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Great Basin--Mojave Desert Region - USGS National Wetlands

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Great Basin–Mojave Desert Region T he Great Basin–Mojave Desert region is a land of striking contrasts. Forming an expansive wedge between the Sierra Nevada, the Transverse ranges, the Rocky Mountains, and the Columbia and Colorado plateaus, the region harbors great biological diversity. This high diversity is produced by a blending of the surrounding region’s flora and fauna with the unique species of the Great Basin and Mojave Desert. The Great Basin–Mojave Desert region is home to the oldest living organisms on Earth, such as Great Basin bristlecone pines, which can live 4,900 years (Schmid and Schmid 1975), and the creosotebush clones in the Mojave Desert, which are an estimated 10,000–11,000 years old (Vasek et al. 1975). Biologists easily distinguish the Mojave Desert and the Great Basin from other regions by their flora and fauna. However, the boundaries between them and the surrounding regions are often unclear. Various sci- entific disciplines define the Great Basin–Mojave Desert region some- what differently (Grayson 1993). Anthropologists define the region by cultural attributes of the aboriginal inhabitants (d’Azavedo 1986), botanists by species composition of the vegetation (Billings 1951; Holmgren 1972; Vasek and Barbour 1990), and geologists by the struc- ture of the land (Hunt 1967). The region includes nearly all of Nevada, much of eastern California, western Utah, southeastern Oregon, and por- tions of southern Idaho. Our descriptions include the hydrographic Great Basin, the floristic Mojave Desert, and the Muddy, Virgin, and White rivers, which are tributaries of the Colorado River with headwaters deep in the floristic Great Basin (Fig. 1). Hydrographic Great Basin The hydrographic Great Basin is the area of internal drainage between the Rocky Mountains and the Sierra Nevada. The waters of streams in this area never reach the ocean but are confined to closed basins. The hydro- graphic Great Basin includes most of the Mojave Desert and exceeds 500,000 square kilometers (Morrison 1991). When the explorer John C. Frémont realized that this region did not drain to the ocean, he coined the term Great Basin (Frémont 1845). Physiographic Region The landforms of the Great Basin and the Mojave Desert define the region as part of the Basin and Range Physiographic Province (Hunt 1967) that extends south to include the Sonoran and Chihuahuan deserts of Arizona and Mexico. The Basin and Range Physiographic Province is characterized by hundreds of long, narrow, and roughly parallel mountain ranges that are separated by deep valleys (Hunt 1967; King 1977; Fiero 1986). These features have evoked several colorful descriptions, includ- ing “an army of caterpillars crawling northward out of Mexico” (Dutton in King 1977:156) and “washboard topography” (Houghton 1978:vii). The Great Basin mountains are geologically recent, and the landforms of the region are a product of the formation of the Rocky Mountains and the Sierra Nevada. The structures of the more than 400 mountain ranges in the region are similar, but their compositions are diverse. Many © D. A. Charlet, University of Nevada
Page 1: Great Basin--Mojave Desert Region - USGS National Wetlands

Great Basin–MojaveDesert Region

The Great Basin–Mojave Desert region is a land of striking contrasts.Forming an expansive wedge between the Sierra Nevada, the

Transverse ranges, the Rocky Mountains, and the Columbia andColorado plateaus, the region harbors great biological diversity. This highdiversity is produced by a blending of the surrounding region’s flora andfauna with the unique species of the Great Basin and Mojave Desert. TheGreat Basin–Mojave Desert region is home to the oldest living organismson Earth, such as Great Basin bristlecone pines, which can live 4,900years (Schmid and Schmid 1975), and the creosotebush clones in theMojave Desert, which are an estimated 10,000–11,000 years old (Vaseket al. 1975).

Biologists easily distinguish the Mojave Desert and the Great Basinfrom other regions by their flora and fauna. However, the boundariesbetween them and the surrounding regions are often unclear. Various sci-entific disciplines define the Great Basin–Mojave Desert region some-what differently (Grayson 1993). Anthropologists define the region bycultural attributes of the aboriginal inhabitants (d’Azavedo 1986),botanists by species composition of the vegetation (Billings 1951;Holmgren 1972; Vasek and Barbour 1990), and geologists by the struc-ture of the land (Hunt 1967). The region includes nearly all of Nevada,

much of eastern California, western Utah, southeastern Oregon, and por-tions of southern Idaho. Our descriptions include the hydrographic GreatBasin, the floristic Mojave Desert, and the Muddy, Virgin, and Whiterivers, which are tributaries of the Colorado River with headwaters deepin the floristic Great Basin (Fig. 1).

Hydrographic Great Basin

The hydrographic Great Basin is the area of internal drainage betweenthe Rocky Mountains and the Sierra Nevada. The waters of streams in thisarea never reach the ocean but are confined to closed basins. The hydro-graphic Great Basin includes most of the Mojave Desert and exceeds500,000 square kilometers (Morrison 1991). When the explorer John C.Frémont realized that this region did not drain to the ocean, he coined theterm Great Basin (Frémont 1845).

Physiographic Region

The landforms of the Great Basin and the Mojave Desert define theregion as part of the Basin and Range Physiographic Province (Hunt1967) that extends south to include the Sonoran and Chihuahuan desertsof Arizona and Mexico. The Basin and Range Physiographic Province ischaracterized by hundreds of long, narrow, and roughly parallel mountainranges that are separated by deep valleys (Hunt 1967; King 1977; Fiero1986). These features have evoked several colorful descriptions, includ-ing “an army of caterpillars crawling northward out of Mexico” (Duttonin King 1977:156) and “washboard topography” (Houghton 1978:vii).

The Great Basin mountains are geologically recent, and the landformsof the region are a product of the formation of the Rocky Mountains andthe Sierra Nevada. The structures of the more than 400 mountain rangesin the region are similar, but their compositions are diverse. Many ©

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506 Status and Trends of the Nation’s Biological Resources — Volume 2

granite ranges occur in the west, basalt rangesoccupy the northwest, rhyolite mountains form

distinguishing feature of the two floristicregions is the presence of creosotebush in theMojave Desert and its absence from the GreatBasin (Billings 1951; Holmgren 1972).Alternatively, big sagebrush dominates much ofthe Great Basin floristic region, but it is mostlyabsent from the Mojave Desert except at mod-erate to high elevations in the mountains. Theseparation of the Mojave from the Colorado andSonoran deserts is less clear because creosote-bush is also dominant in these deserts.


The climate of the Great Basin–MojaveDesert region is one of the most varied andextreme in the world (Hidy and Klieforth 1990).The Sierra Nevada is primarily responsible forcreating this arid, continental climate by captur-ing moisture from Pacific storm fronts beforethe moisture reaches the desert (Houghton et al.1975); similarly, the Rocky Mountains interceptstorms from the Gulf of Mexico (Hidy andKlieforth 1990). Local weather patterns arecomplicated by the mountain ranges that upliftthe dispersed moisture, creating mountainstorms (Hidy and Klieforth 1990). Thus, precip-itation increases with elevation (Billings 1951),and average annual precipitation can be highly

Fig. 1. The Great Basin–MojaveDesert region.

Pyramid Lake


Walker Lake








Great SaltLake

Great Basin–Mojave DesertCounty linesState lines

Virgin RiverPl







Colorado River

the center, and limestone mountains dominatethe east and southwest. In addition to this diver-sity of substrates, there is great topographicrelief throughout the region, the greatest ofwhich is 4,494 meters in the 135 kilometersbetween Badwater in Death Valley and the sum-mit of Mount Whitney, both in California. In atransect across the Great Basin at 39° north lat-itude, Grayson (1993) found that the differencein elevation between mountaintops and valleybottoms ranged from 1,158 to 2,316 meters,with an average relief of 1,768 meters.

The topographic relief in the region createspowerful elevation gradients to which all theorganisms in the region respond. As elevationincreases, air density decreases and solar radia-tion and precipitation increase. The interactionof these factors produces different temperatureregimes at different elevations, which signifi-cantly affect the distribution of plants (Billings1970) and the animals that depend on them(Hall 1946). This mountainous terrain thus pro-vides many opportunities for a multitude oforganisms with diverse life strategies.

Floristic Region

The Mojave Desert can be distinguishedfrom the Great Basin by the presence and abun-dance of its different plant species. Again,the boundaries are imprecise. The principal

variable over small distances.The region can be separated climatically into

hot and cold deserts. The lower, hot, MojaveDesert receives most of its precipitation as rain,and the higher, cold, Great Basin Desertreceives most of its moisture as snow(MacMahon 1988a). This distinction is compli-cated by a strong east–west gradient that allowsmore summer rains from the Gulf of Mexico toreach the eastern regions of the Great Basin andMojave than their western regions. This patternchanges around 40° north latitude, where seasonal precipitation regimes are nearly equal between the Warner Mountains in thewest and the Jarbidge Mountains in the east(Charlet 1991).

Climatic History and ItsInfluence

The Pleistocene Epoch, in which severalmajor glaciation and deglaciation eventsoccurred, lasted from about 3 million to about11,000 years before present (Flint 1971).During the Pleistocene Epoch, enormous lakesand marshes formed in many of the valleys(Mifflin and Wheat 1979; Williams 1982;Benson et al. 1990; Morrison 1991; Fig. 2),and glaciers spotted the high mountains(Blackwelder 1931, 1934; Flint 1971; Piegat1980; Porter et al. 1983; Osborn 1989). The

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modern distribution and ecology (interactionsof organisms with the environment) of theorganisms of the Great Basin–Mojave Desertregion largely reflect the effects of increasedprecipitation and cooler temperatures of the Pleistocene Epoch (Reveal 1979; Whartonet al. 1990).

the Great Basin–Mojave Desert region duringthe Holocene Epoch (Reveal 1979), whiledrought- and heat-intolerant species have beenforced farther north or higher into the coolerand wetter conditions of the mountains, or were extirpated (Billings 1978). This dry periodhas affected the animals (Brown 1978), the people (Bettinger 1991; Thomas 1997), and theplants (Billings 1978) of the region.

Gould (1991) maintains that the HoloceneEpoch is simply another interglacial periodbefore renewed glaciation. This view echoesthat voiced earlier by Wright (1983:xi), “there islittle doubt that another major glaciation willensue.” In the face of either a new glaciationperiod or human-induced global warming, howwill the biota of the Great Basin–Mojave Desertregion respond? To understand the answer, wemust examine the spectacularly varied andnumerous isolated communities of the regionwith history in mind.

Montane Islands in a Sea of Shrubs

The last 3 million years of climatic fluctua-tions left a biological legacy on the land-scapes of the Great Basin–Mojave Desertregion. Remnants of ancient pluvial lakes

Great Basin–Mojave DesertPleistocene glaciersPleistocene lakesPleistocene marshes

CentralBasins Bonneville


Lahontan System



Plants and animals of the region respondedto the Pleistocene climate by changing their rel-ative distributions and abundances (Brown1971, 1978; Billings 1978; Harper et al. 1978;Wells 1983; Charlet 1991). Therefore, commu-nity composition changed as species moved insearch of more congenial climates (C. L.Nowak 1991), adapted in place, or becameeither locally or globally extinct (Grayson1993). Species were generally forced down-slope and southward in response to Pleistoceneconditions (Axelrod 1950, 1976, 1983; VanDevender and Spaulding 1979; Wells 1983;Mehringer and Wigand 1990; Wigand et al.1994).

The Holocene Epoch of the last 11,000 years is characterized by the recession of mountain glaciers and a warmer and often drierclimate than existed during the PleistoceneEpoch. Drought- and heat-tolerant species fromsouthern deserts have moved northward into

include Pyramid, Walker, Carson, Eagle, andMono lakes; each has its unique plants and animals. The many mountaintops are refugia for species left behind from the PleistoceneEpoch in communities that are vastly differentfrom the adjacent desert valleys. Within 10 kilometers, a single basin-range unit can hostenvironments that range from treeless alpinebogs and rocky slopes to montane coniferousforests, diverse mountain shrublands, pygmywoodlands of pinyon pine or juniper, lowerslopes of sagebrush and grasses, lake shoresthat support an entirely different array of shrubs and flowers, barren sand dunes, orplayas (saline flats that sometimes form shallowlakes). Dozens of montane habitat islands in theregion are now separated from each other bydeserts.

Transition Zones

Contact between the flora and fauna of tworegions creates transition zones (Meyer 1978;MacMahon 1988b). These zones have charac-teristics of both regions and constitute a uniquetype of habitat. Although transition zones maybe important sites for plant evolution (G. L.Stebbins 1974), few researchers have examinedthe animals of these zones in detail (Pianka1967, 1970; Parker and Pianka 1975; Robinson1988; Wilson 1991).

Fig. 2. Estimate of the maximum extent of marshes,pluvial lakes, and mountain glaciers during the PleistoceneEpoch in the Great Basin–Mojave Desert region. Datafrom Blackwelder (1931, 1934), Flint (1971), Piegat(1980), Williams (1982), Porter et al. (1983), and Osborn(1989).

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508 Status and Trends of the Nation’s Biological Resources — Volume 2

The eastern boundary of the Great Basin–Mojave Desert region is sharply defined by thehigh elevations of the Wasatch Mountains andthe Colorado Plateau. The western boundaryalong the Sierra Nevada is even sharper and hasbeen called one of the world’s sharpest bound-aries between biological regions (Billings1990). However, the northern and southernboundaries are subtler, and the transition zonesthere are much broader.

The most complex transition zone is about115 kilometers wide and occurs between theGreat Basin and the Mojave Desert (Beatley1976; O’Farrell and Emery 1976; Fig. 3). Thiszone contains a mixture of smaller transitionzones and sharp habitat edges (Billings 1949,1951; Beatley 1974a,b, 1976; Meyer 1978; El-Ghonemy et al. 1980a,b; Turner 1982;Callison and Brotherson 1985; Young et al. 1986;MacMahon 1988b; West 1988; Tueller et al.1991), which result from increasing elevation,decreasing temperatures, and a south–north shift in predominant precipitation from rain tosnow (Billings 1951; MacMahon 1988a; Tuelleret al. 1991).

Many terrestrial animals reach their northernor southern distribution limits in this zone (Hall1946; Holmgren 1972; Stebbins 1985), and theboundaries of several subspecies occur here,

Another gradual transition zone occurstoward the northern edge of the region, wheresagebrush declines in favor of perennial bunch-grasses. Pristine conditions in the north onceincluded such a high grass component that thesecommunities are commonly referred to as sage-brush–steppe or shrub–steppe. The gradationcontinues north until grasses dominate thesteppe of the Columbia Plateau (West 1988).

Status of Major Communities

Aquatic Communities

Riparian Communities

Riparian communities occur along the majorwatercourses in most intermountain valleys ofthe Great Basin–Mojave Desert region and inassociation with isolated springs, seeps, andsmaller streams. In the Great Basin, ripariancommunities are dominated by various mixturesof cottonwood, aspen, and willow species thatare used by the native Humboldt beaver (Fig. 4).Thickets of chokecherry are common, and occa-sional patches of silver buffaloberry provideimportant overwintering sites for NorthAmerican porcupines (Sweitzer 1990).Impressive groves of cottonwoods occur along

such as the northern and southern desert hornedlizards and the northern and desert side-blotched lizards (Stebbins 1985). The turnoverof some bird and mammal species is also abruptbetween the Great Basin and the Mojave Desert(Hall 1946; Behle 1978).

the larger rivers (Fig. 5). In the low-lyingMojave Desert, these dominants are largelyreplaced by mesquite, cat-claw acacia, and vel-vet ash. Other riparian associations dominatedby saltgrass and iodine bush occur on salinesoils and along streams such as Salt Creek inDeath Valley (Fig. 6).

Although extremely small in total area,riparian communities in this region are criticalcenters of biodiversity. More than 75% of thespecies in the region are strongly associatedwith riparian vegetation (U.S. GeneralAccounting Office 1993), including 80% of thebirds (Dobkin 1998) and 70% of the butterflies(Brussard and Austin 1993). Several butterfliesare completely restricted to these habitats (for

AlpineMontanePinyon–juniper Western juniperSagebrush–grassShadscaleMojavean zoneAbsolute desert/water

Fig. 3. Vegetation zones in theGreat Basin–Mojave Desertregion. Compiled from Shreve(1942), Billings (1951), Holmgren(1972), Ertter (1992), and D. A.Charlet (University of Nevada,unpublished data).

Fig. 4. Humboldt beaver at the headwaters of theHumboldt River, Jarbidge Mountains, Nevada.

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Regional Trends of Biological Resources — Great Basin 509

ecological generalists that are adapted to suchhighly disturbed conditions (Dobkin 1998).

A striking example of generalist species indisturbed areas is found along the Virgin,Mojave (Lovich et al. 1994), Humboldt, andWalker rivers and in many other parts of theregion, where riparian communities are nowlargely dominated by aggressive, nonindige-nous tamarisks. These trees reduce the abun-dance of native riparian vegetation, and theirrapid migration is profoundly altering speciescomposition and community functions(Vitousek 1986). Tamarisks use water more effi-ciently than other trees do and tolerate salineconditions (Busch and Smith 1993). Tamarisksare fire-tolerant, thus increased fire frequencyfavors them over the native riparian vegetation(Anderson et al. 1977; Busch and Smith 1993).Tamarisks are also more tolerant of boron, atoxic element that concentrates in soil after fires(Busch and Smith 1993), than are native willowand cottonwood species. These postfire adapta-tions permit tamarisks to dominate ripariancommunities at the expense of the native trees(Anderson et al. 1977).

Stream flow also is altered by the encroach-ment of tamarisks into riparian communities.Tamarisks create narrower stream channels(Blackburn et al. 1982) and promote local

Fig. 5. Cottonwood groves along the Walker River inAntelope Valley, California.

Fig. 6. A Mojave Desert riparian system at Salt Creek,Death Valley National Park, California.

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example, the Apache nokomis fritillary butterflyand the Weidemeyer’s admiral butterfly).Riparian corridors also provide migration path-ways for many species, and subalpine and mon-tane conifers descend to much lower elevationsin riparian situations than in upland stands(Charlet 1996, 1997). Much of this vegetationhas been destroyed or degraded by water diver-sions, agricultural development, and livestockgrazing (U.S. General Accounting Office 1993),which is a major cause of riparian habitat degra-dation in the Great Basin (Platts 1990; Chaneyet al. 1993; National Research Council 1994).Effects of grazing include a reduction of natur-al vegetation, stream channel widening andaggradation, and lowered water tables(Kauffman and Krueger 1984; Armour et al.1991; Chaney et al. 1993; Fleischner 1994). Astructurally diverse flora in riparian areas thathas not been grazed supports a broad assem-blage of wildlife species, whereas reductions ofshrub and herbaceous cover following heavycattle grazing modify many bird and small-mammal communities (Schulz and Leininger1991; Dobkin 1994a). The effect of livestock onriparian habitats in the Great Basin is often so severe that those habitats no longer representnatural vegetation (Ehrlich and Murphy 1987;Chaney et al. 1993), and the associated faunalcommunities support largely widespread,

flooding (Graf 1980). Although expensive (Graf1980; Barrows 1993), the eradication oftamarisks has been shown to rejuvenate marsh-es, such as those along Eagle Borax Spring inDeath Valley (Vitousek 1986).

Wet-Meadow Communities

Permanently wet meadows, like persistentstreams, are scarce in this arid region, yet theycontribute significantly to the region’s plantdiversity (Linsdale et al. 1952; Loope 1969;Charlet 1991). These habitats also are frequent-ly destroyed by livestock grazing and waterdiversion (Hammond and McCorkle 1983).Several species at risk are characteristic of thesecommunities. For example, many populationsof the silver-bordered fritillary butterfly havebeen lost because springs were capped (Pyle etal. 1981).

Terminal-Wetland Communities

A distinguishing feature of most of the GreatBasin–Mojave Desert region is the drainage ofits waters to terminal basins rather than into theocean. This feature creates isolated terminallakes, marshes, and playas (Fig. 7), many ofwhich support unique aquatic species. Althoughmost are small and only seasonally filled withwater, these wetlands are surprisingly numerousand critically important to the biological diver-sity and ecology of the region (Table 1).

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510 Status and Trends of the Nation’s Biological Resources — Volume 2

Water levels in Great Basin–Mojave Desertwetlands are lowered by diversions for agricul-ture and urban growth; for example, Nevada lost

large populations of migratory waterbirds. Thisfauna has not been studied extensively and mayinclude several undescribed species. The effectsof off-highway vehicles on these organisms areunknown but are probably detrimental (R. C.Stebbins 1974).

Mojavean Zone

The Mojavean vegetation zone (Fig. 8)encompasses most of the geographic area of theMojave Desert except the higher mountains.The flora and communities in this zone arecharacterized by great diversity, and only half ofits 545 plant species also occur in the SonoranDesert (Vasek and Barbour 1990). Several veg-etation types are recognized in the Mojaveanzone: creosotebush scrub, saltbush scrub, shad-scale scrub, blackbrush scrub, Joshua-treewoodland, and annual vegetation (Vasek andBarbour 1990). The highest mountains withinthe boundaries of the Mojavean zone (the SheepRange, and White, Spring, and Panamint moun-tains) exhibit most of the Great Basin vegeta-tion zones and thus resemble high mountains inthe Great Basin.

Las Vegas, the fastest growing city in theUnited States, lies in the northeastern part of theMojave Desert. This growth and the lack oflocal persistent streams create an unprecedented

Fig. 7. Terminal wetland ecosys-tems in the Great Basin–MojaveDesert region.

Pyramid Lake

Walker Lake


Owen'sLake Lake



Ruby Lake Great SaltLake

Lake Albert


Perennial lakesMarshesWet only during flood yearsDry only during extreme droughtGreat Basin–Mojave Desert


52% of its wetlands between 1780 and the mid-1980’s (Dahl and Johnson 1991). Low water lev-els are accompanied by an increase in total dis-solved solids (TDS) and increasing accumula-tions of heavy metals such as boron, selenium,arsenic, and mercury. Many of the region’s wet-lands, including some of the larger ones likeHoney Lake, now go dry during low-water years.

All the wetlands of the region are important towaterbirds. Mono Lake and the Great Salt Lakealready have high levels of TDS, and if TDS con-centrations increase further, these lakes may losetheir tremendous populations of brine shrimp andbrine flies, which are key food sources formigrating waterbirds. Further increases in TDSlevels at Walker Lake (associated with lowerwater levels) could prove fatal to fishes and elim-inate them from the lake (Koch et al. 1979; C. A.Stockwell, Biological Resources ResearchCenter, University of Nevada, Reno, unpublishedmanuscript).

Terrestrial Communities

Absolute Desert

Absolute desert communities (Table 2; Fig.3) include playas, salt barrens, and sand dunes.When playas fill from rare rains or snow melt,an invertebrate fauna develops that consists of various crustaceans (for example, fairyshrimp) and insects, which in turn supports

demand on groundwater in the area.Exploitation of groundwater in the Pahranagatand Moapa valleys has significantly altered thehydrology of the area and has harmed ripariancommunities (Runeckles 1982; Franklin et al.1987; Busch and Smith 1995). The fishes arerestricted to springs and streams in the area.Tamarisk has invaded and now dominates manyriparian areas, causing additional problems.Ironically, “Las Vegas” means “the meadows”in Spanish, but the inspiration for the name van-ished early in the city’s sprawling growth. Thissprawl covers a large portion of the Mojaveanzone vegetation with concrete and places at riskthe desert tortoise, many other reptiles, and theCalifornia bearpaw poppy.

Shadscale Zone

The shadscale vegetation zone is named afterits dominant plant species and is the third largestin total area in the region (Table 2; Fig. 3). Inaddition to shadscale, this area is populated bymany other species of widely spaced, drought-tolerant shrubs that are usually thorny with smallleaves. In the western Great Basin and in theBonneville Basin, this zone occurs primarily onslightly saline soils on the periphery of ancientlake beds. In the Mojave Desert and in themountains of southern Utah, it also occurs onrocky upland sites that are too dry to supportsagebrush (Billings 1949). Greasewood is the

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Table 1. Status of terminal wet-lands in the Great Basin–MojaveDesert region.

Terminal wetland StatusNevada

Artesia Lake Dry during 1987–1994; currently refilling

Carson Lake Selenium and boron are contaminants during low-water years; Public Law 101–618 should effect an increase of water levelsand quality (U.S. Fish and Wildlife Service 1992)

Duck Flat Episodically driesFallon National WildlifeRefuge Dry during 1987–1994; currently refilling

Fernley Sink Dry during 1987–1994; currently refillingHumboldt WildlifeManagement Area

Water levels were low from 1987 to 1994; total dissolved solids were high enough to kill duck chicks; high arsenic and selenium levels caused problems through bioaccumulation; current water levels are high

Mason Valley WildlifeManagement Area The hatchery outflow provides a reliable source of water; effects of a recent drought are being alleviated

Pyramid Lake Very reduced lake levels; reduced inflows prevented spawning by the cui-ui and the Lahontan cutthrout trout until 1995Ruby Lake andRuby Marsh

Fed by 200 springs; water level in 1994 was 61 centimeters below targeted level; heavy metals were expected to becomeharmful

Soda Lake Some effects from drought; high brine fly production; important to migrating red-necked phalaropesStillwater WildlifeManagement Area

Water levels were low from 1987 to 1994; selenium, boron, and salinity levels were high; Public Law 101–618 should effect anincrease of water levels and quality during drought years; water levels currently high

Washoe Lake Dry during the 1930’s and 1960’s and during 1991–1994; currently high water level; historically very productiveWinnemucca Lake Dry since 1938

Walker Lake Rapidly decreasing water levels until 1994, when total dissolved solids approached levels that are fatal to fishes; water levelcurrently stabilized

Southeastern Oregon

Abert Lake Lowest water level in past 18 years because of drought and water diversion; relatively clean; naturally high level of total dissolved solids; brine fly populations are low because of increased total dissolved solids

Alkali Lake Chemical residue in water because of nearby disposal siteAlvord Hot Springs Dry during recent droughtBacus Lake Water levels follow those of Malheur Lake

Malheur Lake High precipitation in 1980–1986 increased the size of the lake; by fall 1992, lake size had been reduced to a historical low; in 1994, water levels continued to decline

Silver Lake Dry since early 1980’s from irrigation diversionsStinking Lake Dry during droughtSummer Lake At 0.25 capacity as of fall 1994; relatively pesticide-free; wetlands are maintained regardless of lake level

Warner Wetlands During 1983–1984, highest water level in recorded history; during 1985–1990, completely dry because of diversions anddrought

common phreatophyte (plant requiring ground-water) in the zone (Holmgren 1972).

The rapid decline in the last 100 years of theimportant browse shrub, winterfat, and itsreplacement by nonindigenous annual speciesare clearly documented with photographs(Rogers 1982). This decline is due to heavygrazing by domestic livestock (Billings 1951;Holmgren 1972). An associated change of par-ticular concern is the increase of the nonindige-nous weed halogeton, which is often fatal todomestic stock (Holmgren 1972). Tamarisk isalso rapidly invading and dominating riparianareas in this zone.

Sagebrush–Grass Zone

Although the principal shrub in this zone isbig sagebrush, several other sagebrush speciesand subspecies also occur and can be locallydominant. Because precipitation is higher in the sagebrush–grass zone than in the shad-scale zone (Billings 1949), this zone can

support a surprisingly high richness of shrubs,grasses, and perennial forbs (Fig. 9). The sage-brush–grass zone is the largest (Table 2) andmost contiguous vegetation zone in the region(Fig. 3).

One of the most significant changes in thesagebrush–grass zone has been the invasion ofintroduced plant species such as cheatgrass,halogeton, and other annuals, at the expense ofthe native bunchgrasses and forbs (Young et al.1972; Rogers 1982). The sagebrush–grass zone

Eastern CaliforniaClear Lake Irrigation reservoir; turbidEagle Lake Steady decline of water level from 1987 to 1994 because of drought; currently refillingGoose Lake No contamination; water diverted for farmingHoney Lake Seasonally dry; possible chemical contamination; currently refillingLake Tahoe 1994 water level was below rim; in June 1995 water level was above rim; record high levels in January 1997Modoc National WildlifeRefuge Irrigation reservoir; no contamination

Mono Lake High level of total dissolved solids; in accordance with legal settlements, water level will be raisedOwens Lake Dry since 1957

Vegetation zone Total area in region(square kilometers)

Absolute desert 29,510Mojavean 98,068Shadscale 91,317Sagebrush–grass 206,071Pinyon–juniper 60,556Western juniper 7,187Montane 39,037Alpine 1,224Total 532,970 99.9a


11.3638.66 17.1318.40 5.54

Percent of region

aDoes not total 100% because of rounding.

Table 2. Total area and percent ofregion occupied by the eight vege-tation zones in the GreatBasin–Mojave Desert region.

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is contracting because of the downward expan-sion of the pinyon–juniper zone in the centralportion of the region (Blackburn and Tueller1970) and by western juniper woodlands in thenorth (Burkhardt and Tisdale 1976; Miller andWigand 1994; Miller and Rose 1995).Sagebrush–grass communities are being frag-mented by agriculture and human population

been negatively affected by livestock, with serious effects in about 30% of the zone (Nosset al. 1995).

Pinyon–Juniper and Western Juniper Zones

The pinyon–juniper zone was defined byBillings (1951) as the lowest in elevation of themontane zones. Here we follow Holmgren(1972) and map the unit as a separate zonebecause of its large size and ecological signifi-cance. Billings (1954a) found that thermal beltsand higher precipitation on the mountain slopescontributed to the formation and maintenance ofthe zone, which is characterized by woodlandsof pinyon pine and several species of juniper invarious combinations. Understories are com-posed of grasses, perennial forbs, and shrubs(principally sagebrush and bitterbrush), andseveral gooseberry species are also common.By far the most abundant pinyon pine is single-leaf pinyon, but Colorado pinyon pine occurs inthe Utah portion of the region (Lanner 1984).Utah juniper is the dominant juniper in thezone, but western juniper and Rocky Mountainjuniper may enter the zone at its higher reachesor along streams. This zone is extensivethroughout the Great Basin south of theHumboldt and Truckee river drainages and issparse in the northern Lahontan River basin; it

Fig. 8. Mojavean vegetation zone,eastern base of the SpringMountains, Nevada.

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increases, both of which increase the probabili-ty of invasion by nonindigenous plant species.This zonal contraction and fragmentation andthe increase of nonindigenous plants are furtheraggravated by livestock grazing and fire sup-pression. The combination of these factorsdecreases grass density and increases shrubdensity (Rice and Westoby 1978; Young andEvans 1978; Anderson and Holte 1981). Morethan 99% of the sagebrush–grass zone has

disappears altogether in Oregon. The zone bare-ly extends into Idaho, east of the HumboldtRiver drainage. Many higher mountains in theMojave Desert (for example, Providence,Panamint, Sheep, and Spring mountains) alsosupport pinyon–juniper woodlands.

Higher precipitation and colder climate dis-tinguish western juniper woodlands from thosewith pinyon or Utah juniper (Wigand and Nowak1992). This zone dominates much of southeast-ern and central Oregon, northeastern California,and extreme northwestern Nevada (Fig. 3).

The pinyon–juniper and western juniperzones are spreading throughout the Great Basin.Tree densities increased dramatically in the last100–150 years (Tausch et al. 1981; Rogers1982). The abundance of Utah juniper clearlyhas increased in the Bonneville Basin since thelate 1800’s (Rogers 1982), and Utah juniper andsingleleaf pinyon are also invading black sage-brush communities in east-central Nevada westof the Bonneville Basin (Blackburn and Tueller1970). Increased tree density is accompanied bya decline in understory density and diversity,and in severely degraded juniper stands virtual-ly all herb species can be excluded (Tausch andTueller 1990; Charlet 1996, 1997). Such a land-scape has little value as livestock forage orwildlife habitat.

Singleleaf pinyon also is spreading wher-ever it occurs in the Great Basin, particularly in western Nevada (Billings 1951). During the

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Fig. 9. Sagebrush–grass vegetation zone, Pueblo Valley, Oregon–Nevada border. The Pine ForestRange is in the distance.

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last 250 years, the species advanced northacross the Truckee River into PeavineMountain, the Virginia Mountains, the JunctionHouse Range, and the Pah Rah Range (Charlet1996). Western juniper is increasing its rangeand abundance in the northern part of the regionin southern and central Oregon (Miller andWigand 1994; Miller and Rose 1995) and in southwestern Idaho (Burkhardt and Tisdale 1976).

This woodland expansion is largely a resultof a combination of fire suppression and over-grazing. These factors lead to a decline ofbrowse and grass species that competitivelyexclude juniper and provide the fuels to carryfires that restrict junipers to rocky sites(Burkhardt and Tisdale 1976). Extreme mea-sures, including chaining (Tidwell 1986), burn-ing (Bunting 1986), and poisoning (Johnson1986) have been used in attempts to control thisexpansion.

Montane Zone

Billings (1951) divided the mountain vegeta-tion above the sagebrush–grass zone into sever-al montane zones in three geographic series.We, however, followed Holmgren (1972) bymapping the pinyon–juniper zone and the alpinezone separately from the remainder of the mon-

also inhabit western Great Basin mountainranges (Charlet 1996).

Along the eastern boundary of the GreatBasin region, forest fragments and extensivewoodlands characteristic of the RockyMountains occur in the Wasatch Range(Billings 1951, 1990; Holmgren 1972). A fewGreat Basin–Mojave Desert mountain ranges(for example, Snake Range, Spring Mountains)

Fig. 10. Quaking aspens along LyeCreek in the Santa Rosa Range,Nevada. Granite Peak is in the distance.

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tane series (Fig. 3). Smaller species of sage-brush (for example, Vasey’s sagebrush and littlesagebrush) are dominant plants above the piny-on–juniper zone and are associated with a dif-ferent suite of shrubs and grasses than those insagebrush–grass communities below the piny-on–juniper zone. These are productive commu-nities that provide important forage for wildlifeand livestock.

A common feature of the montane zone isthe widespread occurrence of quaking aspens(Fig. 10). In many northern mountain ranges,aspens form extensive pure forest stands.Because these ranges usually contain few or noconifer (cone-bearing) species, aspens provideimportant habitats for birds and other wildlife.Curlleaf mountain-mahogany also providesimportant shelter and nesting sites and usuallyoccurs in rocky, ridgeline situations. A close rel-ative, the Mexican cliffrose, replaces it in theMojave Desert mountains in similar but hotterand drier situations.

Forests of western juniper occur in the mon-tane zone on the northern slope of SteensMountain, Oregon (Holmgren 1972), and thespecies also extends into the montane zone of afew western Great Basin mountain ranges(Vasek 1966; Charlet 1996). High-diversityconiferous forests are abundant in the CarsonRange, a spur of the Sierra Nevada along thewestern boundary of the region. Only 2 of the15 conifer species in the Carson Range do not

support montane coniferous forests, but theseare usually small and have few conifer species.However, subalpine woodlands (Fig. 11), withvarious mixtures of whitebark, bristlecone, lim-ber, and lodgepole pines, and other conifers,occur in most of the higher mountains of theregion (Charlet 1996).

Several forest trees have been extirpated dur-ing this century. For example, Douglas-fir hasbeen lost from two mountain ranges in northern

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Fig. 11. Subalpine woodlands and meadows of the montane zone at Cougar and Prospect peaks,Jarbidge Mountains, Nevada.

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Nevada since 1937 (Loope 1969; Charlet 1996),and California white fir probably was extirpatedfrom Petersen Mountain, Nevada, by a fire in1994 (Charlet 1996). Many conifer stands andtheir associates are susceptible to extirpationthroughout the region, and fires may leave baldmountains (Billings and Mark 1957) in areasthat can only marginally support trees. The lossof trees subsequently causes a rapid deteriora-tion of conditions that are suitable to forestdevelopment (Billings and Mark 1957).

Increased success of subalpine coniferseedlings at higher elevations has been observedthroughout the region. For example, whitebarkpine and subalpine fir are moving toward thesummits of the Jarbidge Mountains on the north-ern border of the region (Charlet 1996). Studiesof tree rings revealed accelerated growth ratesduring the last 100 years in upper treeline treessuch as limber pine in the Toquima Range andGreat Basin bristlecone pine in the WhiteMountains (M. Rose, Desert Research Institute,Reno, Nevada, personal communication).

Alpine Zone

The alpine zone begins at the limit of theupper treeline. In the Great Basin–MojaveDesert region, the zone is tiny and restricted toonly a few of the highest mountain ranges (Fig.

Reduced snow cover is detrimental for manyplant species in alpine communities such asEschscholtz’s buttercup, which occurs aroundsnowbanks; white bog-orchid, swamp laurel,and sphagnum moss, which are associated withbogs; and louseworts, shooting stars, and ladies’tresses, which are characteristic of springs(Charlet 1991).

Biodiversity Hot Spots

The Nevada Natural Heritage Program(1992) recognizes 101 locations in Nevada asareas with many rare species—biodiversity hotspots—based on the total number of sensitivespecies of all groups. Eighteen of these siteswere identified and ranked on the basis ofworldwide endangered subspecies and varieties,protection urgency, and management urgency(Nevada Natural Heritage Program 1992).Detailed maps of sensitive species of all taxo-nomic groups have been prepared for the SpringMountains of southern Nevada (The NatureConservancy 1994). The Spring Mountains areof particular concern for conservation becausethey are one of only two ranges (the other is theWhite Mountains) that contain all the vegeta-tion zones in the Great Basin–Mojave Desertregion. As a consequence, the Spring Mountainsand the White Mountains have a great many

3). True alpine tundra is present in small localareas in the Ruby (Loope 1969), Sweetwater(Lavin 1981), White (Billings 1978), Toiyabe(Linsdale et al. 1952), and Toquima (Charlet1997) mountain ranges. Alpinelike habitats alsooccur on many of the mountain ranges that donot have an upper treeline, such as the SpringMountains and the Santa Rosa and Pine Forestranges in Nevada and Steens Mountain inOregon. In most of the mountains with an alpinezone, more alpine area is occupied by talusslopes, cliffs, rocky ridges, and peaks than byalpine tundra. Nevertheless, small patches oftundra, alpine bogs, and springs host manyalpine plant species that occur nowhere else inthat mountain range. The alpine flora of theGreat Basin, with about 600 species, is asdiverse as that of either the Rocky Mountains orthe Sierra Nevada at equivalent latitudes(Charlet 1991). This richness of alpine speciessuggests that there was a greatly expandedalpine zone throughout much of the region dur-ing the Pleistocene Epoch.

Subnormal precipitation and higher temper-atures during 1971–1994 (H. Klieforth, DesertResearch Institute, University of NevadaSystem, Reno, personal communication)reduced snow cover and caused the demise ofmany perennial snowfields in the region. Thisallowed subalpine woodlands to encroach onthe alpine zone, reducing its already tiny size.

species. The Spring Mountains are also uniquebecause about 20% of the endemic plant speciesof Nevada—those found only in that state—occur there. In the exploratory study of theSpring Mountains, 23 areas of concern weremapped and described as biodiversity hot spots(The Nature Conservancy 1994).

Recognition of areas with particularly highbiological diversity, regardless of the status ofthe taxa, is only at its beginning stages in theGreat Basin–Mojave Desert region. Most of thisexploratory work has focused on lists of speciesfor local areas. Checklists for vascular plantswere prepared for a few highly diverse moun-tain ranges, and 988 species were identified inthe White Mountains (Morefield et al. 1988),more than 625 species in the Ruby andSweetwater mountains (Charlet 1991), andmany more than 500 species in other highranges such as the Jarbidge, Warner, Toiyabe(Charlet 1991), Toquima, Santa Rosa, andIndependence mountains (Charlet, unpublisheddata). Future mapping efforts of the NevadaBiodiversity Initiative are expected to revealother areas of high biological diversity.

Montane Islands

Ecological islands of montane and alpinevegetation occur throughout the GreatBasin–Mojave Desert region. These isolated,generally small communities have been known

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to exist for many years (Sudworth 1898, 1913)and have been the subject of serious inquiry fordecades (Billings 1950, 1951, 1954b; Vasek1966; Critchfield and Allenbaugh 1969; Loope1969; Brown 1971, 1978; Little 1971; Johnson1975, 1978; Axelrod 1976; Harper et al. 1978;Reveal 1979; Thompson and Mead 1982; Wells1983; Critchfield 1984a,b; Axelrod and Raven1985; Wilcox et al. 1986; Grayson 1987;Charlet 1991; Morefield 1992). The wide distri-bution of these habitats and the large number ofspecies they support add greatly to the biologi-cal diversity of the region and to the potentialresilience of the region in response to climaticchange (Wharton et al. 1990; Charlet 1991).

Contemporary populations of 16 montanemammal species are presently isolated onmountains, and probably have been since thePleistocene Epoch (Brown 1971, 1978;Grayson 1987; Mead 1987). Palmer’s chipmunkis the only full species of mammal that isendemic to the Great Basin, while 15 endemicsubspecies of 8 other species are montane iso-lates (Hall 1981; Table 3). Fossils indicate thatpopulations of montane mammals occurredthroughout the region during the PleistoceneEpoch (Grayson 1987). Exploratory research onthe extinction of the region’s montane mammalshas included comparisons of current species

unique populations; this threat has clear impli-cations for the management of high-altitudeenvironments in the Great Basin (Grayson1987).

Lowland Aquatic and Riparian Endemism

Fishes are the best-known native aquaticspecies in the region. Aquatic invertebrates maybe equally diverse (Hershler and Pratt 1990),but relatively little is known about them.Riparian butterflies, however, have been wellstudied and show rather striking endemism inthe region, far more so than in montane areas(Austin 1985, 1992). In the northern GreatBasin, 27 species of butterflies are closely asso-ciated with lowland riparian vegetation. Ofthese, 9 (33%) have differentiated into 23endemic subspecies. The most extensive sub-speciation has occurred in the common woodnymph butterfly; nine subspecies are endemicto the Great Basin (Austin 1992). These butter-flies evidently evolved in response to Holocenedroughts that resulted in the isolation of remnant waters and their associated ripariancommunities.

Sand Dunes

At least 52 unconsolidated sand dunes occurin the region, 32 in Nevada (Fig. 12). These

assemblages above 2,300 meters in individual

mountain ranges with species assemblages inthe Rocky Mountains and Sierra Nevada(Brown 1971, 1978; Patterson 1984; Pattersonand Atmar 1986; Belovsky 1987). The studiesrevealed that warm, dry periods during theHolocene Epoch reduced population sizes ofmontane mammals in the region and led to theextinction of many of their populations. If extir-pated, relict mammal populations that are iso-lated on montane islands probably could notrecolonize under current climatic conditions.Such extirpations may eliminate genetically

dunes were formed during the Holocene Epoch (Morrison 1964; Smith 1982; Lancaster1988a,b; 1989; Dohrenwend et al. 1991) and areunique habitats because they are rare, small, ofrecent origin, and spatially dynamic. The dunes

Scientific name Common nameEutamias panamintinus panamintinus Panamint chipmunk

E. panamintinus acrusE. umbrinus inyoensis Uinta chipmunkE. umbrinus nevadensisE. palmeri Palmer’s chipmunkE. amoenus monoensis Yellow-pine chipmunkSpermophilus lateralis certus Golden-mantled ground squirrelS. lateralis bernardinusNeotoma cinerea lucida Bushy-tailed woodratMarmota flaviventris parvula Yellow-bellied marmotM. flaviventris fortirostrisOchotona princeps nevadensis PikaO. princeps tutelataO. princeps sheltoniO. princeps albataMicrotus longicaudus latus Long-tailed voleZapus princeps curtatus Western jumping mouse

Table 3. Mammal taxa confined to montane habitats in theGreat Basin–Mojave Desert region (Hall 1981).

Sampled sand dunesUnsampled dunesGreat Basin/ Mojave DesertPleistocene lakesPleistocene marshes

Fig. 12. Sand dunes of theNevada portion of the GreatBasin–Mojave Desert region.

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516 Status and Trends of the Nation’s Biological Resources — Volume 2

support a diverse group of animals (Hall 1946;Brown 1973) and plants (Pavlik 1985, 1989)that have responded with special adaptations tothe unique challenges of this harsh environment(Fig. 13).

Beetles are the best-studied group of sanddune invertebrates (Fig. 14). Most are eitherpredators or are dependent as adults on fineorganic debris, and the larvae of many speciesfeed on plant roots. Most dune beetles dependon vegetation around the base of the dunes foradult or larval forage, mating sites, and protec-tive cover. Ten Nevada sand dune beetle speciesare species of concern (U.S. Fish and WildlifeService 1994a), including Giuliani’s dunescarab from the Big Dune, the large aegialianscarab from the Big and Lava dunes, and sever-al unnamed species of scarabs on the Big,Crescent, and Sand Mountain dunes. Additionalsand dune insects that are species of concernfrom the region include the Sand Mountain pal-lid blue butterfly, Nelson’s miloderes weevil,

endemic altered buckwheat, a candidate for fed-eral listing, occurs exclusively in these disjunctSierran communities. The closest relative of thisspecies, Lobb buckwheat, is restricted to highelevations in the Sierra Nevada. Seventy-fourspecies of wild buckwheat and many more sub-species and varieties are native to Nevada(Reveal 1985, 1989; Kartesz 1988). One of thefeatures of the buckwheat genus that contributesto its high diversity is its ability to occupyunusual soils. For example, Tiehm buckwheat isa candidate species that occurs in clays in theSilver Peak Range of western Nevada, SulphurSprings buckwheat is from the Ruby Valley ofnortheastern Nevada, and the endangeredSteamboat buckwheat grows at Steamboat HotSprings in western Nevada.

In the Mojave Desert, gypsum forms anunusual soil that supports many narrowlyendemic plant species especially adapted tothese conditions (Meyer 1986), including theendangered California bearpaw poppy. TheBureau of Land Management is currently map-ping these edaphic units in detail (S. D. Smith,University of Nevada, Las Vegas, personal com-munication). Endemics that are restricted tolimestone are important in canyons east ofDeath Valley (Raven 1988).

the Kelso giant sand treader cricket, and theKelso Jerusalem cricket (U.S. Fish and WildlifeService 1994a). Many of these species are high-ly endemic (confined to one or a few dunes). Small geographic distributions andhabitat destruction by off-highway vehicles(Luckenbach and Bury 1983) are the greatestthreats to their persistence.

Edaphic Communities

Communities that resemble the montaneconiferous forest vegetation of the SierraNevada are scattered throughout much of thewest-central Great Basin in a matrix of pinyon–juniper woodlands (DeLucia et al.1988). More than 140 of these communitiesoccur in edaphic sites (Billings 1950, 1990).These sites are well removed from the mainbody of Sierran forests and range from 1 to 15hectares (DeLucia and Schlesinger 1990). The

Status of the Biota


Fungi are essential for community healthbecause they decompose dead organic material,making nutrients again available for plants.Unfortunately, the fungi of the GreatBasin–Mojave Desert region are virtuallyunknown. A survey of stalked puffballs wasstarted in 1975 to help fill this void. About 700 collections of this group and all otherencountered macrofungi have been made,but vast areas of the region have not yet beencovered (Fig. 15).


Although large numbers of sensitive plantspecies are found in local areas, sensitive plantsdisplay no distinctive geographic or ecologicalpatterns in the Great Basin–Mojave Desertregion. Sensitive species occur in all areas andvegetation zones (J. Morefield, Nevada NaturalHeritage Program, Reno, personal communica-tion), and rare and endemic plant species occurin many types of habitats, from alpine summitsto sand dunes. Many sensitive plants arerestricted to edaphic habitat islands such asalkaline, limestone, or gypsum soils in thesouthern part of the region and soils derivedfrom volcanic ash in the north. Throughout the

Fig. 14. Eusattus muricatus, adarkling ground beetle at SandMountain, Nevada.

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Fig. 13. Eureka Dune in EurekaValley, Death Valley NationalPark, California. Eureka Dune isthe tallest sand dune in NorthAmerica.

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Wildlife Service 1994a), was recently providedby a mining company in northeastern Nevada.This relative of the sweet pea was known onlyfrom a single population in the IndependenceMountains (Barneby 1989). The mining com-pany had proposed expansion of its explorationinto the area occupied by Grime’s vetchling andprovided a helicopter to conduct aerial surveysof the species. Ground inspection of sites spot-ted from the air revealed 60 additional popula-tions of the species (Morefield, personal com-munication).

Many plant species in alpine and montanehabitat islands are uncommon in the region butare not candidates for listing because they areabundant outside of the Great Basin. Althoughthe species are not globally threatened, thepotential for dispersal and recolonization in theGreat Basin and Mojave Desert in a differentclimatic regime is now small.


Invertebrates make up 95% of the animalson Earth and are critical for ensuring communi-ty health (Mason 1995). Invertebrates generallyexperience the environment on much smallerspatial and shorter temporal scales than largeranimals. As a result, invertebrates are extraordi-narily susceptible to natural and human-caused

Great Basin–Mojave DesertStalked puffball locations

Fig. 15. Collection sites for stalked puffballs, a fungus, inNevada and adjacent California.

region, hot springs often host narrowly restrict-ed native species.

Populations of most sensitive plant speciesare presumably stable (Morefield, personal communication), but there are several excep-tions. The California bearpaw poppy on gypsumsoils in the Mojave Desert is seriously threat-ened by the expanding urbanization in the LasVegas Valley. The persistence of the endangeredSteamboat buckwheat, known only fromancient hot springs south of Reno, is of concern.This site is on private land that was recentlydeveloped for a geothermal power plant, and thespecies is therefore not protected by theEndangered Species Act of 1973. However,Nevada law (Nevada R.S. 527.260–527.300)prohibits the destruction of sensitive specieslisted by the state forester fire warden except byspecial permit. This statute was invoked to pro-tect the species, and a cooperative venture wasinitiated between the developer, The NatureConservancy, the State of Nevada Division ofForestry, and the Nevada Natural HeritageProgram. As a result, the development proceed-ed, and a small portion of the Steamboat buck-wheat population was transplanted to a nearbylocation. So far, the transplanted buckwheatsare doing well, but a large portion of the plant’shabitat is still on private lands that may bedeveloped.

Assistance with the preservation of Grime’svetchling, a species of concern (U.S. Fish and

habitat modifications (Mason 1995). Larvae andadults of the same species may rely on entirelydifferent resources, including food plants,microhabitats, and solar insolation regimes(Weiss et al. 1988; New 1991). Consequently,many invertebrates are sensitive not only tohabitat alterations that change vegetation struc-ture but also to those that perturb microclimate(Thomas 1983, 1991; Dobkin 1985; Dobkin etal. 1987; Murphy et al. 1990). Therefore, landuse patterns are important for the persistence ofinvertebrate populations at the regional scale andat the level of microhabitats (Hammond andMcCorkle 1983; Dobkin et al. 1987; Weiss et al.1988; Kremen 1992). Thomas (1984:337) notedthat some invertebrates may be “extremely par-ticular about the niches they occupy,” and that“seemingly minor” modifications in land man-agement or habitat structure may have majorrepercussions on the suitability of a given sitefor eggs and larvae. Land management is alsocritical to invertebrates that depend on a level ofplant quality and abundance specific to a givensuccessional stage (Scott 1986; Murphy et al.1990; Thomas 1991).

The restricted mobility of many inverte-brates that inhabit the diverse habitat islands in the Great Basin–Mojave Desert region renders them increasingly vulnerable to habitat disturbance. Many invertebrates havehighly specialized adaptations, and the reestab-lishment of extirpated populations becomes less

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probable in degraded and fragmented habitats.Thus, invertebrates are of substantial conserva-tion concern.

Terrestrial Invertebrates

Several groups of terrestrial invertebrates arewell known in the region. The ants and butter-flies are perhaps the most collected and studiedgroups of insects (Austin 1992; Wheeler andWheeler 1986). The mosquitoes were studiedby Chapman (1966), and the grasshoppers andcrickets are well represented in collections of179 known species and more than 3,000 identi-fied specimens (J. B. Knight, NevadaDepartment of Agriculture, Reno, personalcommunication). However, most systematicentomologists consider collections of almost allother insect groups from the Great Basin to beinsufficient, and most investigators report new,undescribed species (Rust 1986; Austin 1992)or geographic range extensions. Until the tax-onomy of the region’s insect fauna is betterknown, an assessment of species of concern isdifficult.

Aquatic Invertebrates

The aquatic invertebrate fauna in the regionshares species with the adjacent regions, espe-cially the Rocky Mountains; species that also

Aquatic invertebrate communities are useful forevaluating water quality because they respondto low-level disturbances, function as monitors(Chandler 1970; Erman 1991), and may be indi-cators of conditions for terrestrial plant commu-nities (Erman 1991). Because the abundances ofaquatic organisms are declining much morerapidly than those of their terrestrial counter-parts (Moyle and Yoshiyama 1994), studies areclearly needed. Although aquatic invertebratesin other areas of the country have been used forevaluating alterations to stream communities,few studies have been done in the GreatBasin–Mojave Desert region.

Vulnerable Invertebrates

Specific information on the conservation sta-tus of almost all sensitive invertebrates in theGreat Basin–Mojave Desert region is complete-ly lacking. For example, only the Ash Meadowsnaucorid is listed as threatened, and 1 grasshop-per, 13 beetles, 27 moths and butterflies, 16snails, 1 mussel, 1 stonefly, and 1 wasp inNevada are species of concern (U.S. Fish andWildlife Service 1994a; Table 4). As studies ofinvertebrates progress, this number will proba-bly increase. A previously undescribed rifflebeetle was recently discovered in Ash Spring inthe Pahranagat Valley and will probably be rec-

occur in the Sierra Nevada are not as widely dis-tributed in the Great Basin (Nelson 1994).Speciation (the evolutionary formation of newspecies) in the region has probably occurred fre-quently because of recurring fragmentation andreconnection of the wetlands during the pastmillion years. This pattern is now being docu-mented for springsnails (Hershler and Sada1987; Hershler 1989, 1994; Hershler and Pratt1990). As with the terrestrial species, hundredsof native aquatic invertebrates probably exist inthe Great Basin but have not been described(see Channel Islands and California DesertSnail Fauna box in California chapter).

General phenomena that affect aquaticinvertebrates have been studied in other areasand probably also apply to the Great Basin. Thecomposition of aquatic insect communitieschanges after the damming of waterways (Ward1984; Williams and Feltmate 1992) and therelease of mining waste (Ward 1984;Wiederholm 1984; Williams and Feltmate1992). Furthermore, changes in water tempera-ture after removal of vegetation, the presence ofheavy metals in the water, and eutrophication oflakes are detrimental to aquatic insects (Ward1984; Wiederholm 1984).

Kremen et al. (1993) suggested that terrestri-al arthropods are appropriate indicators of theneed for conservation, and aquatic insects areprobably equally appropriate indicators of theneed for conservation of aquatic communities.

ommended for federal listing (Schmude andBrown 1991).

Two examples of vulnerable invertebratesinclude the Apache nokomis fritillary and theGiuliani’s dune scarab. The Apache nokomisfritillary is a showy, monarch-sized butterflythat is confined to wet areas, including ripariancorridors, seeps, and damp meadows across theGreat Basin. The species of violet on which thelarvae feed and the thistles from which theadults take nectar are confined to these moisthabitats. Although the butterfly is physicallycapable of dispersing over large distances,observations and genetic studies have revealedthat it rarely ventures far from its small, insularpatches of habitat. Thus, each colony is a semi-isolated population that is rarely recolonizedafter extirpation. Several colonies were

Table 4. Sensitive aquatic insects in the GreatBasin–Mojave Desert region.

Common name Scientific name

Lake Tahoe benthic stonefly Capnia lacustraAsh Meadows naucorid Ambrysus amargosusDeath Valley agabus diving beetle Agabus rumppiDevil’s Hole warm spring riffle beetle Stenelmis calida calidaMoapa warm spring riffle beetle S. calida moapaTravertine bandthigh diving beetle Hygrotus fontinalis

Amargosa naucorid Pelocoris shoshoneamargosus S



a S = species of concern (previously treated as Category 2 species; U.S. Fish and Wildlife Service 1994a); T = threatened

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destroyed by agricultural development in recentyears. A related although still unnamed sub-species of the Apache nokomis fritillary fromthe Carson Valley is a species of concern (U.S.Fish and Wildlife Service 1994a).

Giuliani’s dune scarab occurs only at BigDune and Lava Dune in the southern GreatBasin of Nevada and, because of its restrictedgeographic range and rarity, it became a candi-date for federal listing in May 1984, although itis now considered a species of concern (U.S.Fish and Wildlife Service 1994a). Surveys ofthe beetle in 1993 and 1994 revealed the pres-ence of fewer than 10 individuals at each surveysite. This unique species is at high risk ofextinction because it is present at only a few iso-lated sites and is abundant nowhere. No oneknows how many other invertebrate species arein similar situations across the region.


Fifty-six species and 75 subspecies of fishesoccurred historically in the Great Basin–MojaveDesert region. Of these 131 taxa, 10 (8%) arenow extinct. Of the remaining 121, 75 (62%)are listed, are candidates for federal listing, orare species of concern (U.S. Fish and WildlifeService 1994a). More than 40% of the speciesand 90% of the subspecies are endemics. The

recent use of rotenone to rid the system of non-indigenous red shiners severely harmed thenative species and may have resulted in the lossof genetic variation in at least the Virgin Riverchub (Demarais et al. 1993).

The survival of native fishes in the ColoradoRiver is also in doubt. The damming of thisriver drastically altered flow and temperatureregimes, and many nonindigenous species havebeen introduced. Most of the larger native fish-es such as the Colorado squawfish, the hump-back and bonytail chubs, and the razorbacksucker are endangered; it is unlikely that an effective preserve for these fishes can beestablished. Although dams could be regulatedto maintain a more natural flow regime, thisaction is politically impossible on a river withoverallocated water resources on which somany people depend.

The extensive isolation and close adaptationto specific circumstances by most desert fishspecies are illustrated by the six distinct sub-species of the Amargosa pupfish: the Amargosa,Tecopa, Ash Meadows, Saratoga Springs, WarmSprings, and Shoshone pupfishes. Each of thesesubspecies occurs in a single spring or smallgroup of springs in the Amargosa Valley ofsouthern Nevada and eastern California (Fig.16). Two of the subspecies, the Ash Meadows

causes of nearly all, if not all, declines in desertfish populations can be traced directly to humanactivities. Deacon (1979) recognized that habi-tat modification (including damming, diverting,and channelizing waterways; overgrazing; andother forms of human disturbance) was thelargest contributor to the endangerment ofdesert fishes. Biotic interactions (includinghybridization, competition, and predation), pri-marily with introduced fish species, are the sec-ond largest contributor to the endangerment ofdesert fishes. Given the pervasiveness of waterdevelopment and introductions of nonindige-nous fishes, probably few fish populations inthe West remain unaffected. Minckley andDouglas (1991:15) succinctly summarize prob-lems faced by desert fishes: “All major streamsin the western United States are dammed, con-trolled, and overallocated. . . . Groundwaters arepumped at rates greatly exceeding those atwhich aquifers can be recharged.” Clearly, thisdoes not bode well for the future of the region’sfishes or for its human inhabitants.

The entire fish fauna of the Virgin River inthe southeastern part of the region is imperiled.The native fauna includes two endangeredspecies (the woundfin and the Virgin Riverchub), one subspecies proposed as threatened(the Virgin spinedace), and at least one othercandidate species. Despite management, thelong-term persistence of native fish populationsin the Virgin River continues to be doubtful. The

and the Warm Springs pupfishes, are listed asendangered. Two more of the subspecies arespecies of concern (U.S. Fish and WildlifeService 1994a). At least one subspecies, theTecopa pupfish, is probably extinct (Sigler andSigler 1994).

The problems of the Amargosa pupfish andall desert fishes can be summarized quite simply: water in the desert is scarce, humanswant more than they require, and the fishes

Fig. 16. The Amargosa Valley and Ash Meadows from Devils Hole Hills, Nevada. This area supports many endemic species.

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must have it. Although desert fishes have a ten-uous hold on survival under natural conditions,occurring only in the few permanent springs,rivers, and lakes of the region, their plight hasbeen exacerbated greatly by human activities.The pumping of groundwater for agriculturenearly eliminated several Amargosa pupfishpopulations, including the Devils Hole pupfishand several other native endemic species(Deacon and Williams 1991). Grazing by cattlein riparian areas has increased sedimentationand siltation, reducing spawning of the threat-ened Lahontan cutthroat trout. Diversion ofmost of the flow of the Truckee River by theNewlands Project in 1906 to support agriculturein the Fallon area eliminated the largest knownstrain of cutthroat trout, the Pyramid Lakestrain, and nearly wiped out another endemicfish, the endangered cui-ui. Anglers also pose athreat to desert fishes, not through direct takebut by stocking waters with nonindigenousgame fishes. Eighty-one species of nonindige-nous fishes have been introduced in the region,and 28 species of nonindigenous game fisheshave become naturalized or are currentlyreleased. Many of these have been implicated inthe decline of native species.

One of the greatest concerns affecting sur-vival of many fishes in the desert is the lack of several sites in northern Nevada where leopard

Common name Status a

Long-toed salamander NTiger salamander NInyo Mountains slender salamander SWestern toad NYosemite toad SGreat Plains toad NBlack toad SSouthwestern toad SAmargosa toad CRed-spotted toad NWoodhouse’s toad NMount Lyell salamander SOwens Valley web-toed salamander SBoreal chorus frog NPacific chorus frog NCanyon treefrog NCalifornia red-legged frog PEBullfrog IMountain yellow-legged frog SRelict leopard frog NNorthern leopard frog NSpotted frog C

a C = Category 1 species; species for which there is adequate informa-tion on which to base a proposal to list as threatened or endangeredunder the Endangered Species Act; S = species of concern (previouslytreated as Category 2 species; U.S. Fish and Wildlife Service 1994a);I = introduced to the region; PE = proposed endangered; N = not listed.

Great Basin spadefoot toad N

Table 5. Amphibians of the Great Basin–Mojave Desertregion, compiled from Stebbins (1985) and Zeiner et al.(1988).

available information on the status of thesespecies. Few long-term studies or monitoring ofany fishes other than those listed or proposedfor federal listing have been made, and the dis-tributions of many native fishes are poorlyknown. For example, many undescribed sub-species of the tui chub and speckled dace mayexist, and the discovery and formal descriptionsof other species or subspecies are possible.


Amphibians are one of the rarest animalgroups in the Great Basin–Mojave Desertregion because of their high water require-ments. Only 22 native species and 2 introducedspecies occur, and the ranges of many of thesebarely extend into the region along the easternSierra–Cascades or along the western ColoradoPlateau and the Wasatch Mountains in Utah.Four native species of frogs and toads are wide-ly distributed throughout: the Great Basinspadefoot, western toad, Pacific chorus frog,and northern leopard frog. Three narrowly dis-tributed natives also are residents: the relictleopard frog, Amargosa toad, and black toad.The introduced bullfrog occurs throughout theregion. Of the 22 native amphibian species, 10(45%) are species of concern or candidates forfederal listing (U.S. Fish and Wildlife Service1994a; Table 5), and several others seem to bedeclining. Surveys were conducted recently at

frogs, spotted frogs (Fig. 17), western toads,chorus frogs, and spadefoots had been abundantearlier in this century. All species except thenonindigenous bullfrog are now difficult orimpossible to find (P. Hovingh, University ofUtah, personal communication).

The Amargosa toad, which is endemic to theMojave Desert, is probably the most imperiledamphibian in this region. Thirty years ago, theestimated population size of this species wasseveral thousand, but now as few as 15 breedingpairs and perhaps fewer than 100 adults mayexist (G. Clemmer, Nevada Natural HeritageProgram, Carson City, personal communica-tion; K. Hoff, Biology Department, Universityof Nevada, Las Vegas, personal communica-tion). Habitat loss and degradation—from graz-ing, off-highway vehicles, development, andintroduced predators—are implicated in thedecline of the Amargosa toad. The species iscurrently a candidate for federal listing (Table5), although conservation at the local level maymake such a listing unnecessary (Clemmer,personal communication).

The shortage of quantitative data on amphib-ian populations in this region is serious.Management of these species is difficult orimpossible without data on current distribu-tions, population trends, and species-specificecological requirements. Amphibian popula-tions may well disappear from the region evenbefore they have been discovered.

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Mojave Desert Reptiles

The Pet Trade and Mojave Desert Lizards

In 1993, 19 commercial pet collectorsreported a harvest of 21,794 reptiles in Nevadaat an estimated total value of $250,000 (S.Albert, Nevada Division of Wildlife, personalcommunication). Collectors took 308 chuck-

the 1970’s. The desert tortoise is a long-livedanimal with a low reproduction rate, and popu-lation persistence depends on the long-term sur-vival of adults (Fig. 18). High adult mortalityseems to be the main cause of the populationdecline. Many factors are responsible for thismortality: direct take by humans (for example,collection for pets or food, shooting, killing andinjuring with motor vehicles); habitat loss,degradation, and fragmentation (for example,from roads, agriculture, and residential develop-ment); trampling by livestock; predation bycommon ravens; diseases; and recent droughts(U.S. Fish and Wildlife Service 1994b).

The reversal of population declines in thedesert tortoise is possible only if pressure fromthese factors is greatly alleviated. Protection ofthe desert tortoise began in 1980 when it wasplaced on the federal list of threatened specieson the Beaver Dam Slope, Utah. In April 1990,the population in the Mojave Desert was listedas threatened (U.S. Fish and Wildlife Service1990). Critical habitat was delineated in 1994(U.S. Fish and Wildlife Service 1994c), follow-ing guidelines set by a desert tortoise recoveryplan, which was released in the same year (U.S. Fish and Wildlife Service 1994b). Therecovery plan recommended six recovery unitsbased on genetic, ecological, and behavioral

Fig. 17. Spotted frogs in the Great Basin.

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or C











wallas (a species of concern; U.S. Fish andWildlife Service 1994a) from Nevada in 1993and may have collected more than 400 in 1994.This volume of collecting may harm chuckwal-la populations and populations of other large,long-lived species such as desert iguanas anddesert collared lizards. Commercially popularspecies such as California king snakes,Panamint rattlesnakes, long-nosed leopardlizards, Gila monsters, and Mojave Desertsidewinders are similarly at risk from pet-tradecollecting.

States adjacent to Nevada eliminated thecommercial take of reptiles for the pet tradethrough law enforcement. Because baselinedata on reptile populations in Nevada are notavailable and the effect of unlimited harvestingof the state’s reptiles has never been quantified,the Nevada Fish and Wildlife Commission hasbeen reluctant to eliminate commercial harvest-ing. However, preliminary studies on severalspecies of large-bodied lizards in southernNevada revealed declines of lizard populationsin areas that are affected by humans (U.S. Fishand Wildlife Service 1989). Leopard lizardabundances are much lower adjacent to roadsand where off-highway vehicles are used than inareas that are removed from such effects.

Desert Tortoise

Declines of desert tortoise populationsbecame a major concern of many biologists in

differences found in the species throughout theMojave Desert. Several desert wildlife manage-ment areas on federal lands in these recoveryunits were proposed to provide protection forthe tortoises.

More than 150,000 square kilometers ofdesert lands recently gained some additionalprotection under the California DesertProtection Act (Public Law 103–433), whichcreated the Mojave Desert National Preserve.Although the preserve includes parts of twomajor desert tortoise populations, many activi-ties that contribute to declines of desert tortoisepopulations—such as residential developmentand agriculture (U.S. Fish and Wildlife Service1994b)—are to continue in the preserve. On the positive side, however, the Death Valleyand Joshua Tree national monuments wereexpanded and gained full national park status.

Fig. 18. A desert tortoise in theMojave Desert. ©

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522 Status and Trends of the Nation’s Biological Resources — Volume 2

In addition, 69 wilderness study areas were des-ignated wilderness areas by the Bureau of LandManagement and were thereby afforded addi-tional protection. The same preserves may pro-tect many other desert species whose popula-tions have declined.


Vulnerable Birds

Although only three of the GreatBasin–Mojave Desert region’s bird species areaccorded official status as endangered or threat-ened (bald eagle, peregrine falcon, and south-western willow flycatcher), an additional 20species that regularly breed here are candidatesfor federal listing or are considered sensitivespecies by either the U.S. Fish and WildlifeService (1994a), the Bureau of LandManagement, or the region’s state wildlifeagencies. The Columbian sharp-tailed grousehistorically occurred in the region but has beenextirpated in recent decades, and the westernyellow-billed cuckoo’s population has beenreduced to only a few individuals. Most speciesof concern are jeopardized by the loss anddegradation of habitats which they use duringbreeding, wintering, or migration.

Almost all of the region’s bird species

breeding season (96 at Hart Mountain, 86 atSheldon), most were represented by only a fewindividuals. The ten most abundant speciesaccounted for more than half of the total abun-dance of breeding birds in riparian habitats onboth refuges (Dobkin 1994b). The overall com-position of birds in riparian habitats was char-acterized by low species richness and dispro-portionate representation of a small number ofabundant, widespread species. The riparianhabitats of the Sheldon National WildlifeRefuge (elevation 1,335–1,855 meters) support-ed less diverse avifaunas than comparable habi-tats on Hart Mountain (elevation 1,750–2,250meters) and were quintessential examples ofdegraded, lower-elevation Great Basin ripariancommunities that are numerically dominated byblackbirds. Long-term studies are needed todetermine whether recovery of quality riparianbird communities will be as slow and unpre-dictable in degraded Great Basin habitats assuggested thus far (Dobkin 1994b, 1998).

Nonriparian, shrub-dominated habitats sup-port nearly 20% of the migratory land birds thatbreed in the Great Basin, and the number ofbreeding bird species there is second only tothat in riparian habitats (Dobkin 1998). In addi-tion, resident species of conservation impor-tance, such as the sage grouse (Dobkin 1995),

depend on wetland and riparian habitats duringat least some phase of their annual cycle.Among the 134 species of migratory land birdsthat breed regularly in the Great Basin, morethan half are associated primarily with riparianhabitats (Dobkin 1998). Throughout the aridand semiarid West, an extraordinary diversity ofbird species depends on these habitats(Carothers et al. 1974; Knopf et al. 1988a,b;Dobkin 1994a), and degradation and destruc-tion of riparian areas are widely viewed as themost important causes of the decline of landbird populations in western North America(Bock et al. 1993; DeSante and George 1994;Ohmart 1994). Restoration of riparian habitatsmust become a top priority for land manage-ment agencies, many of which are now aware ofthe importance of protecting and restoring thesehabitats for birds and other wildlife (Thomas et al. 1979; Warner and Hendrix 1984; Johnsonet al. 1985). However, information about the response of bird populations to riparianhabitat restoration is scarce (Krueper 1993;Ohmart 1994).

In conjunction with the cessation of live-stock grazing on the Hart Mountain andSheldon national wildlife refuges in the north-western Great Basin, a long-term study was ini-tiated in 1991 to examine the relation of birdpopulations to riparian habitat recovery(Dobkin 1994b; Dobkin et al. 1995b). Althoughmany species used riparian habitats during the

are linked inextricably to sagebrush-dominatedsteppe. Although abundances of migratoryshrub–steppe birds often vary greatly in timeand space (Wiens and Rotenberry 1981; Wiens1989), DeSante and George (1994) detectedwidespread population declines of grasslandand shrub–steppe birds throughout the westernUnited States and attributed these declines todestruction of grasslands and livestock over-grazing in shrub–steppe habitats (Fleischner1994). As indicated by Bock et al. (1993),almost all shrub–steppe-nesting birds are prob-ably harmed by livestock grazing. The extrememodification of vegetation structure and plantspecies composition from overgrazing createscommunities that are poor in both plant and birdspecies. To understand the dynamics ofshrub–steppe birds, restored examples areurgently needed of the shrub–steppe communi-ties that existed before livestock were intro-duced into the region. Such communities aredominated by native perennial grasses and forbsand have only moderate shrub densities (Bocket al. 1993; Dobkin 1995).

Population declines of many bird speciesthat nest in forests in eastern North Americahave been linked to fragmentation of forest and woodland habitats (Robbins et al. 1989; Terborgh 1989; Ehrlich et al. 1992;Bohning-Gaese et al. 1993; Rich et al. 1994),but few studies have been conducted on theeffects of habitat fragmentation on western

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birds (Dobkin 1994a). Habitat fragmentation,degradation, and loss are threats to birds in theriparian and shrub–steppe communities in theGreat Basin. The distribution patterns of manyriparian species are area-dependent, and manysuch species are lost from smaller riparian frag-ments, such as the Toiyabe Mountains ofNevada (Dobkin and Wilcox 1986). Inshrub–steppe habitats of the Snake River Plainsin southern Idaho, species such as the sagethrasher, Brewer’s sparrow, and sage sparrowselect nesting territories based on landscape-scale patterns of size and distribution of shrubhabitat patches (Knick and Rotenberry 1995).Fragmentation of native shrub–steppe by majorwildfires that now let nonindigenous plantspecies invade may be an important factor in thedecline of bird populations of the shrub–steppeand grasslands in the region. The potentialinfluence of habitat fragmentation in coniferouswoodlands and forests of the Great Basin hasnot been examined.

The shrinking and disturbance of suitablehabitat also present problems for many birdsduring migration. On their annual migration,large numbers of birds of prey and many shore-bird species move through the Great Basin. Thelarger saline and alkaline lakes of the region arecritical stopover points for waterbirds such as

limitations, statistical analyses of Breeding BirdSurvey data have become increasingly complex(Peterjohn and Sauer 1993; Peterjohn et al.1994). Population sizes of some bird species inthe Great Basin have fluctuated during a 23-year period, but no trend was statistically sig-nificant in the recent 10-year period (Fig. 19).However, population sizes and trends in popula-tion sizes have not been estimated for allspecies, and the reliability of the estimatedtrends for some species is not absolute.

Many species with downward trends in pop-ulation size are associated primarily or exclu-sively with shrub–steppe or riparian habitats. Inthe shrub–steppe, such species include the

Population increase Population decrease Trend unknown







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Fig. 19. Long-term population-size trends of 110 species ofNeotropical migratory land birdsin the Great Basin, based on rank-ings from population-trend indicesdeveloped with Breeding BirdSurvey data (Dobkin 1998).Asterisks denote inclusion ofspecies that exhibited upward anddownward trends in two differentportions of the region. For mostspecies, data are insufficient toestablish reliable trends.

the eared grebe, Wilson’s phalarope, and red-necked phalarope (Jehl 1994). The large con-centrations of these and other birds depend onthese lakes for rest and food and would be seri-ously threatened by the shrinkage or human-caused disturbance of this habitat.

Neotropical Migrant Birds

Recent interest in the management ofNeotropical migrant land birds (Finch andStangel 1993; Dobkin 1994a) arose when thedeclining abundance of many of these species inthe eastern United States was recognized.Neotropical migrants depend on habitat in theirNorth American nesting areas and on habitat intheir wintering areas in the Neotropics, whichinclude the Caribbean, Central America, andSouth America. In the Great Basin–MojaveDesert region, Neotropical migrants are a het-erogeneous group that includes more than halfof all nesting bird species.

To assess changes in the abundances ofNorth American birds, the U.S. Fish andWildlife Service established the Breeding BirdSurvey. Annual surveys have been conducted inthe western United States since 1968. Data fromthe survey have been used to determine trendsin the population sizes of some western birds(Robbins et al. 1986), but the reliability of the data on most species in the GreatBasin–Mojave Desert region is questionablebecause of limited sampling. To overcome these

northern harrier, mourning dove, horned lark,loggerhead shrike, green-tailed towhee, vespersparrow, and sage sparrow. In pinyon–juniperwoodlands with a sagebrush and bunchgrassunderstory, such species are the commonnighthawk, northern flicker, gray flycatcher,northern mockingbird, chipping sparrow, andScott’s oriole. Trends in the population sizes ofthe rock wren, sage thrasher, Brewer’s sparrow,black-throated sparrow, and western mead-owlark were upward in one portion of the GreatBasin and downward in another.

Species with downward population-sizetrends and associated most closely with riparianhabitats include the killdeer, violet-green swal-low, warbling vireo, yellow warbler, lazulibunting, savannah sparrow, song sparrow,yellow-headed blackbird, and Brewer’s black-bird. The population-size trend of the red-tailedhawk has been downward for 23 years.

Three riparian species with upward popula-tion-size trends are the broad-tailed humming-bird, red-winged blackbird, and Bullock’s ori-ole. Other species with upward population-sizetrends are the American kestrel, house wren,mountain bluebird, American robin, and black-headed grosbeak—all of which frequentlyoccur in riparian habitats and in pinyon–juniperor mountain-mahogany woodlands—and thenorthern rough-winged swallow, cliff swallow,and barn swallow, which are aerial insectivoresand feed over riparian meadows and other open

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524 Status and Trends of the Nation’s Biological Resources — Volume 2

habitats. More species with upward population-size trends are the brown-headed cowbird, anest parasite of many Neotropical migrants, andseveral wide-ranging birds of prey such as the turkey vulture, Swainson’s hawk, goldeneagle, and prairie falcon. The lark sparrow wasthe only songbird species with an upward population-size trend in the shrub–steppe.

Data have been largely insufficient to deter-mine population-size trends of nearly any of the80 species of Neotropical migrant land birdsthat occur in the Mojave Desert (Fig. 20). Thepopulation sizes of the mourning dove and the loggerhead shrike, whose abundances are declining widely in western North America(DeSante and George 1994), are decreasing in the Great Basin. Upward population-sizetrends were found for the red-tailed hawk, ash-throated flycatcher, northern mockingbird, andblack-throated sparrow.


Because of the tremendous past and contin-uing loss of wetlands, many waterbird speciesin the Great Basin–Mojave Desert regionshould be considered sensitive. Data on water-fowl are available from long-term surveys, butfew data are available on other species.

Long-term surveys of waterfowl indicatethat none seems to be in immediate danger ofextirpation. The only waterfowl species that areconsidered sensitive in North America andoccur in the Great Basin–Mojave Desert regionare the trumpeter swan and the canvasback.Swans have a relatively stable, small population(about 25; Alcorn 1988), and canvasback popu-lations seem to be stable.

Common loons and several species of grebesmigrate through Nevada. Their primarystopover site, Walker Lake, is in danger ofbecoming too saline to support fishes, their pri-mary food. The largest breeding colony ofAmerican white pelicans in the United States ison Anaho Island in Pyramid Lake, and the sizeof this population is believed to be decreasing.Survey records from the 1960’s and 1970’sshowed that the population sizes of double-crested cormorants also were declining.

Population-size trends in wading birds are

Fig. 20. Long-term population-sizetrends of 80 Neotropical migratoryland birds in the Mojave Desert,based on rankings from popula-tion-size trend indices developedwith Breeding Bird Survey data



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Population increase Population decrease Trend unknown

The preponderance of downward population-size trends of birds of theshrub–steppe indicates continuing problemswith the health of these communities. As notedelsewhere (DeSante and George 1994), declin-ing abundances of several of these species mayalso be linked to habitat degradation on winter-ing grounds in the southwestern United Statesand northern Mexico. The population-sizetrends were downward for several riparian specialists and upward for only two riparianspecialists (broad-tailed hummingbird andBullock’s oriole). These trends are further evi-dence of the continuing deterioration of theriparian habitats of the Great Basin.

Although more than 90% of the region’sNeotropical migrant land bird species were seenduring Breeding Bird Surveys, inadequate sam-ple sizes precluded the determination of popula-tion-size trends of almost all species, especiallythose in the Mojave Desert. Some species can-not be sampled adequately with the standardtechniques of the Breeding Bird Survey, regard-less of sampling effort. The monitoring of suchspecies requires specialized techniques.

largely unknown. Population sizes of the greatblue heron declined in the late 1980’s in associ-ation with an extended drought, but white-facedibis populations seem to be increasing. Noinformation is available on population-sizetrends of two species of concern, the Americanbittern and the western least bittern.

Surveys of shorebirds in western NorthAmerica are inadequate, and a determination ofthe importance of habitats in the Great Basin formany of these species is difficult. Most data areprovided by surveys in staging areas, and littleis known about species that migrate in smallgroups. Wetlands of the Great Basin–MojaveDesert provide critical stopover habitat duringmigration for great numbers of the Wilson’s andred-necked phalaropes, long-billed dowitcher,and American avocet, and for smaller numbersof the least and western sandpipers. Significantnumbers of the breeding snowy plover, long-billed curlew, American avocet, black-neckedstilt, spotted sandpiper, and common snipeoccur in the western Great Basin. Of these, thesnowy plover is probably of greatest concern.The western snowy plover has been declining inabundance throughout its range, includingNevada (from 878 in 1980 to 349 in 1988 and139 in 1991; see box on Western Snowy Ploversin California chapter) and in southeasternOregon. The Franklin’s gull and the black ternoccur in the Great Basin, and their numbers are

(Dobkin 1998). Because of inade-quate data, reliable trends could bedetermined for only 6 of 80species.

4 2 0 0


1982–1991 0

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declining in many parts of their ranges. No dataare available on which to base population-sizetrends of either species in the region, but bothseem to warrant concern.


One hundred fifty species of mammals occurin the Great Basin–Mojave Desert region(Zeveloff 1988). Thirty (20%) of these speciesare either species of concern, candidates for list-ing as endangered under the EndangeredSpecies Act of 1973, or are already listed asendangered (U.S. Fish and Wildlife Service1994a,d; Table 6). The Mojave ground squirrel,Tehachapi pocket mouse, and Owens Valleyvole are small rodents whose entire geographicranges are in the Mojave Desert. The Mojaveground squirrel occupies a restricted range inthe northwestern Mojave Desert in California(Hafner and Yates 1983), and the Owens Valleyvole is restricted to the Owens Valley of south-eastern California. The flora and fauna of thenorthwestern portion of the Mojave Desert wereisolated in a wet relict habitat during the lastglaciation and consequently became an ecolog-ically unique area (Hafner 1992). This corner ofthe Mojave Desert borders on the expandingLos Angeles metropolitan area, and encroach-

Table 6. Mammal species of concern, previously treated as Category 2 status of the U.S. Fish andWildlife Service, in the Great Basin–Mojave Desert region. “States present” refers to the occur-rence of the species in this region.

Species States present Amargosa vole CaliforniaPygmy rabbit Nevada, Oregon, Idaho, UtahSpotted bat Nevada, Idaho, Utah, California, ArizonaGreater western mastiff-bat Nevada, California, ArizonaPalmer’s chipmunk NevadaHidden Forest Uinta chipmunk NevadaNorth American lynx Nevada, Oregon, CaliforniaNorth American wolverine Nevada, Idaho, Utah, CaliforniaCalifornia wolverine Nevada, CaliforniaAllen’s big-eared bat Nevada, Utah, California, ArizonaSierra Nevada snowshoe hare Nevada, CaliforniaSouthwestern otter Nevada, Utah, ArizonaCalifornia leaf-nosed bat California?, Nevada?Desert Valley kangaroo mouse NevadaFletcher dark kangaroo mouse NevadaPahranagat Valley montane vole NevadaAsh Meadows montane vole NevadaSmall-footed myotis Nevada?Long-eared myotis Nevada, California, Utah, ArizonaFringed myotis Nevada, Oregon, Idaho, CaliforniaCave myotis Nevada, ArizonaLong-legged myotis Nevada, Oregon, Idaho, California, Utah, ArizonaYuma myotis Nevada, Oregon, Idaho, California, Utah, ArizonaBig free-tailed bat Nevada, California, Utah, ArizonaPale Townsend’s big-eared bat Nevada, Oregon, Idaho, Utah, ArizonaPreble’s shrew Nevada, Oregon, CaliforniaFish Spring pocket gopher NevadaSan Antonio pocket gopher NevadaMojave ground squirrel CaliforniaOwens Valley vole CaliforniaSierra Nevada red fox California

ing urban development is believed responsiblefor the declining abundance of the Mojaveground squirrel. Little is known about the ecol-ogy, the current population size, or the distribu-tion of the Owens Valley vole.

The Sierra Nevada red fox, Pacific fisher,California wolverine, and Sierra Nevada snow-shoe hare are montane mammals whose distrib-utions in the Great Basin are confined to theeast slope of the Sierra Nevada. The decliningabundances of the Pacific fisher and Californiawolverine are attributed to trapping in the latenineteenth and early twentieth centuries (Ingles1947; Hall 1981; Jameson and Peeters 1988; R.M. Nowak 1991). Humans perceived thewolverine as a nuisance and eliminated thespecies (Ingles 1947; R. M. Nowak 1991).

The California bighorn sheep is a species ofconcern (U.S. Fish and Wildlife Service 1994a).The historical distribution of this subspeciesextended from northeastern California intonorthern Nevada and eastern Oregon (Hall1981), but by the early part of the twentieth cen-tury, sheep populations were declining becauseof hunting (Zeveloff 1988). Bighorn sheep werereintroduced into the Great Basin by Nevadaand Oregon wildlife agencies in an attempt toreestablish them as a game species. Californiabighorn sheep from populations in BritishColumbia are thriving at the Hart MountainNational Wildlife Refuge where the currentpopulation exceeds 450 individuals. Forty-two

hunting tags were issued in 1994 (R. Cole, HartMountain National Wildlife Refuge, Oregon,personal communication).

Gaps in Knowledge

Recent research in Nevada revealed signifi-cant extensions of the known distributions of 14of 22 conifer species that are native to the state(Charlet 1996). For example, the range of thewestern juniper was extended to 43 additionalmountain ranges (Fig. 21), and the range ofwhite fir was extended to seven more mountainranges (Fig. 22). Many of these stands inNevada were unknown to scientists, althoughconifers are easy to see on the landscape evenfrom long distances, and their visual promi-nence is mirrored by their ecological impor-tance wherever they occur. If not even the loca-tions of the trees are known, how much can beknown about other floral and faunal distribu-tions and community processes of the region?Conifer species represent only 0.07% of themore than 3,000 known plant species in Nevada(Kartesz 1988). Among the Nevada flora, 127plants are listed, are candidates for federal list-ing, or are species of concern (U.S. Fish andWildlife Service 1994a,d), but virtually nothing

Pacific fisher CaliforniaCalifornia bighorn sheep Nevada, Oregon, CaliforniaTehachapi white-eared pocket mouse California

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526 Status and Trends of the Nation’s Biological Resources — Volume 2

such information would greatly assist managersof public lands with balancing mandates for theconservation of biological diversity and formultiple use by humans.



Most of the 300 described caves in the GreatBasin are in limestone mountains (McLane1975; Fiero 1986). Anthropological informationhas been a major focus of cave explorations inthe region because of the long history of caveuse by Native Americans (Thomas 1985).Woodrat middens, in which many importantfossils have been found, are an additional richresource in cave habitats throughout the region(Betancourt et al. 1990; C. L. Nowak 1991;Wigand and Nowak 1992; Nowak et al. 1994).

The Lehman Caves in Nevada and theTimpanagos Cave National Monument in Utahwere developed for tourism (Fiero 1986). TheLehman Caves receive 46,000 visitors yearly,and changes in the cave community resultingfrom such heavy use are a source of concern(Great Basin National Park 1988). Invertebratesin caves are rarely surveyed (Desert ResearchInstitute 1968), so it is difficult to determine ifany invertebrate groups are at risk. Large

Great Basin–Mojave DesertNevada mountain rangesWhite fir locations (Charlet 1996)

Fig. 21. Distribution of westernjuniper in Nevada. Red dots indi-cate known distributions based oncollections (Charlet 1996). Yellowpolygons are distributions asreported by Little (1971).

Nevada mountain rangesWestern juniper locations (Charlet 1996)Western juniper range(Little 1971) Great Basin–Mojave Desert

is known about them except that they are prob-ably rare. The basic information about plant dis-tributions is skeletal at best, yet plants are theprimary producers in communities, and allorganisms ultimately depend on them.Information about the ecology and evolution ofmost organisms has not been gathered, although

colonies of bats in caves were killed or extirpat-ed when the caves were mined for guano; batsthat hibernate or raise young in caves are espe-cially susceptible to disturbance (Pierson andBrown 1992; Dobkin et al. 1995a; Fig. 23).


Tunnel mines that were excavated since themid-1800’s provide potential roosting sites for19 of the 22 bat species in the region (Burt andGrossenheider 1976; Pierson et al. 1991). InNevada, most of the more than 50,000 knowninactive mines are unsuitable roosts, although afew may house significant colonies. Inactivemines in California provide most known colonysites for the cave myotis and Townsend’s big-eared bat. This situation is probably mirrored in

White fir range (Little 1971)

Fig. 22. Distribution of white fir inNevada. Red dots indicate knowndistributions based on collections(Charlet 1996). Yellow polygonsare distributions as reported byLittle (1971).

Fig. 23. Long-eared myotis, a species of concern, atEmerald Lake, Jarbidge Mountains, Nevada.

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the Great Basin (Pierson et al. 1991), althoughthese species make extensive use of naturalcaves in forested lava flows adjacent to theregion in Oregon and Idaho (Genter 1986;Dobkin et al. 1995b). The loss of mines ascolony sites for rearing young and for hiberna-tion is magnified by the degradation of naturalroosts in caves through human disturbancessuch as recreation or guano mining (Pierson andBrown 1992). In addition, cyanide ponds usedfor gold extraction at active gold mines havebeen implicated in the deaths of many bats whodrink the contaminated water (Clark andHothem 1991).

The Abandoned Mines Program of theNevada Division of Minerals identifies andsecures abandoned mines that pose a hazard tohuman safety. Approximately half of the 3,000mines that were closed in Nevada since 1987were not inspected for wildlife and becameunusable to bats and other wildlife after theywere filled (D. Driesner, Bureau of AbandonedMine Lands, Nevada Division of Minerals,Reno, personal communication). Since October1993, however, mines have instead been closedby fencing and signs where possible, to retainwildlife habitat. Education of land managementagencies about the use of inactive mines by batsin Nevada increased the frequency of bat sur-

will not come to the sites if they detect humans(Charlet and Rust 1991); however, it is unclearhow these birds are affected when they are dis-placed by humans.

Sand Dunes

Despite the uniqueness of the flora andfauna of sand dunes across the Great

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Fig. 24. Golden eagle leaving abog after drinking and bathing atthe headwaters of the HumboldtRiver, Jarbidge Mountains,Nevada.

veys and proactive management. The Bureau ofLand Management entered a cooperative agree-ment with the Nevada Division of Mines aboutmanagement of orphaned mine sites on Bureauof Land Management land (Bureau of LandManagement 1994). Population and natural his-tory data of the region’s bats will becomeincreasingly important as managers focus onthe 11 bat species in the region that recentlybecame species of concern (Burt andGrossenheider 1976; U.S. Fish and WildlifeService 1994a).


Although eagles were believed to requireonly the water they obtained from their prey(Brown and Amadon 1968), Charlet and Rust(1991) observed golden eagles drinking,bathing, and preening at high mountain bogsand springs in the northern Great Basin (Fig.24). Five eagles at one time were seen on theground on three occasions (Charlet and Rust1991), and more than 45 landings at one bogoccurred in an 8-hour period (Charlet, unpub-lished data). To date, golden eagles have beenobserved drinking at high mountain springs in15 mountain ranges in the northern and centralGreat Basin (Charlet and Rust, unpublisheddata). Several of these bogs are in areas thatattract many visitors or are claimed for mining.Trails pass near some of the bogs, and peoplecamp at others to use the water. Golden eagles

Basin–Mojave Desert region, little is knownabout these island-like habitats or the organismsthat inhabit them. The locations of some dunesin the region are not even portrayed on U.S.Geological Survey maps. Virtually nothing isknown about the spatial dynamics of thesedunes and how these dynamics affect the bio-logical communities. Biological inventoriesexist only from the most heavily used dunes,often from only one survey (Rust 1994).Popular dunes in the region (for example, SandMountain and Nevada and Little Sahara dunes,Utah) may receive several thousand visitorsduring one holiday weekend, but the ecologicaleffects of this heavy recreational use have beenassessed in only a few cases. Hardy andAndrews (1979) and Luckenbach and Bury(1983) linked the effects of off-highway vehi-cles to the loss of vertebrate and invertebratespecies richness, a reduction in vertebrate andinvertebrate populations, and a disruption ofmating behaviors in insects that depend ondune-margin vegetation for mating habitat.More information is needed on the effects ofoff-highway vehicles on the dune biota and onhow these effects can be minimized or mitigat-ed. Nearly all sand dunes in the region are onland that is administered by the Bureau of LandManagement, and this agency should take thelead on conserving the unique biota.

A good start to conservation of sand dunebiota was made with the Sand Mountain pallid

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528 Status and Trends of the Nation’s Biological Resources — Volume 2

blue butterfly. This species was originally dis-covered in a small patch of Kearney buck-wheat—the plant on which its caterpillarsfeed—near the parking lot at the base of SandMountain, Nevada. Sand Mountain is a largedune used extensively for recreational use ofoff-highway vehicles and was officially desig-nated by the Bureau of Land Management forthat purpose. Because the distribution of thebutterfly at Sand Mountain was not known and because trampling and traffic seemed tothreaten the species, it became a species of con-cern (U.S. Fish and Wildlife Service 1994a).

When biologists visited the site in summer1994, they found that much of the Kearneybuckwheat at the parking lot site had been beat-en down by vehicles. At this point the optionsseemed to be fencing off the remaining plants orclosing part of this popular recreational area,and recreationists feared the whole dune wouldthen be closed. However, a recreationist over-heard the agency biologists and took on the taskof finding additional food plants. And indeed hefound them—the plants were widely distributedall around the base of the dune. Thus, consider-able potential habitat for the butterfly existed inareas that are rarely visited by off-highwayvehicles. The question remained whether thebutterfly also existed in these areas. Their pres-

(about 10% of the state flora) are now estab-lished (J. Kartesz, Department of Biology,University of North Carolina at Chapel Hill,personal communication), and their biomasssometimes exceeds that of native species(Hunter 1990). Although most of the effect ofnonindigenous plant species in the regionoccurs in sagebrush–grass, pinyon–juniper, andwetland zones, it is apparent to some extent inevery community.

Several highly invasive nonindigenousspecies are recent introductions, and others,such as the yellow star-thistle, are poised on itsedges. The medusahead is considered a particu-lar threat because of its ability to invade undis-turbed sites (R. J. Tausch, U.S. Forest Service,Intermountain Research Station, Reno, Nevada,and J. A. Young, Agricultural Research Service,University of Nevada, Reno, personal commu-nication). Red brome continues to spread in theMojave Desert (Beatley 1966; Hunter 1990,1991). The barbwire thistle resembles andhybridizes with the Russian thistle, thus thebarbwire thistle’s introduction and dispersalduring the past 20 years went virtually unno-ticed until recently (J. A. Young, U.S. ForestService Intermountain Research Station, per-sonal communication).

The invasion of cheatgrass caused drastic

ence at these sites was verified later by biolo-gists during the butterfly’s flight season. TheBureau of Land Management developed a man-agement plan that includes monitoring the but-terfly and its food plant and directing off-highway vehicle traffic away from the vegetat-ed dune fringes, which are rarely used for recre-ation anyway. As long as the off-highway-vehicle community cooperates, the butterflyand its food plant should be secure without afederal listing.

Potential Ecological Crises

Invasion by Nonindigenous Plants

Europeans brought to the Great Basin–Mojave Desert region their farming techniques,plants, and animals; these created different dis-turbance patterns than the native biota of theregion had previously experienced (Mack1986). The newly disturbed environments creat-ed excellent seedbeds for highly invasivespecies like the Russian thistle, tumble mustard,cheatgrass, halogeton (Holmgren 1972), filaree(Mack 1986), and tamarisk (Robinson 1965).

The total effect of nonindigenous plantspecies in the region is unknown, but in NorthAmerica invasions of nonindigenous grasses aremost severe in the arid western United States(D’Antonio and Vitousek 1992). In Nevadaalone, at least 315 nonindigenous plant species

changes in fire regimes and secondary plantsuccession (Piemeisel 1951) across at least amillion hectares. Species composition(Piemeisel 1951) and competitive interactionsin the affected plant communities are greatlymodified by this invasion (Melgoza et al. 1990;D’Antonio and Vitousek 1992). Because of therapid accumulation of flammable fuels fromstalks, cheatgrass-infested areas experiencemore frequent fires (Pickford 1932; Whisenant1990) and a longer fire season (Roberts 1990)than areas without grass or with sparse grasscover. The pattern of fire damage was altered:smaller, patchier burns that were typical beforeEuropean settlement were replaced by fires thattend to destroy vegetation over large, more con-tinuous stretches of land (Whisenant 1990;D’Antonio and Vitousek 1992). When fire-intolerant native plant species are burned, theresident fungi necessary for their establishmentand growth also are lost, favoring the establish-ment of nonindigenous species such as theRussian thistle, tumble mustard (Goodwin1992), and cheatgrass (Yensen 1981).

Although some areas that are dominated bycheatgrass can be reclaimed by native species ifleft undisturbed (Hironka and Tisdale 1963), thevulnerability of these sites to burning and sus-ceptibility to midsummer erosion nearlyensures the continued persistence of cheatgrass(Yensen 1981). Thus, the naturalization ofcheatgrass in sagebrush–grass communities is a

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Human-Induced Changes in the Mojave and Colorado Desert Ecosystems: Recovery and Restoration Potential

Large parts of the Mojave and Coloradodesert (see chapter on Southwest) ecosys-tems have been affected by humans and theiractivities, especially in the last 100 years.Urbanization, agriculture, off-highway vehi-cle use, construction of roads and utility cor-ridors, livestock grazing, and military train-ing activities have all created measurablechanges in the structure and stability of theecosystem. Air pollution, although difficultto measure accurately (Reible et al. 1982),has noticeably damaged plants in theMojave and Colorado deserts (Fisher 1978).The effects of mining have been limited tospecific sites in the region. Like all ecosys-tems, the Mojave and Colorado deserts areresilient to some stresses but not to others.Unfortunately, the weakly defined, thin soillayers that characterize the region are easilydisturbed. This vulnerability, coupled withthe extreme aridity and temperature rangesof the region, means that conditions neces-sary for germination and survival of manyplants occur only infrequently, and recoveryof plant populations to predisturbance con-ditions is very slow (Table).

before disturbance. The creosotebush-dominated shrub–steppe ecosystem thatmany recognize as characteristic of theMojave and Colorado deserts (Burk 1977;Vasek and Barbour 1977) may have existedonly for the last 10,000 years or so (Grayson1993). Before the arrival of Europeans, thisvast area was characterized by communitiesof widely spaced, long-lived, fire-sensitiveshrubs, including creosotebush, burro-weed,Joshua-trees, various cacti, and other lessfamiliar associates. Since then, the com-bined effects of human activities haveincreased erosion, the destruction of soil-stabilizing microorganisms, soil com-paction, fire frequency, habitat destructionand fragmentation, the number of non-indigenous species, and the number ofthreatened and endangered species.

Much of what is known about ecologicalsuccession in the Mojave and Coloradodeserts comes from research on recoveryfrom human disturbances in the region. Therate of change in plant communities overtime, or succession, is a function of the typeof disturbance, its magnitude and frequency,

communities may take place in a span of 9 to 78 years (Vasek 1979; Prose et al. 1987;Webb et al. 1987), but 1,000 or more yearsmay be required for disturbed sites to recov-er to predisturbance vegetation structure andspecies composition (Webb and Newman1982), though cover of colonizing plantsmay equal predisturbance cover values with-in 20–50 years.

Construction of UtilityCorridors

In the Mojave and Colorado deserts, therates of increase and composition of colo-nizing plant species vary considerably fol-lowing construction of utility corridors forpower lines and pipelines, demonstratinghow difficult it is to predict succession rela-tive to adjacent undisturbed areas. Groundcover of short-lived perennial species actu-ally increases in areas of severe disturbance,under the central wires, and along the edgesof maintenance roads. After 33 years, thereis a noticeable but incomplete recovery of

Understanding the changes that any system has undergone requires baselineinformation on conditions that existed

and, to a lesser extent, the type of plant com-munity affected. Short-term, partial recov-ery of Mojave and Colorado Desert plant

vegetation (Vasek et al. 1975a). Naturalrevegetation (to 41% ground cover) by long-lived perennials was observed 12 years afterconstruction of a pipeline by trenching, pil-ing, and refilling (Vasek et al. 1975b).Disturbed and control areas appear to havesimilar cover, biomass, and densities ofplants following partial recovery; however,long-lived species are poorly represented ondisturbed sites (Lathrop and Archbold1980a,b).

Impacts of MilitaryActivities

Large areas of the Mojave and Coloradodeserts have been, and continue to be, affect-ed by military training activities. The recov-ery of such areas of the eastern MojaveDesert was studied almost 36 years after theregion was first subjected to military activi-ties (Lathrop 1983a). Disturbed areasincluded tent sites, roads, and tank tracks.All of these areas exhibited reduced plantdensity and cover relative to control areas.Reductions of cover and density were great-est in tank tracks and least in tent areas.Recovery to predisturbance levels of coverand density varied according to disturbancetype. Tent areas showed the greatest recov-ery, and roadways showed the least, reflect-ing the intensity of disturbance. Recovery intank tracks was intermediate. Diversity of

Disturbance Location Recovery time (years) ReferenceTank tracks (military) Eastern Mojave 65,a 76b Lathrop (1983a)Tent areas (military) Eastern Mojave 45,a 58b Lathrop (1983a)Dirt roadways (military) Eastern Mojave 112,a 212b Lathrop (1983a)Tent sites (military) Eastern Mojave 8–112c Prose and Metzger (1985)Tent roads (military) Eastern Mojave 57–440c Prose and Metzger (1985)Parking lots (military) Eastern Mojave 35–440c Prose and Metzger (1985)Main roads (military) Eastern Mojave 100–infinityc Prose and Metzger (1985)Military Eastern Mojave 1,500–3,000d Prose and Metzger (1985)Town sites Northern Mojave 80–110,e 20–50,b 1,000+f Webb and Newman (1982)Pipeline Southern Mojave Centuriesg Vasek et al. (1975b)Power line Southern Mojave 33h Vasek et al. (1975a)Fire Western Colorado Desert 5b,i O’Leary and Minnich (1981)Off-road vehicle use Western Mojave Probably centuries Webb et al. (1983)Pipeline (berm and trench) Mojave Desert 100j Lathrop and Archbold (1980a)Pipeline (road edge) Mojave Desert 98j Lathrop and Archbold (1980a)Power line pylons and road edges Mojave Desert 100j Lathrop and Archbold (1980a)Under power line wires Mojave Desert 20j Lathrop and Archbold (1980a)

aRecovery time to reach control density.bRecovery time to reach control cover.cEstimated recovery time for creosotebush to reach control densities.dEstimated recovery time (if at all) to reach original vegetative structure, assuming establishment of control densities.eCompaction recovery time.fTotal estimated recovery time.g30–40 years assuming linear rates of succession; 3,000 years until formation of large creosote bush clonal rings.hIncomplete recovery time in areas of high impact.iTime for appearance of perennial seedlings.jBiomass recovery assuming that successional vegetative growth is approximated by a straight line. Recovery of long-livedspecies is estimated to take at least three times longer than indicated.

Table. Estimated natural recovery times (years) for California desert plant communities subjected tovarious human-induced disturbances.

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530 Status and Trends of the Nation’s Biological Resources — Volume 2

dominant perennials also varied betweendisturbed and nondisturbed areas, but resultswere clouded by low species richness at thestudy sites and few individuals of subdomi-nant species. However, diversity in disturbedtransects at the Camp Ibis study site was lowrelative to control sites. The more intense orfrequent the use of the site, the less similarthe species composition was to that of undis-turbed control sites.

Overall, recovery of plant density is slowrelative to increases in cover: the number ofindividuals present at a site changes littlefollowing recovery from disturbance, butsurviving individuals cover larger areas.Lathrop (1983a,b) concluded that recoveryof the disturbed eastern Mojave sites tosome original level of community composi-tion and stability may not occur in the fore-seeable future. Similar observations andconclusions were reached by Prose andMetzger (1985) and Prose et al. (1987) atabandoned military camps in the easternMojave. Long-lived species such as cre-osotebush were dominant in all controlareas, but their cover and density werereduced in disturbed areas. Dominant plantsin disturbed areas included pioneer speciessuch as burro-weed and burrobrush. Groundcover by pioneer species in disturbed areaswas equal to or greater than their cover in

damage is generally high in all areas exceptbarren sand dunes (but see Bury andLuckenbach 1983) and the clay flats ofplayas (Dregne 1983). Soil damage causedby off-highway vehicles is environmentallysignificant because desert soils may take10,000 years to develop. From this estimate,Dregne (1983) concluded that it was futile tospeak of disturbed soil recovery in time peri-ods related to human occupancy of theaffected areas (Fig. 1).

A major effect of off-highway vehicles isthe physical destruction of plants. Plants aredestroyed when their stems and foliage, rootsystems, and germinating seeds are crushed.Lathrop (1983b) examined aerial pho-tographs of nine disturbed and undisturbedareas in the Mojave Desert to assess theeffects of off-highway vehicle use. Perennialplant density and cover were dramaticallyreduced in areas disturbed by vehicles, andtotal plant cover and density were less than15% of that in three undisturbed controlsites examined.

Weeds and FireLike the rest of the western United

States, the Mojave and Colorado desertshave been hit hard in the last century by invasive nonindigenous plants.

The proliferation of nonindigenousannual plants has dramatically increased thefuel load and frequency of fires in parts ofthe Colorado Desert in recent years(O’Leary and Minnich 1981; Brown andMinnich 1986). The frequency of fires in theColorado Desert of California is furtherenhanced by the proximity of previouslyburned areas (Chou et al. 1990). Nativeperennial shrubs are poorly adapted to fire,as evident in their low rates of recovery. Inthe upper Coachella Valley on the east scarpof the San Jacinto Mountains near PalmSprings, California, burned creosotebushscrub is replaced by open stands of brittle-bush, native ephemerals, and nonindigenousannual grasses (Brown and Minnich 1986).

Although fire had a role in the evolutionof the desert plant community, it was proba-bly minor, with limited effect and long inter-vals between fires. With the invasion ofspecies that serve as fine fuels, like non-indigenous annual grasses, the fire cycle hasbeen significantly shortened and fires aremore likely to spread. The result has beenthe conversion of desert scrub landscapes to“weedscapes” dominated by nonindigenousplants.

Livestock Grazing

control sites.

Differences in vegetative structurebetween control and disturbed plots weredue to soil compaction, removal of the toplayer of soil, and alteration of drainagechannel density at the military sites (Prose et al. 1987). Penetrometer measurementsshow that the compaction caused by a singlepass by a “medium” tank can increase aver-age resistance values in the upper 20 cen-timeters of soil by 50% relative to adjacentuntracked soil; values of up to 73% wererecorded. Dirt roadways could not be pene-trated below 5–10 centimeters because ofextreme compaction. Physical modificationsto the soil beneath tank tracks extended to a depth of 25 centimeters and outward from the track edge to 50 centimeters (Prose1985).

Effects of Off-HighwayVehicles

Off-highway vehicles have also dis-turbed large areas of the Mojave andColorado deserts. The effects of these vehi-cles have been well documented and includedestruction of soil stabilizers, soil com-paction, increased wind and water erosion,noise, decreased abundance of lizard popu-lations (Busack and Bury 1974), anddestruction of vegetation (Webb andWilshire 1983). Susceptibility of soils to

Nonindigenous annual grasses have becomethe dominant understory plants in areas for-merly occupied by native perennial grasses(D’Antonio and Vitousek 1992). Large areasof the Mojave and Colorado deserts areinfested with Mediterranean grasses, cheat-grass, and other exotics (Beatley 1969;Bowers 1987; Hunter 1991).

The effects of livestock grazing in theMojave and Colorado deserts, while contro-versial, have been locally significant(General Accounting Office 1992). No published studies have yet fully documentedthe impact of grazing by livestock or estimated the time required by heavilygrazed areas of the desert to recover to

Fig. 1. Large areas of the Mojave and Colorado desert ecosystems have been affected by off-highwayvehicles. The road scars shown in this photograph will be clearly visible for decades or longer.





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pregrazing levels of plant diversity, density,and cover (Oldemeyer 1994).

Webb and Stielstra (1979) observed that,relative to ungrazed control areas, soils inthe Mojave Desert exhibited greater com-paction in areas where sheep bedded andgrazed. Compaction was greatest in theupper 10 centimeters of soil but was alsoobserved at lower depths. Surface soilstrampled by grazing animals lose stabilizerscomposed of microorganisms, whichincreases erosion potential (Fig. 2).

The Role of RestorationEstablishment and growth of native

plants are naturally slow processes under theextreme conditions of the desert, and distur-bance makes these conditions even moresevere. Natural recovery is thus extremelyslow (Table) and does not necessarily resultin communities that resemble predistur-bance conditions. Revegetation and restora-tion can help mitigate many of these nega-tive impacts and speed recovery.

Unfortunately, our ability to restoredegraded habitats relies on current technolo-gies that are sharply constrained by theharsh conditions imposed by the desert envi-ronment. Furthermore, the costs of large-scale restoration are prohibitive, and thechances of long-term success are low orunknown. Brum et al. (1983) estimated thatpower line corridors in the Mojave Desertcould be restored for $9,221 per hectare byusing seeding and irrigation. Estimates ofcosts involved in restoring degraded land atthe Yucca Mountain site in Nevada weremuch higher, ranging from $73,969 to$115,754 per hectare, depending on thenature of the disturbance (Malone 1991).However, recent advances in desert restora-tion technology offer hope for the success oflocalized restoration efforts (Bainbridge andVirginia 1990; Bainbridge et al. 1995).Given the sensitivity of desert habitats andtheir slow rate of natural recovery, the best management option is to limit theextent and intensity of disturbances as muchas possible.

See end of chapter for references

ch, U


“catastrophic change to a new system” (Turneret al. 1993:223). The conversion of sagebrush–steppe to annual grasslands makes these range-lands virtually worthless (Yensen 1981;Morrow and Stahlman 1984; Roberts 1990),and the cost of rehabilitation is often higherthan the value of the land on the open market(Roberts 1990).

Degradation of Springs

Isolation and small size render many springcommunities in the Great Basin–Mojave Desertregion particularly vulnerable to disturbanceand loss. Groundwater pumping in MojaveDesert areas, such as Pahrump Valley, causedthe drying of springs, complete loss of habitat,and extinction of subspecies of native fishes.Pumping in Ash Meadows nearly led to the lossof springs in the 1970’s, but the first interven-tion of the Endangered Species Act of 1973 pre-vented these losses (Soltz and Naiman 1978).Continuing development of hot springs for electric power in the Great Basin also poses

questions about the persistence of spring habitats. Presently, pumping of groundwaterfrom gold mines is one of the greatest threats tospring communities. In the north-central regionof Nevada in particular, large open-pit goldmines are rapidly altering groundwater condi-tions in many areas, and many spring communi-ties there are at risk. Although relatively fewfish populations may be lost by these practices,the invertebrate faunas of the affected areas arepoorly known, thus the effects on these organ-isms cannot be determined.

On a smaller scale, the continuing develop-ment of springs for livestock by ranchers andstate and federal agencies also poses a threat tothe continued existence of spring biota. Theseactions typically involve fencing of the areaimmediately adjacent to springs, placing aspringbox over the water source, and piping most or all of the water off the site intolivestock tanks. Although some of the riparianvegetation may be retained with such practices,the essential flowing character of the spring islost, and often no exposed water remains on the


Jeff LovichU.S. Geological Survey

Biological Resources DivisionDepartment of BiologyUniversity of California

Riverside, California 92521-0427

Fig. 2. Grazing can have locally significant effects in the Mojave Desert, particularly around wateringtanks. Notice the almost complete absence of perennial plants in the immediate vicinity of the tank.Soil compaction in these areas is very high relative to undisturbed areas.



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surface. Populations of endemic species ofspringsnails have been lost under such circum-stances (G. L. Vingard, University of Nevada,Reno, personal observation), and the conse-quences for other invertebrates must also besevere.

Livestock grazing continues to pose anotherserious threat to spring communities. Heavytrampling by livestock often reduces the sub-strate to mud, can completely eliminate riparianvegetation, and alters flow characteristics.Although the magnitude of these effects on thespring biota is largely unknown, it is probablygreat because of the complete alteration of thevegetation and substrate structure.

In springs throughout the region, introduc-tions of nonindigenous organisms—particularlyfishes, snails, crayfishes, and frogs—also havehad adverse effects. Fish species have been lostin some springs, and changes in the invertebratefauna have been substantial in others (Soltz andNaiman 1978). It is difficult to assess the magni-tude of the effects from introduced nonindige-nous species on spring biota because data onnative organisms are lacking. Many populationsand species will probably be destroyed beforetheir presence has even been documented.

Development in the Las Vegas Valley

yield that would otherwise be lost to evapora-tion (Las Vegas Valley Water District 1992,1993). Although hydrologic models are used to predict steady-state groundwater flow and to ascertain the effects of groundwater with-drawals, no one knows the level of successreached by predictions of the magnitude of these effects over the long term and on aregional scale.

Developments on the Truckeeand Carson Rivers

The Truckee River originates as overflowfrom Lake Tahoe (Fig. 25) in the Sierra Nevada,flows through the Truckee Meadows (nowoccupied by the Reno-Sparks metropolitanarea), and terminates at Pyramid andWinnemucca lakes. In 1906, water was divertedfrom the Truckee River for the NewlandsProject in the Fallon area, the first project in aneffort to make the desert bloom in the UnitedStates. While an agricultural economy was cre-ated in Fallon, Pyramid Lake dropped 25meters, the endemic cui-ui became endangered,the Pyramid Lake cutthroat trout went extinct,and Winnemucca Lake completely dried shortlyafter it was established as a national wildliferefuge for waterbirds. Because of the loss of

Future losses of species and decreased bio-logical diversity attributable to water diversionsseem inevitable in light of estimated humanpopulation growth throughout the GreatBasin–Mojave Desert region and particularly in Clark County, southern Nevada. Water sup-plies in Clark County today include theColorado River (300,000 acre-feet or 365.9 million cubic meters per year), groundwater inLas Vegas Valley (35,000 acre-feet or 42.7 million cubic meters per year net), and waste-water reuse. Forecasts indicate that at currentrates of use, existing supplies will meet localneeds until the year 2013; however, by the year 2020, the population of Clark County isexpected to increase by 63%, from 919,388 in1993 to an estimated 1,450,409 (Clark County1994). To meet the water needs of this popula-tion, the Las Vegas Valley Water District filedapplications to obtain surface water from the Virgin River and groundwater diversionsfrom approximately 20 basins (Table 7); thisincludes all of the unappropriated perennial

Winnemucca Lake, the Stillwater NationalWildlife Refuge near Fallon became one of theonly resources for migrating shorebirds in thearea. The Stillwater marshes were naturally fedby the Carson River, but most of those watersare also diverted for irrigation. Added to thesecomplications are the demands on the TruckeeRiver from the rapidly growing Reno–Sparksarea. These demands conflict with those of thePyramid Lake Paiute tribe, the Fallon farmers,and the Stillwater refuge.

Heroic water importation schemes to solveReno’s insatiable thirst have included draininggroundwater from central and eastern Nevadamines, sending this water down the HumboldtRiver, and pumping it to Reno. Pumpinggroundwater from Honey Lake Valley in northeastern California for delivery to Renowas also proposed. Estimates on the yield of theaquifer were made by using the rate of wateruse by the greasewood in Honey Lake Valley (L. Crowe, Air Quality Management, County of Washoe, Nevada, personal communication).Water used by greasewood was consideredwasted, but this water could support additionaldevelopment in Reno. Yet, that water supportsnot only greasewood, but an entire natural community that includes at least six species of scale insects from four families (D. R. Miller, Agricultural Research Service,U.S. Department of Agriculture, Logan,Utah, personal communication) and several

Water sources Acre-feet per year

Currently consumedColorado River 300,000Groundwater (from Las Vegas Valley basin) 35,000Total consumed 335,000

In permit applicationVirgin River 125,000Groundwater (from approximately 20 basins) 180,000Total in permit application 305,000 (376,070,000)




(Cubic meters per year)

Table 7. Sources and amounts ofwater currently consumed byClark County and those sources in the permit application processwith the Nevada Division of WaterResources (Las Vegas Valley WaterDistrict 1992; Clark County 1994;Nevada Division of WaterResources 1995, personal commu-nication).

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desert rodents (W. Longland, AgriculturalResearch Service, Reno, Nevada, personal communication).

Walker River Basinand Walker Lake

The Walker River basin is a medium-sizeddrainage in eastern California and westernNevada. The eastern and western forks of theWalker River originate on the eastern slope ofthe Sierra Nevada in California, flow intoNevada via the Smith and Mason valleys,merge, and eventually terminate in Walker Lake(Fig. 26). During the last 100 years or so,upstream water diversions for irrigation createda vigorous agricultural economy in the Smithand Mason valleys, but these diversions alsodiminished flows into Walker Lake. The lake’slevel has dropped considerably, and the concen-tration of total dissolved solids has increased tothe extent that the lake will not be able to support fishes much longer (Koch et al. 1979;Horne et al. 1995; Stockwell, unpublished manuscript).

One hundred twenty percent of the averageflow in the Walker River is allocated to upstreamuses, primarily for agriculture in Smith andMason valleys (California Department of Water

water probably will not become available in thebasin unless it consistently receives much high-er than average precipitation, various waterredistribution schemes have been proposed bygroups intent on preserving the current WalkerLake community. These schemes include thepurchase of water rights from willing sellers,voluntary contributions to instream flow, ormore draconian measures such as reallocation.However, the economy of the Smith and Mason

Fig. 25. Lake Tahoe, Nevada andCalifornia. Overflow from LakeTahoe is the primary source of theTruckee River.

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of N



Resources 1992). Thus, a runoff of 120% of normal—or about 420,000 acre-feet (512.2 mil-lion cubic meters)—is necessary for the rightedallocations of all upstream users. Under droughtconditions, flows are about 40%–60% of average,and only negligible amounts of water reachedWalker Lake from 1986 to 1994. Under theseconditions, the total dissolved solids in the lakewill soon reach a level that will shift a fish-dominated community to one dominated byinvertebrates (Stockwell, unpublished manu-script). One effect of this shift will be the disap-pearance of the fish-eating birds that depend onthe lake’s resources during migration; anotherwill be the loss of a major recreational fishery thatis important to the economy of Hawthorne, atown at the southern end of the lake.

The only feasible way to forestall thesechanges is to increase flows into Walker Lake.If 80,000–90,000 acre-feet (97.6–109.8 millioncubic meters) of water were to reach the lakeannually, the total dissolved solids would fluc-tuate around the present level, which is margin-al for fish life. An annual inflow of more than109.8 million cubic meters would result in agradual reduction of the total dissolved solids(Stockwell, unpublished manuscript). However,providing about another 100,000 acre-feet (122million cubic meters) of water annually forWalker Lake would require annual flows of520,000 acre-feet (634.2 million cubic meters),or 157% of average. Because this amount of

valleys would probably suffer if substantialamounts of irrigation water were diverted intothe lake.

Fig. 26. Walker Lake, Nevada,viewed from the summit of Mount Grant. ©

D. A

. Cha




ity o

f Nev


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534 Status and Trends of the Nation’s Biological Resources — Volume 2

Furthermore, irrigation in these valleys mayhave created habitats that also support impor-tant elements of biological diversity. Upstreamriparian and wetland habitats originally coveredonly a fraction of the land area in the Smith andMason valleys, but they expanded considerablyunder irrigation. At present, modern irrigationpractices (such as replacement of the originalearthen ditches by concrete-lined ditches) andthe recent drought have resulted in the degrada-tion or loss of riparian segments. The extent towhich these habitats support important ele-ments of biological diversity must be quantifiedso that the overall effects of potential waterredistribution can be predicted.

A second potential effect of water redistrib-ution on upstream biological diversity may bethe conversion of former grazing or agriculturallands into residential areas. If irrigation is nolonger possible, the most profitable course for alandowner is to sell the land to a developer.Residential development creates habitat typesthat are not used by most native species, andpets (particularly house cats) have a substantialnegative effect on bird, reptile, and small mam-mal populations. If the density of houses in adevelopment is sufficiently high, most nativeanimals disappear altogether.

The Walker River basin serves as an example


Peter F. BrussardDavid A. Charlet*

Department of Biologyand Biological Resources

Research CenterUniversity of NevadaReno, Nevada 89557

David S. DobkinHigh Desert Ecological Institute

15 SW Colorado, Suite 300Bend, Oregon 97702

StateU.S. Forest ServiceTribal landsU.S. Bureau of Land Management Department of DefenseLakesPrivate

Fig. 27. Land ownership in the Walker River basin,California and Nevada.

for many others facing resource managementdilemmas in the arid West. The land in theWalker River drainage basin has a variety ofpublic and private ownership (Fig. 27), includ-ing two states with their respective fish andwildlife agencies, the U.S. Forest Service, theBureau of Land Management, the Departmentof Defense, private landowners, and an Indianreservation. Resident organisms include athreatened species (the Lahontan cutthroattrout) and many migratory birds, and all vegeta-tion zones of the Great Basin are represented inthe Walker River basin. Will this area becomethe focus of yet another conflict over land andwater use or an example of cooperative ecolog-ical restoration?

ConclusionsBiological diversity in the Great Basin and

Mojave Desert region is concentrated in rem-nant waters, montane islands, and specializedhabitats. Throughout the region, there is muchendemism, mostly at the subspecies but also atthe species level. However, the human popula-tion in Nevada is growing at one of the fastestrates in the nation (6.7% annually, which repre-sents a doubling time of 10 years), and the portions of the region in adjacent states aregrowing nearly as fast. Most people who moveinto the region are from nondesert areas and donot understand the fragile ecology of the desertand the value of its biological heritage.

In addition to being subjected to the new,severe effects brought on by population growth,the biota of the region already has been severe-ly harmed by water development, mining, graz-ing, and the introduction of nonindigenousspecies. Many rare species are at risk in theGreat Basin–Mojave Desert region, but evencommon species are now imperiled by humanenterprises. All of these factors profoundly altercommunity structure, function, and integrity inmany ways. In the face of these acceleratingchanges, it is difficult to be optimistic. Unlessmajor changes are made in the interaction ofpeople with natural communities in thisregion—one of the last large expanses of wildland in the nation—hope for the retention of thenatural character and important ecological roleof the Great Basin–Mojave Desert is small.


We gratefully acknowledge the assistance of the following individuals and their institu-tions: G. Clemmer, K. Cooper, and J. Morefieldof the Nevada Natural Heritage Program,Carson City; R. Cole, Hart Mountain NationalWildlife Refuge, Oregon; S. Bassett, T.Edwards, and C. Homer, U.S. GeologicalSurvey, Utah Cooperative Fish and WildlifeResearch Unit, Department of Fisheries andWildlife, Utah State University, Logan; R.Hamlin, U.S. Fish and Wildlife Service, Nevada

Contributing authors

Lianne C. BallKathleen A. Bishop

Hugh B. BrittenErica FleishmanScott A. Fleury

Tom Jenni*Tom B. Kennedy

Christine O. MullenMary M. Peacock

Don PrussoMichael Reed

Lynn RileyRichard W. Rust

Janice L. SimpkinGary Vinyard

Ulla G. YandellBiological Resources Research

CenterUniversity of NevadaReno, Nevada 89557

Ron MarlowDepartment of BiologyUniversity of NevadaReno, Nevada 89557

*(currently employed by NevadaNatural Heritage Program

Carson City, Nevada)

*current address:Community College of

Southern NevadaDepartment of Science S2B3200 East Cheyenne AvenueNorth Las Vegas, NV 89030

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Regional Trends of Biological Resources — Great Basin 535

State Office, Reno; J. Kartesz, Department ofBiology, University of North Carolina, ChapelHill; A. Launner and J. Reaser, Center forConservation Biology, Stanford University,Stanford, California; Q. Ly and M. Rahn,Biological Resources Research Center,University of Nevada, Reno; T. Charlet,Department of Biochemistry, University ofNevada, Reno; K. Geluso, Department ofBiology, University of Nevada, Reno; J. Nachlinger, The Nature Conservancy,Northern Nevada Project Office, Reno; P. Wigand and M. Rose, Quaternary Sciences,Desert Research Institute, Reno, Nevada; G. Stephens, Idaho Conservation Data Center,Idaho Department of Fish and Game, Boise;

S. Smith, Department of Biology, University of Nevada, Las Vegas; and J. Young,U.S. Department of Agriculture, AgriculturalResearch Service, Reno, Nevada. The text was improved by the comments of four anonymous reviewers. Research at HartMountain and Sheldon National WildlifeRefuges was supported by the U.S. Fish and Wildlife Service and by the High Desert Ecological Research Institute of Bend, Oregon. Research in Nevada was supported by the Nevada Biodiversity Initiative and the Biological ResourcesResearch Center and the Nevada AgriculturalExperiment Station, University of Nevada,Reno.

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