rates for each manage-ment area, specifically for adult tortoises


found dead on plots and with an estimatedtime since death of # 4 yr; we used theequation d 5 D/N, where D is the number ofdead adults/4 yr, and where N is the numberof live and dead adult tortoises on plots.

Comparisons of predator activity and an-thropogenic impacts among managementareas.—We compared counts of CommonRavens (Ravens) and mammalian predator scat(Mammals) found on plots. We similarlycompared surrogate variables for anthropogen-ic impacts, measured specifically as counts ofsheep scat (Sheep); vehicle tracks (Vehicles);trash (Trash); shooting debris, including cas-ings, shells, and targets (Firearms); and mines(Mines). We calculated amount of surfacedisturbance (m2) for partially and completelydenuded areas, dirt roads, and vehicle routes,trails, and tracks. For variables with abundantcounts (. 10 total per management area), weused generalized linear models (McCullagh andNelder, 1989) with a log link function to analyzedifferences among management areas (glmfunction; R Development Core Team, 2013).We used quasi-Poisson distributions in ourmodels when overdispersion was evident (i.e.,variance . mean m). Thus, we corrected theSEs by modeling variance as m 3 ø (mean 3overdispersion parameter; Zuur et al., 2009).For variables with low counts, we conductedexact binomial tests between pairs of areas todetermine significant differences (binom.testfunction; R Development Core Team, 2013).We considered variables with P , 0.10 asstatistically significant for the models and used90% confidence intervals.

Models of tortoise presence across thestudy area.—We used generalized linearmodels with a logit link for binary outcomesof tortoise presence to examine the relation-ship between tortoise presence and thecovariates for the three management areascombined and for each management areaseparately (R Development Core Team,2013). Tortoise presence was indicated bythe occurrence of live tortoises, shell-skeletalremains, and tortoise sign on each plot.Tortoise sign can be used as a surrogate forlive tortoises, because a positive correlationhas been demonstrated between tortoise signand tortoise densities (Krzysik, 2002). Here-after live tortoises, shell-skeletal remains, and

tortoise sign collectively are called tortoisesign. We treated tortoise sign as a binaryresponse variable where 0 5 no live or deadtortoises or sign detected on the plot, and 1 5live, dead, or sign detected on the plot.

We evaluated all variables for indications ofoutliers that had .10% influence on correla-tions with tortoise sign using the Pearson rinfluence feature in Systat (Systat Software,Inc., 2007). We removed one plot on theTortoise Natural Area that had large trashcounts, apparently as a result of an oldhabitation or mining operation. All explanato-ry variables except Mines were log-trans-formed (log10) to normalize their distributionsand reduce the influence of unusually largevalues. We conducted correlation analyses andcalculated generalized variance inflation fac-tors to assess multicollinearity (vif function;Fox and Weisberg, 2011). We omitted surfacedisturbance from models because of highcorrelations with other predictors (r . 0.8).Because multicollinearity can obscure theeffects of predictors when used in combina-tion, we used two complementing strategies tomodel Desert Tortoise presence. Our firstapproach was to model the relationshipbetween Desert Tortoise presence with eachindividual predictor alone in a separate model,but including management area and vegeta-tion association effects in every model tocontrol for differences in plot types. Oursecond approach was to select a set ofvariables that best predict Desert Tortoiseoccurrence; we performed backward removalmodel selection by starting with a full modelwith all predictors, including interactioneffects with management area, and sequen-tially dropping variables that were not signif-icant according to chi-square tests at the 0.1significance level (drop1 function; R Devel-opment Core Team, 2013). We arrived at afinal model when no further variables couldbe dropped, and estimated the probability andodds of Desert Tortoise presence with respectto significant effects (lsmeans function; Lenth,2013). For continuous predictors with modelcoefficients b, we estimated the percent changein the odds of Desert Tortoise presence as(exp[b] 2 1) 3 100%.

Models of tortoise abundance for eachmanagement area.—Using a separate gener-


alized linear model for each of the threemanagement areas, based on a Poisson (orquasi-Poisson, if overdispersion was evident)distribution and log link function, we modeledtortoise sign counts to examine tortoise abun-dance in relationship to vegetation associationsand anthropogenic and predator variables. Aswith our presence models, we performedbackward removal starting with full modelsthat included the same variables: Vegetation,Sheep (private lands only), Vehicles, Trash,Firearms, Mines (Tortoise Natural Area andcritical habitat only), Ravens, and Mammals.Only one sheep scat was found in the TortoiseNatural Area and no sheep scat was found incritical habitat. No mines were found in theprivate lands. We dropped variables using Fand chi-square tests, for quasi-Poisson andPoisson models respectively, until only vari-ables significant at the 0.1 level remained. Forcontinuous predictors with model coefficientsb, we used b to calculate the percent change inmean tortoise sign.

RESULTS

Precipitation

Precipitation during the hydrologic year(1 October 2010–30 September 2011) was.218 mm for the fall–winter of 2010–2011 (1October–30 March), and was above normal.The 2011 records from the National ClimateData Center were incomplete with missingvalues, and therefore precipitation could havebeen higher. Rainfall was recorded in everymonth between 1 October and 30 March.Wildflowers and therefore tortoise forageplants were abundant.

Perennial Vegetation

We identified 39 species of perennialshrubs and four species of perennial grasseson the plots. Creosote bushes and white bur-sage were common to dominant within all fourassociations. We assigned each plot to one offour vegetation associations (Table 3; Fig. 2):(1) creosote bush/white bur-sage (hereaftercreosote bush) had only two abundant speciesamong the 26 species found on these plots;(2) creosote bush/white bur-sage/Andersonbox-thorn (hereafter box-thorn) had a total offive abundant species, including cheesebushand goldenhead; (3) creosote bush/white bur-sage/Mojave indigo bush (hereafter indigobush) had nine dominant species (Andersonbox-thorn, cheesebush, goldenhead, Nevadaephedra, Mojave California buckwheat, anddesert trumpet, Eriogonum inflatum); and (4)creosote-bush/white bur-sage/Nevada ephe-dra (hereafter Nevada ephedra) was the mostdiverse, with 11 dominant species (golden-head, Mojave indigo bush, Mojave Californiabuckwheat, Anderson box-thorn, winter fat,hop-sage, and Mojave aster [Xylorhiza torti-folia]). The vegetation associations differedsignificantly in elevations (F3,236 5 16.158, P5 0.001), with Nevada ephedra occurring athigher elevations than the other three vege-tation associations, and indigo bush occurringat a higher elevation than box-thorn (TukeyHSD, P 5 0.034; Table 3). More plots incritical habitat and on private lands wereassigned to the creosote bush association, theleast diverse association with only two abun-dant species, than to the more diversevegetation associations with more abundantspecies (exact binomial P , 0.05; Table 3).

TABLE 3.—Vegetation associations found in the three management areas, eastern Kern County, California, indescending order by total plots assigned. Number of species includes perennial species (shrubs, herbaceous perennials,and bunch grasses). Bold values for creosote bush indicate that significantly more plots were assigned to this associationthan to other vegetation associations in the respective management areas. Elevation values sharing a superscript capitalletter were not statistically different at P , 0.05.

Vegetation associationname

Total plots assigned

No. ofspecies

No. and %abundant species

Elevation range(m)

Elevation mean(m) 6 SE

TortoiseNatural Area

Criticalhabitat

Privatelands

Total plotsassigned

Creosote bush 20 37 49 106 26 2 (7.7%) 590–960 788 6 10.6BC

Box-thorn 31 21 26 78 36 5 (13.9%) 613–1027 763 6 10.6C

Indigo bush 22 8 2 32 31 9 (29.0%) 682–1002 821 6 13.3B

Nevada ephedra 7 14 3 24 30 11 (36.7%) 753–1210 924 6 26.2A


Live Tortoises

The field team located 17 live tortoises onthe 240 plots: 12 in the Tortoise Natural Area,2 in critical habitat, and 3 on private lands.They observed an additional 27 tortoises offplots, when walking from one plot to anotheror to and from vehicles (Tortoise NaturalArea, 11; critical habitat, 12; private lands, 4).In critical habitat, they observed a tortoise ona vehicle route, a juvenile on a motorcycletrail, and a third tortoise on a road.

The density and confidence intervals foradult tortoises on all 240 plots combined forthe entire study area were 5.5 (5.4–5.6)/km2.When we analyzed densities of adult tortoisesseparately for the three types of managedlands, the results differed significantly fromone another by management area: for theTortoise Natural Area, 10.2 (9.9–10.4) tortois-es/km2; for critical habitat, 2.4 (2.3–2.6)tortoises/km2; and for private lands, 3.7 (3.6–3.8) tortoises/km2. When we estimated densi-ties and confidence intervals for all sizes oftortoises separately for the three types ofmanaged lands, the results were 14.8 (14.6–15.1)/km2 for the Tortoise Natural Area, 2.4(2.3–2.6)/km2 for critical habitat, and 3.7 (3.6–3.8)/km2 for private lands.

Adults composed the majority of thesamples for both on- and off-plot tortoises.For the 17 on-plot tortoises, the compositionof the tortoises by size–age class was 11 adults$208 mm carapace length, 2 small adults, 2large immature tortoises, and 2 juveniles(Table 4). The sex ratio of female-to-malesubadult and adult tortoises was 9:4 and notstatistically different from 1:1 (P 5 0.27).Juvenile and immature tortoises were ob-served only on the Tortoise Natural Area

plots. For the 27 off-plot tortoises, the size–ageclass composition was similar, with 21 adults, 1small adult, 2 immature tortoises, and 3juveniles, and the female-to-male sex ratiowas 10:12, which was not statistically differentfrom 1:1 (P 5 0.83). Overall, for both on- andoff-plot tortoises, the female-to-male sex ratiowas 19:16 and also not significantly differentfrom 1:1 (P 5 0.74). Of the 34 adult tortoisesthat could be assigned an age class, 38% wereyoung and growing, 24% were of middle age,and 38% were in the old-age classes.

Of the 44 tortoises observed during thesurveys, 34 received comprehensive healthevaluations. We could not handle the remain-ing tortoises because of federal permit con-straints, but instead recorded some data foreach individual on the basis of field observa-tions. Two adult male tortoises, one on privatelands and one in the Tortoise Natural Areamanagement area, had moderate clinical signsof upper respiratory tract disease characteris-tic of mycoplasmosis (damp or wet beak fromexudate or bubbles from the nares). Bothtortoises also had other clinical signs, such asocular discharge, crusts on the palpebrae andperiocular area, mucus on the globe, andexposed conjunctiva. The nares of 19 othertortoises were partially or completely occlud-ed, potentially from dried exudate or fromplant sap or dirt and mud from drinkingduring rainstorms.

Without exception, signs of predator attackswere evident on all 25 adult tortoises that wehandled. Twelve of the 25 had moderate tosevere damage to the gular horn; for ninetortoises the gular horn was severely reducedor chewed away completely. Some signs oftrauma (extensive chewing) appeared typical

TABLE 4.—Size–age classes of live Desert Tortoises and shell-skeletal remains occurring on and off plots on theDesert Tortoise Research Natural Area, critical habitat, and private lands in the study area in the western MojaveDesert, eastern Kern County, California.

Plot location Tortoise Natural Area, on plot (off plot) Critical habitat, on plot (off plot) Private lands, on plot (off plot)

Dead Dead Dead

Size–age class Live #4 yr .4 yr Live #4 yr .4 yr Live #4 yr .4 yr

Juvenile 2 2 (1) (3) 1Immature 2 (1) 2 (1) 2 (1) (1)Subadult 1 8 (1) 1 (1) 1 3 (2)Adult 7 (10) 1 (1) 9 (11) 1 (7) 8 (3) 8 (5) 3 (4) 1 1 (1)Totals 12 (11) 5 (2) 17 (12) 2 (12) 12 (4) 11 (7) 3 (4) 1 (1) 1 (1)


of domestic dog attacks. One adult tortoisehad a healed injury from crushing, potentiallyby a vehicle. Two juvenile tortoises had antheads attached to soft parts of the integument.

Shell-Skeletal Remains and Death Rates

We found shell-skeletal remains of 47tortoises on plots, and observed 27 off plots(Table 4). We collected more remains from theTortoise Natural Area and critical habitat plotsthan from private land plots. Eighteen of the 47(38.3%) on-plot remains were from tortoisesdead # 4 yr: 11 adults, 4 immatures, and 3juveniles. We assigned the 11 on-plot adulttortoises to two general categories: (1) un-known and found in desert woodrat (Neotomalepida) middens (n 5 5), and (2) traumaticdeaths (n 5 6). We separated traumatic deaths(fractured scutes, bones) by cause, wherepossible: three showed signs of mammalianpredation (one, domestic dog), one had bothpellet holes (gunshot death) and punctures,one was probably a vehicle kill, and one waskilled by a predator at a site with a CommonRaven perch and mammalian predator scatsand prey remains. Three of the seven recentjuvenile and immature remains showed signs ofhaving been killed by Common Ravens or smallmammals; we could not determine the causesof death for the other four. Causes of death foroff-plot tortoises were similar and includedpredation by canids and Common Ravens,gunshot and vehicle kills, and unknown.Remains with evidence of gunshots occurredboth within the Tortoise Natural Area andcritical habitat; remains of tortoises likely tohave been killed by vehicles were in theTortoise Natural Area and on private lands.

Crude annual death rates for adults for the4 yr preceding the survey differed by man-agement area. The lowest rate was in theTortoise Natural Area, with 2.8%/yr, followedin ascending order by private lands with 6.3%/yr and critical habitat with 20.4%/yr.

Tortoise Sign on Plots

We observed sign on 55.0% (44/80) of theTortoise Natural Area plots, 47.5% (38/80) ofcritical habitat plots, and 12.5% (10/80) ofprivate land plots (Fig. 3). Sign counts werehighest (n 5 190) on the Tortoise NaturalArea plots, followed in descending order by

critical habitat plots (n 5 90) and private landplots (n 5 20). Scats (n 5 159) were the mostcommon type of sign observed, followed byburrows and pallets (n 5 77) and shell-skeletalremains (n 5 47). The Tortoise Natural Areahad ,2 3 the total sign count of criticalhabitat and ,9 3 the count of private lands.Shell-skeletal remains accounted for 25.6% ofon-plot sign in critical habitat, compared with11.6% at the Tortoise Natural Area and 10.0%on private lands.

Predators

We observed nine species of potential avianpredators both on and off plots: the CommonRaven (n 5 193), Loggerhead Shrike (Laniusludovicianus, n 5 5), Red-tailed Hawk (Buteojamaicensis, n 5 4), Golden Eagle (Aquilachrysaetos, n 5 4), Burrowing Owl (Athenecunicularia, n 5 2), Greater Roadrunner(Geococcyx californianus, n 5 1), NorthernHarrier (Circus cyaneus, n 5 1), AmericanKestrel (Falco sparverius, n 5 1), and PrairieFalcon (Falco mexicanus, n 5 1). CommonRavens composed 91.5% of the observations.We saw more avian predators on the TortoiseNatural Area (75 on plots, 13 off plots)compared with critical habitat (12 on plots,20 off plots) and private lands (32 on plots, 41off plots). We found remains of a juveniletortoise below the perch of a Common Ravenoff plot at the Tortoise Natural Area. Sightingsof Common Ravens were more common onplots in the Tortoise Natural Area and privatelands than in critical habitat (Table 5; P 50.002 and P 5 0.018, respectively).

We detected three species of mammalianpredators by finding concentrated areas ofmarking sites, dens, and den complexes onplots: kit fox (Vulpes macrotis, n 5 22), coyote(Canis latrans, n 5 15), and badger (Taxideataxus, n 5 1); we assigned seven additionalobservations to canid sign, because it wasunclear if the scat was from a coyote or kit fox.Signs of concentrated activity by mammalianpredators (i.e., dens, marking sites) also weremore common on plots within the TortoiseNatural Area (n 5 25) than on plots withincritical habitat (n 5 11) or private lands (n 59). By far the most common sign was scat orgroups of scat deposited by coyotes and kitfoxes (Table 5). We found no remains of


tortoises in 1222 predator scats or at 26 of thepredator dens. We observed one kit fox on aplot and one coyote off plot within theTortoise Natural Area during the surveys.Although we noted twice as many scat from

mammalian predators in the Tortoise NaturalArea compared with critical habitat andprivate lands (Table 5), the differences in scatcounts between the three management areaswere not significant (P 5 0.15).

FIG. 3.—Locations of Desert Tortoise sign on the 240 1-ha plots, including live tortoises, remains of dead tortoises,cover sites (burrows and pallets), and scat in three management areas in the western Mojave Desert, eastern KernCounty, California.

TABLE 5.—Total counts for observations of Common Ravens and mammalian predator scats and five anthropogenicvariables in the three types of management areas for Desert Tortoises in the western Mojave Desert, California. Boldfont emphasizes a management area with a variable that is significantly higher than the same variable in one or both ofthe other management areas (P , 0.05).

Type of disturbance on plots Total counts from principal sources of anthropogenic impacts (no. plots affected)

(Variable names) Desert Tortoise Research Natural Area Critical habitat Private lands

Common Ravens (Ravens) 88 (40) 32 (24) 73 (36)Mammalian predator scat (Mammals) 652 (55) 239 (47) 331 (30)Sheep scat (Sheep) 1 (1) 0 (0) 85,208 (77)Vehicle tracks (Vehicles) 11 (6) 180 (42) 1407 (73)Trash, general (Trash) 362 (49) 370 (40) 1324 (73)Shooting debris: casings, shells, targets (Firearms) 191 (18) 108 (18) 209 (20)Mining pits, excavations (Mines) 6 (4) 6 (6) 0 (0)


Human Uses

Counts of historic and recent anthropogen-ic impacts and amounts of area partially orcompletely denuded of vegetation differed bymanagement area (Table 5). In general, plotson private lands had higher counts and moresurface area disturbed by human uses than theother two areas. Total surface area disturbedwas 8507 m2 on the Tortoise Natural Area,7082 m2 on critical habitat, and 32,530 m2 onprivate lands. Evidence of sheep grazing wasalmost nonexistent on plots in the TortoiseNatural Area and critical habitat but signifi-cantly more prevalent on private land plots (P, 0.01). Trash was a common occurrence onplots but counts were highest on private lands(P 5 0.01), where 91% of plots had trash. Onthe Tortoise Natural Area, trash was generallyold and associated with old homesteads andabandoned mines (broken bottles, rustedcans, old pieces of metal, wood). In criticalhabitat and on private lands, visitors common-ly deposited trash at camp sites, in areas withrecreation use, along roadsides, and at edgesof urban areas. Counts of debris from shooting(Firearms), another form of trash, weresimilar on all three management areas (P 50.67); some were very old. Mining was limitedto six sites each at the Tortoise Natural Areaand critical habitat and was minimal, yetstatistically greater than the absence of miningon private lands (P 5 0.03). With fewexceptions, mining consisted of small, shallowpits and bulldozed areas ,400 m2. Weconsidered one pit, covered with boards, ashazardous to tortoises.

Vehicle tracks were least common on theTortoise Natural Area, although not statisti-cally different from the critical habitat (P 50.10), and significantly more common onprivate lands (P , 0.01; Table 5). The amountof surface area disturbed by vehicle tracks waslowest (467 m2) on Tortoise Natural Areaplots, higher on critical habitat plots (4109 m2),and highest on private land plots (10,074 m2).Counts of areas denuded or partially denudedof vegetation by vehicles were lowest in theTortoise Natural Area and critical habitat (n 50) and highest on private land plots (n 5 297);total area disturbed was 18,820 m2 on privatelands. One partially denuded area, probablynonvehicle in origin, covered 400 m2; 12

similar partially or completed denuded areascovering 1736 m2 occurred on private landplots. Dirt road counts, whether graded orungraded, were similar on the three manage-ment areas; total area disturbed was higheston the Tortoise Natural Area (7640 m2) andlower on critical habitat and private lands(2863 m2 and 2320 m2, respectively). Theimportant difference between the TortoiseNatural Area and other management areas isthat dirt roads within Tortoise Natural Areaboundaries have received little or no use since1980. Only landowners with inholdings insideTortoise Natural Area boundaries can drive totheir parcels (a rare event). As a result, shrubsare in the process of colonizing most of theseroads.

Models

We began by evaluating relationships be-tween tortoise presence and single predictors.Anthropogenic and predator variables weregenerally negatively correlated with DesertTortoise presence, except for Mammals,which was positively correlated; however, therelationships of these single predictors withDesert Tortoises were imprecise, and Sheepwas the only predictor with a significantrelationship when we considered only onepredictor (Table 6). Although no other singlepredictor had a strong relationship with theDesert Tortoise, when we included all vari-ables together in the full model, Sheep andmanagement area were the only two predic-tors with a high variance inflation factor (6.9and 10.7, respectively), which suggests multi-collinearity between these variables. After weconducted backward removal model selection,Sheep remained as a significant predictor,with a 21% decrease in odds of occurrence oftortoise sign per twofold increase in Sheep (P5 0.06), whereas management area showeddifferences only with respect to the effects ofMammals within areas (Table 7; Appendix 1).The occurrence of tortoise sign also varied byvegetation association, with box-thorn havingthe highest probability of occurrence (0.46),about double that of Nevada ephedra andindigo bush (Table 7).

When we modeled the three managementareas separately, we found differences inmean numbers of tortoise sign, with the


Tortoise Natural Area having the highest sign(2.2 per plot), followed by critical habitat(0.87) and private lands (0.09–0.43, dependingon vegetation association; Table 8; Appendix1). Desert Tortoises in the Tortoise NaturalArea had no positive or negative associationswith predators, livestock, or other human-related impacts. In critical habitat, tortoisesign was negatively associated with Vehicles(28% decrease per twofold increase in tracks);in private lands, tortoise sign was negativelyassociated with Sheep (14% decrease pertwofold increase in Sheep). However, num-

bers of tortoise sign were also positivelyassociated with certain anthropogenic im-pacts, including Firearms in critical habitat(52% increase per twofold increase in countsof Firearms) and Trash on private lands (26%increase per two-fold increase in Trash), andwith Mammals in both critical habitat andprivate lands (41% and 46% increase, respec-tively, per twofold increase in mammalianpredator sign). In private lands, we foundmore tortoise sign in the box-thorn vegetationassociation than in creosote bush (P 5 0.003);however, we were unable to test the Nevada

TABLE 7.—Estimated probabilities of the presence of Desert Tortoise sign (live and dead tortoises and other sign) ineastern Kern County, California, including three management areas: Tortoise Natural Area, critical habitat, and privatelands. Odds are the ratio of probability of presence divided by probability of absence, and change multiplicatively withincremental changes in predictors of binomial logistic models. These estimates and their chi-square tests were calculatedby the final binomial logistic model of presence and absence of Desert Tortoise sign after backward removal ofnonsignificant variables (P . 0.10) from a full model with all predictors. Significance levels: * 5 P , 0.10, ** 5P , 0.05. Vegetation associations sharing a superscript capital letter were not significantly different (P . 0.05).

Probability and odds of Desert Tortoise sign, or % change in odds ofDesert Tortoise sign per increment in predictor

Predictor variable df x2 P Estimate (90% interval)

Vegetation association 3 7.18 0.066* 0.46 (0.35 to 0.56)A

0.83 (0.54 to 1.30)Probability, box-thorn

Odds0.35 (0.26 to 0.45) AB

0.53 (0.35 to 0.81)Probability, creosote bush

Odds0.25 (0.14 to 0.39)B

0.33 (0.17 to 0.65)Probability, indigo bush

Odds0.20 (0.10 to 0.36)B

0.25 (0.11 to 0.56)Probability, Nevada ephedra

OddsLog(Sheep) 1 3.53 0.06* 221% (23.5 to 236) % change, per 23

increment in SheepManagement area 2 0.539 0.764Log(Mammals) 1 0.35 0.554Management area 3

log(Mammals)2 6.11 0.047**

TABLE 6.—Estimated coefficients from a binomial logistic model on the presence of Desert Tortoise sign (live anddead tortoises and other tortoise sign) based on seven predictor variables (five anthropogenic uses and two types ofpredator sign). Each coefficient was estimated in isolation by separate models of presence and absence of DesertTortoise sign, with only management area and vegetation factors included in all models to control for long-termcategorical effects. Regression coefficients (b) are presented with SE, z-statistic (z), P-values (P), the 90% confidenceinterval, and the percent change in the odds of Desert Tortoise sign occurrence for a given change in predictor. * 5significant at P , 0.10.

Predictor variable

Percent change in odds of Desert Tortoise sign

b SE z P % change 90% interval Predictor change

Log(Ravens) 21.056 0.675 21.56 0.118 227.2 (248 to 2) 23 increment in RavensLog(Mammals) 0.196 0.332 0.59 0.556 6.1 (210 to 25) 23 increment in MammalsLog(Sheep) 20.726 0.397 21.83 0.068* 219.6 (234 to 22) 23 increment in Sheep scatLog(Vehicles) 20.231 0.431 20.54 0.592 26.7 (225 to 15) 23 increment in Vehicle tracksLog(Trash) 20.284 0.321 20.88 0.376 28.2 (222 to 8) 23 increment in TrashLog(Firearms) 20.017 0.432 20.04 0.968 20.5 (220 to 23) 23 increment in FirearmsMines 20.384 0.533 20.72 0.472 231.9 (272 to 64) 1 additional Mine


ephedra or the indigo bush vegetation associ-ations for differences because the plots werefew and no tortoise sign occurred on them(Tables 3 and 8).

DISCUSSION

The topic of protected areas—their role inconserving biodiversity, maintaining popula-tions of rare and endangered species andhabitat integrity, and overall managementeffectiveness—has been a common theme inconservation science during the last decade(e.g., Leverington et al., 2010; Geldmann etal., 2013; Le Saout et al., 2013). Our studyexplores the effectiveness of three manage-ment strategies for two protected areas, theTortoise Natural Area and adjacent criticalhabitat, as well as recently acquired privatelands, in delivering positive conservationoutcomes for the Desert Tortoise by evaluat-ing the tortoise population, predators andpredation, and habitat condition in the contextof historic land uses and management plans.Our findings represent a spectrum based onland-use histories and protective measures.

Desert Tortoise Populations

Between the late 1970s and early 1990s,densities of all sizes of tortoises declined

precipitously in the western Mojave Desert(Table 1; Berry et al., 1986a, 1986b; Berry andMedica, 1995; Brown et al., 1999). At theTortoise Natural Area and in Fremont Valley,population declines of .90% were document-ed. The causes were numerous (USFWS,1994a). Vandalism, vehicle kills on and offroads, and illegal collecting occurred (e.g.,Berry, 1986; Berry et al., 1986a, 1986b).Mycoplasmosis contributed to populationdeclines in the late 1980s and early 1990s(Jacobson et al., 1991; Brown et al., 1999).Hyperpredation by the Common Raven,described ,30 yr ago by Campbell (1983),has been and continues as a source ofmortality to juvenile and immature tortoises(Berry et al., 1986a; Boarman, 1993; Boarmanand Berry, 1995; Kristan and Boarman, 2003).The estimate of population density, 5.5 (5.4–5.6) subadult and adult tortoises/km2 for theentire study area during spring of 2011, issimilar to and within the densities reportedby the USFWS using distance sampling, atechnique of estimating densities at a land-scape scale for critical habitat in the westernMojave Desert (USFWS, 2010). The USFWSreported annual density estimates for adulttortoises ($180 mm carapace length, includessubadult tortoises) from 2001 through 2007

TABLE 8.—Estimated mean number of Desert Tortoise sign for each management area: Tortoise Natural Area, criticalhabitat, and private lands in eastern Kern County, California. These estimates and their chi-square tests were calculatedfor each management area separately by the final Poisson model (quasi-Poisson for the Tortoise Natural Area), afterbackward removal of nonsignificant variables (P . 0.10) from a full model with all predictors. Significance levels: * 5P , 0.10, ** 5 P , 0.05, *** 5 P , 0.010. Incr 5 increment; Na 5 not applicable.

Mean Desert Tortoise sign, or % change per increment in predictor

Management area Variable df x2 P Estimate (90% interval)

Tortoise NaturalArea

None 2.2 (1.6 to 2.9) Mean

Critical habitat 0.87 (0.61 to 1.2) MeanLog(Vehicles) 1 3.91 0.048** 228% (23.9 to 245) % change, per 23 incr in vehicle

tracksLog(Firearms) 1 5.81 0.016** 52% (17 to 97) % change, per 23 incr in firearmsLog(Mammals) 1 9.34 0.002*** 41% (18 to 69) % change, per 23 incr in predator

sign

Private lands Vegetationassociation

3 12.3 0.006*** Na Mean, Nevada ephedraNa Mean, indigo bush

0.09 (0.04 to 0.20) Mean, creosote bush0.43 (0.27 to 0.71) Mean, boxthorn

Log(Sheep) 1 3.56 0.059* 214% (23.2 to 224) % change, per 23 incr in sheepLog(Trash) 1 3.65 0.056* 26% (3.6 to 53) % change, per 23 incr in trashLog(Mammal) 1 5.27 0.022** 46% (15 to 85) % change, per 23 incr in

mammals


for the entire western Mojave Desert: meanfigures ranged from 3.8 to 6.1 (3.0–8.5)/km2.In 2011, in a survey of critical habitat in theFremont–Kramer management area, theUSFWS estimated a density of 3.5 adulttortoises/km2 (USFWS, 2012). Therefore, den-sities have remained disturbingly low, com-pared with those reported for periods 26 to32 yr earlier using mark–recapture studies inthe region, e.g., from 40 to 92 subadult andadult tortoises/km2 (Berry et al., 1986a, 1986b;Berry and Medica, 1995).

During our study, densities of live tortoiseswere significantly higher inside the TortoiseNatural Area fence than outside in criticalhabitat (,63) or private lands (,43), andsign counts further corroborate the finding. Incontrast, critical habitat and private lands hadsignificantly lower densities of adult tortoisesand lower tortoise sign counts by .50%.Overall, in all three management areas, thetortoise population was composed primarily ofadults in a relatively even sex ratio. The fieldteam observed live juvenile and immaturetortoises on plots inside the Tortoise NaturalArea and off plots in critical habitat, indicatingthat some individuals in these age classes werepresent.

Crude annual death rates for adult tortoiseswere lowest in the Tortoise Natural Area(2.8%/yr), followed by private lands (6.3%/yr)and critical habitat (20.4%/yr). The high deathrates in critical habitat were of particularconcern: shell-skeletal remains composed25.6% of on-plot tortoise sign, whereas inboth the Tortoise Natural Area and privatelands, shell-skeletal remains were ,12% ofthe total tortoise sign. When causes of deathcould be determined, they included vehicles,gunshot, and predation by ravens and mam-mals. We have estimated that subadults andadults in stable populations of this long-livedspecies probably had annualized death ratesof adult age classes of ,2%/yr in the past(Turner et al., 1987).

Subsidized Predators

Counts of Common Ravens and Mammalswere highest in the Tortoise Natural Area,followed in descending order by private landsand critical habitat. The modeled relationshipsbetween tortoise abundance and predators

didn’t follow this pattern, however. Themodels showed no significant associationsbetween tortoise abundance and Ravens inany management area, whereas the results ofmodeling effects of Mammals on tortoiseabundance varied by management area. Themost protected area, the Tortoise NaturalArea, had no significant associations, positiveor negative, with predators, in contrast tocritical habitat and private lands, where theassociation between tortoise abundance andMammals was positive. However, more thanone-third of live tortoises in the study showedsigns of predator attacks by mammals (coy-otes, kit fox, and dogs), and many tortoiseshell-skeletal remains showed signs of preda-tion by mammals as well as Common Ravens.Of particular concern was the moderate tosevere damage to shells of 12 of 34 livetortoises; predators severely reduced orchewed off the gular horn in nine cases.Severe injury to the gular horn is typical ofdog attacks, which are more common within5 km of settlements than in remote areas(Andrea Carlson and K.H. Berry, personalobservations). A potential confounding factoris that dead tortoises, killed by predators, areincluded as part of the total tortoise sign usedin the models. Furthermore, both the mam-malian predators and Common Ravens havethe potential to severely limit recovery oftortoise populations and potentially to drivelocal populations to extinction (Kristan andBoarman, 2003; Esque et al., 2010).

The explanations for the significant positiverelationships between tortoise abundance andMammals in two of the management areas(critical habitat, private lands) may be com-plex and involve other, unmeasured factors.Subsidies and attractants for mammals in theform of food, trash, and water may beimportant. Although standing water does notoccur within the management areas, year-round food and water are available at one city,two towns, and two settlements within 4 km ofthe study area edge. The study area also is inproximity to heavily used recreation areas,each with tens of thousands of visitors peryear: Jawbone Canyon and Dove Springs off-highway vehicle areas, Red Rock CanyonState Park, the Spangler Hills off-highwayvehicle area, and the El Paso Mountains


(Fig. 1). Trash was available in all threemanagement areas but generally was old inthe Tortoise Natural Area.

The higher numbers of predator observa-tions in the Tortoise Natural Area may berelated to available prey of wild fauna andcover, as well as lack of human disturbancesand interference. Studies by Brooks (1995,1999) support these observations. Brooksreported a greater abundance and diversity ofnocturnal rodents, biomass of seeds, andabundance and richness of bird and lizardspecies inside the fenced Tortoise Natural Areaboundaries than outside. Other scientists havereported positive effects of fenced protectedareas on birds and carnivores, e.g., populationsof two species of foxes were higher inside afenced protected area than outside (Lenain etal., 2004). The presence of humans may also bea factor, with protected areas offering anescape or greater physical distance from people(Ikuta and Blumstein, 2003; Gehrt et al., 2009).

Desert Tortoise Habitat andAnthropogenic Impacts

In the models, tortoise sign was negativelycorrelated with two anthropogenic predictorvariables, each in a different managementarea: counts of livestock scat on private landsand vehicle tracks in critical habitat. Scientistshave previously identified these predictorvariables as contributing to deterioration ofhabitat and mortality of tortoises (Busack andBury, 1974; Berry, 1978; Webb and Stielstra,1979; Nicholson and Humphreys, 1981;USFWS, 1994a; Bury and Luckenbach, 2002;Berry et al., 2008; Keith et al., 2008). At thetime of our survey in 2011, the Tortoise NaturalArea had been protected from sheep grazingfor 31 yr, since 1980, and from most vehicle usefor 38 yr, since 1973. In contrast, sheep grazingended in critical habitat in 1990, the year theDesert Tortoise was federally listed as threat-ened (USFWS, 1990). Recreation vehicle usewas also intensive in critical habitat until theearly 1990s, but unauthorized travel off existingroutes has continued for decades. Privatelands, unless fenced, have no protection fromsheep grazing or vehicle use and thus receiveheavy use.

Livestock, particularly sheep, have grazedwestern Mojave Desert lands for over a

century (Wentworth, 1948). They have alteredcomposition of perennial and annual vegeta-tion by preferentially consuming edible forbs,perennial grasses, and shrubs (e.g., species ofsaltbush, hop-sage, white bur-sage, winter fat,Anderson and Cooper’s box-thorn). Damagewas most severe in the vicinity of wateringsites, whether through stock tanks (e.g.,Brooks et al., 2006) or watering trucks. In astudy on effects of grazing on annual andperennial vegetation, Brooks et al. (2006)reported significant declines in cover, speciesrichness, and density of perennial plants withincreasing proximity to watering sites. Thedeclines in cover were due primarily todeclines of the small to mid-sized shrubs,e.g., box-thorn, hop-sage, cheesebush, andNevada ephedra.

Vehicle use off-highway or off paved roadsfor recreation or other purposes similarlyaffects perennial vegetation (e.g., Busack andBury, 1974; Lathrop, 1983; Bury and Luck-enbach, 2002; Brooks and Lair, 2009). Whereusers concentrate camping and racing activi-ties and where route and track densities arehigh, the result is partial or complete denu-dation of perennial shrubs, especially theseveral species of small shrubs occurringbetween the larger creosote bushes.

The long-term degradation and loss ofperennial shrubs by grazing and vehicles prob-ably account for significantly more plots in thecreosote bush association on private lands andcritical habitat than in the Tortoise Natural Area.The creosote bush association is the simplest andleast diverse of the four vegetation associations inour study area, with only two abundant speciescompared with 5 to 11 abundant species in theother vegetation associations. In contrast, tor-toise sign had the highest probability of occur-rence in the box-thorn association, regardless ofmanagement area.

Both livestock grazing and vehicle use alsohave negative effects on availability of nativeannual forbs, important forage for the tortoise,through consumption, trampling, crushing, andcompaction (Berry, 1978; Jennings, 1997, 2002;Oftedal, 2002). Grazing and vehicle traveladditionally disturb soils, thereby enhancingopportunities for alien annual plants to thrive atthe expense of the natives. For example, coverand richness of native annuals declined with


increasing proximity to livestock watering sites(Brooks et al., 2006), and density of dirt roadswas positively correlated with richness of alienannual species and biomass of the alien forb,filaree, Erodium cicutarium (Brooks and Ber-ry, 2006).

Grazing and vehicle use more directly affecttortoises by damaging or destroying burrows,reducing canopies of shrubs used for shelterfrom predators and temperature extremes,and injuring or killing tortoises (Berry, 1978;Nicholson and Humphreys, 1981; Bury andLuckenbach, 2002). Paved and dirt roads,designated off-highway vehicle routes, andtracks also provide access to users who mayshoot tortoises (Berry, 1986).

The significant effects of two other humanactivities were limited to private lands (Trash)and critical habitat (Firearms). The modelsshowed a positive association between trashand Desert Tortoise abundance on privatelands. The finding of a positive associationdiffers from other, similar studies. Keith et al.(2008) reported that plots with tortoise signhad lower counts of trash than plots withouttortoise sign, and Berry et al. (2006) notedthat tortoise mortality was correlated withtrash counts. Effects of trash on tortoises mayhave both positive and negative elements,and the relationship may change over timeand location. Trash can be an indicator ofheavy human use and habitat deterioration,as well as an attractant to subsidized preda-tors. Trash also may attract tortoises, whomay eat foreign objects out of hunger,accidental ingestion, or curiosity (Walde etal., 2007), or, if the object is sufficiently large,use it as cover. Consumption of trash as acause of illness and death is well known toveterinarians who work with chelonians,whether tortoises, freshwater turtles, or seaturtles (Donoghue, 2006; Wyneken et al.,2006). Some trash, such as partially crushedcans, collects water during rain events, andcan be a source of drinking water for wildtortoises. Debris from firearms may havesimilar positive and negative aspects. Thepositive association of debris from firearmswith abundance of tortoise sign in criticalhabitat may have ominous implications:25.6% of tortoise sign was composed of deadtortoises in this management area.

One surface-disturbing activity, mining, wasminimal in the study area and held nosignificant associations with tortoise abundancein the models. Historically, mining was animportant land use in the western MojaveDesert, especially in the Rand Mining District(eastern edge of our study area) and the nearbyEl Paso Mountains from the 1860s to recenttimes (Starry, 1974; Vredenburg et al., 1981;Chaffee and Berry, 2006). Mining has not beenauthorized in the Tortoise Natural Area sincethe Congressional withdrawal from the 1872mining laws in 1980, but our surveys showedthat some hazardous sites remain.

Overall, this study confirms that the Tor-toise Natural Area, with higher densities ofDesert Tortoises, has benefited from theprotective fence and elimination of grazingand vehicle use. Despite the emergence andspread of a chronic, infectious disease (myco-plasmosis) throughout the western MojaveDesert (Jacobson et al., 1991; Homer et al.,1998; Brown et al., 1999), adult tortoisedensities at the Tortoise Natural Area weresignificantly higher than in critical habitat notonly in our study area but also throughout theWest Mojave Recovery Unit (USFWS, 2010,2012). The mammalian predators and avianpredators also may have benefited fromprotection in the Tortoise Natural Area onthe basis of our counts, as well as results fromprevious studies (Brooks, 1995, 1999). In ourstudy, models of tortoise sign and presencesuggest that relationships with these predatorgroups were neither positive nor negative. Inother studies, vertebrate species have receivedbenefits from fenced protected areas byreducing human disturbances and limitingtransmission of wildlife-borne diseases (Ikutaand Blumstein, 2003; Lenain et al., 2004;Ferguson and Hanks, 2012). In a globalanalysis of the effectiveness of protected areamanagement, Leverington et al. (2010) iden-tified several factors related to overall successin maintaining biodiversity: legal establish-ment, design, legislation, and boundary mark-ing. One critical question, addressed in the1970s for the Tortoise Natural Area, waswhether posting of signs alone was a sufficientand effective method for marking the bound-ary and eliminating unauthorized recreationalvehicle use and sheep grazing. Because


posting was ineffective, the Bureau of LandManagement constructed the hog-wire fencein 1979–1980. Also ineffective was posting ofsigns to limit recreational vehicle use toexisting routes and specific areas in the criticalhabitat portions of our study area, even afterdecades of effort by the Bureau of LandManagement (e.g., Goodlett and Goodlett,1992; USBLM, 2002, 2006; US District Court,2009). As a result, the Bureau of LandManagement closed and fenced a portion ofthe critical habitat in 2002, and this arearemained closed with the exception of a year(2008–2009), when the area was reopened.Because of continued noncompliance by off-highway vehicle users, the area was againclosed. The many years of sheep grazing andoff-highway vehicle use degraded the habitatand therefore are likely contributors to thelower abundance and higher mortality oftortoises in critical habitat in 2011.

MANAGEMENT APPLICATIONS

Historic and recent human uses affect thefunctioning of ecosystems (Foster et al., 2003).The three management areas provide exam-ples of how these legacies have becomeembedded in the sites and how DesertTortoises, in turn, may have been affected.The legacies are persistent and need to beconsidered in framing expectations for recov-ery of the Desert Tortoise and in planninghabitat restoration. Other important consider-ations are the types and levels of protectionneeded, enforcement of protective measures,and long-term monitoring.

In 1994, the USFWS outlined severalactivities that were incompatible with recov-ery of the Desert Tortoise and recommendedthat these activities be prohibited in recoveryareas (USFWS, 1994a). Examples include allvehicle activity off designated roads, uncon-trolled dogs out of vehicles, dumping andlittering, domestic livestock grazing, anddischarge of firearms (except for hunting ofbig game or upland game birds at specifiedtimes). The USFWS recommended emergen-cy closures of dirt roads and routes wherehuman-caused mortality of tortoises was aproblem, as well as protective fencing andregular and frequent patrols of law enforce-ment personnel in areas with unauthorized

vehicle use and vandalism. They made specificrecommendations for the region where ourstudy occurred, including reducing popula-tions of Common Ravens to limit predation onjuvenile Desert Tortoises. A few of theserecommendations have been implemented;others remain to be accomplished. The resultsof our studies provide scientific support forimplementing recommendations on livestockgrazing, vehicle use off designated roads, andcanid and raven predation. Areas such as theTortoise Natural Area, fenced to prohibitentry of vehicles and livestock, appear to havebenefited the Desert Tortoise compared withunfenced areas, if higher tortoise densitiesand lower mortality rates are used as mea-sures. The recent fencing of parts of criticalhabitat to reduce unauthorized vehicle usemay have similar benefits for Desert Tortoiserecovery in the future. The high levels oftrauma from canids on live tortoises are likelyto inhibit recovery and demonstrate the needfor better control of dogs and management ofwild canids, at least on a local basis. Dog-prooffencing is an option to consider. Likewise,when Desert Tortoise densities are very low,predation by Common Ravens and mammalsinhibits recovery. Government agencies haveundertaken control of coyotes and CommonRavens to reduce hyperpredation of DesertTortoises in limited areas of the Mojave andwestern Sonoran deserts but not at our studyarea or in the vicinity (USFWS, 2008a, 2008b).Other causes of death to tortoises, such asshooting and vehicle trauma, still remain to beaddressed. The USFWS recovery recommen-dation on trash and litter has not beenimplemented on a landscape scale; volunteerscollect litter in local areas, however.

Acknowledgments.—We thank K. Anderson, C. Bed-well, J. Boswell, S. Hanner, C. Hatton, A. Keller, S.Moore, and A. Spenceley for fieldwork. A. Emerson Coblecontributed to project design and S. Ellis and C. Woodsassisted in locating historical plans. C. Darst, M. Harvey,S. Schwarzbach, J. Weigand, and two anonymousreviewers provided comments that improved the manu-script. We thank the Desert Tortoise Preserve Commit-tee, Inc., M. Kotschwar Logan, and J.Y. Lee for locationsof and access to private lands. Funding was provided bythe Off-Highway Motor Vehicle Recreation Division ofthe California Department of Parks and Recreation to theBureau of Land Management and US Geological Survey.K.H.B. held permits for handling tortoises from theCalifornia Department of Fish and Game (801063-04, SC-003623) and USFWS (TE-006556-16) under US Geolo-


gicial Survey-approved animal care and use protocols. Anyuse of trade names is for descriptive purposes only anddoes not imply endorsement by the US Government.

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.Accepted: 14 July 2014


APPENDIX I

Coefficients for generalized linear models for Desert ortoise presence—in all three management areas and forabundance in three separate areas. Colons in the predictor variable represent an interaction effect between twopredictors. Coefficients for presence and private lands models were tested using z-statistics. Coefficients for the TortoiseNatural Area and critical habitat models were analyzed with quasi-Poisson models and were tested using t-statisticsbased on 78 and 76 df respectively. Regression coefficients (b) are presented with SE, z-statistic (z), and P-values (P).Levels of significance: * P , 0.10, ** P , 0.05, *** P , 0.010.

Response variable Predictor variable b SE z, t P

Presence in three areas Intercept (box thorn and Tortoise NaturalArea)

0.934 0.400 2.33 0.020

Veg. (creosote bush) 20.456 0.365 21.25 0.211 **Veg. (indigo bush) 20.926 0.465 21.99 0.046 **Veg. (Nevada ephedra) 21.195 0.544 22.20 0.028 **Log(Sheep) 20.795 0.411 21.93 0.053 *Management area (critical habitat) 20.888 0.471 21.88 0.059 *Management area (private lands) 21.043 1.043 21.00 0.317Log(Mammals) 20.509 0.442 21.15 0.250Management area (critical habitat):

log(Mammals)1.504 0.737 2.04 0.041 **

Management area (private): log(Mammals) 1.644 0.860 1.91 0.056 *Abundance in

TortoiseNatural AreaIntercept 0.772 0.177 4.36 ,0.001 ***

Abundance in criticalhabitat

Intercept 20.396 0.308 21.29 0.202Log(Vehicles) 21.075 0.572 21.88 0.064 *Log(Firearms) 1.387 0.526 2.64 0.010 **Log(Mammals) 1.152 0.361 3.19 0.002 ***

Abundance in privatelands

Intercept (box thorn) 20.491 0.760 20.65 0.518 *Veg. (creosote bush) 21.567 0.529 22.96 0.003Veg. (indigo bush) 217.108 3168 20.01 0.996 **Veg. (Nevada ephedra) 216.789 2282 20.01 0.994Log(Sheep) 20.516 0.248 22.08 0.037 **Log(Trash) 0.764 0.393 1.94 0.052 *Log(Mammals) 1.248 0.485 2.57 0.010 **

Tortoise


protection benefits desert tortoise abundance · 2016. 3. 19. · protection benefits desert...

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