Conservation of Desert Wildlife Management Areas (DWMAs) in Clark County Nevada

Tony Krzysik, Shipley Group
February 2005

Introduction

Four Desert Wildlife Management Areas (DWMAs) were selected in the northeastern Mojave Desert by U.S. Fish and Wildlife Service (USFWS) and the Desert Tortoise Recovery Team to enhance the recovery of desert tortoise populations so that they would eventually be removed from federal listing under the Endangered Species Act (ESA). The DWMAs are protected along highways or major dirt roads by low wire-mesh fencing that retains even hatchling desert tortoises (Figure 1, Interstate 95, just south of Boulder City). All of Piute-Eldorado, most of Coyote Springs and Mormon Mesa (the rest are in Lincoln County), and most of Gold Butte-Pakoon (the rest is in Arizona) are located in Clark County Nevada. It is the intent of Clark County and USFWS that the habitat in these DWMAs are conserved and managed not only for desert tortoise and other sensitive species, but also for Mojave Desert’s native biological populations, natural community landscape patterns, and ecological processes. A prime motivation is the prevention of future species or population listings under the ESA. Clark County has already prepared a Multiple Species Habitat Conservation Plan (MSHCP), an Environmental Impact Statement (EIS), and a legal mandate “Clark County Conservation of Public Land and Natural Resources Act of 2002, P.L. 107-282”. The express purpose of this act is “To establish wilderness areas, promote conservation, improve public land, and provide for high quality development in Clark County, Nevada, and for other purposes.” The purpose of this project is to identify and develop practical and scientifically valid conservation strategies to meet the conservation and management goals of Clark County for: federal and state listed species, native biodiversity and habitats, wildlife, and natural landscape patterns. Current conservation efforts will center on the four DWMAs.

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Human populations and their associated infrastructure have grown exponentially in the Las Vegas region over the last 35 years (Figures 2-4). The result has been dramatic habitat loss and fragmentation; lowering of water tables and the subsequent desiccation of surface waters, including springs and seeps; wildlife mortality from roads and pets; and direct mortality and harassment of wildlife by humans. The development of the planned Aerojet – Coyote Springs Residential Homes/Golf Courses project in the center of the Coyote Springs and Mormon Mesa DWMAs constitutes an ecological and conservation planning disaster. The spatial footprint of this project is approximately 42,000 acres, centers on the upper Muddy River watershed, and lies in both Clark and Lincoln counties. However, the environmental degradation of this project far extends its “Ecological Footprint” beyond its spatial context; because of its physical size, habitat fragmentation, extraordinary development of water resources and associated disposal of wastewater, location in a major Mojave riparian zone, producing regionally unparalleled traffic on I93, associated diverse and high impact human and pet disturbances, and severe irreparable landscape context impacts. This incredibly poorly planned development is in the landscape of two major DWMAs, the USFWS Desert Wildlife Refuge, a Wilderness Area, and ecologically important BLM lands.

Clark County’s DWMAs are ecologically vibrant and biological treasures in the Mojave Desert landscape. They are home to the substantial vertebrate, invertebrate, and plant diversity of the Mojave Desert. Interestingly, because of landscape complexity, plant community diversity, and the relatively large extent of aquatic resources, the northeastern Mojave Desert has the richest vertebrate fauna in the entire Mojave Desert (Figure 5). This analysis includes mammals, birds, reptiles, and amphibians, but not fish. Although the desert tortoise (Gopherus agassizii) is a focal species in the DWMAs, many other sensitive, rare, indicator, and ecologically relevant animal and plant species are present. Many charismatic vertebrates are found in the DWMAs: mountain lion (Felis concolor), desert bighorn sheep (Ovis canadensis nelsoni), bobcat (Lynx rufus), ringtail cat (Bassariscus astutus), Golden Eagle, Peregrine Falcon, Gila monster (Heloderma suspectum), and western diamondback rattlesnake (Crotalus atrox). The Gila monster, one of only two poisonous lizards in the world, is a beautiful rare reptile that is reclusive and not often seen  (Figure 6, taken by Jim Morgan in central Arizona). 

Not many people are aware of its presence in southern Nevada. Surprisingly, because of the native aquatic ecosystems present, including hydroriparian (perennial rivers or streams), springs, seeps, and wetlands; at least 11 native species of frogs and toad were once found in “presettlement” DWMAs. However, because of extensive water resources development including: dams, irrigation diversions, ground water pumping, draining of wetlands, and introduction of exotic species (especially fish and bullfrogs [Rana catesbeiana]), very few of these species are probably present at this time. Nevertheless, the once believed extinct relict frog (Rana onca) has recently been found once again in Clark County springs near Lake Mead.

This newsletter will focus on the ecology and landscapes of the Piute-Eldorado DWMA. 

Piute-Eldorado DWMA 

The Piute-Eldorado DWMA not only possesses the highest desert tortoise population densities in Nevada, but also represents the largest area of high quality tortoise habitat in the state. Desert tortoise populations are very patchy in distribution at Piute-Eldorado, as they typically are throughout their range, but USFWS in their 1994 recovery plan has estimated their densities at 40-90 adults per square mile. Additionally, this DWMA is contiguous with other high tortoise density DWMAs, Ivanpah and Fenner in California. Other than high habitat quality the other most important element for conservation of landscape biological viability is spatial integrity and connectivity.

The Piute-Eldorado DWMA represents one of the finest examples of Mojave Desert rolling valley and plains bajadas (ancient coalesced alluvial fans). This is one of the primary topographical features of the Southwest’s Basin and Range physiography. These broad desert plains between the mountains are very extensive, and because of their aridity and harsh climate their persistence was taken for granted. Nevertheless, they are currently under severe threat, and their continuity and habitat quality are being increasingly compromised by sprawling development and recreational OHVs (off- highway vehicles). Piute Valley, just west of Cal-Nev-Ari is an excellent example of this Mojave ecosystem (Figure 7, note Piute Range in the distance on Nevada-California border). This is a broad valley of creosote/bursage scrub, is in excellent condition, and represents outstanding desert tortoise habitat. Creosote/bursage scrub on gentle rolling bajadas is considered “classic” desert tortoise habitat. Creosote/bursage scrub habitat in Eldorado Valley is similar (Figure 8, south of Boulder City). Looking to the west, note traffic on Interstate 95 and McCullough Range in the distance. 

Creosote bush (Larrea tridentata) is the taller green shrub, and burroweed (white bursage, Ambrosia dumosa) is the smaller, rounder, and more abundant gray-stemmed shrub devoid of leaves (Figures 7 and 8). These two Mojave community dominants face the challenge of aridity in dramatically different ways. Burroweed simply sheds its bluish-green leaves (as we see in Figures 7 and 8) and “shuts-down” its physiological activities and water transpiration losses through its leaves, patiently waiting for the rainy season. Creosote bush, on the other hand, can physiologically tolerate up to 50 percent tissue water loss and still function normally. No other plant on earth has this capability. Also Creosote bush has both a deep tap-root to obtain ground water and an extensive network of shallow roots to rapidly absorb precipitation. Typically plants do not use both of these strategies to such a large extent. For example mesquite (Prosopis sp.) only has a large very deep tap-root, while cacti only use an extensive network of shallow fine roots.

The soils in Piute-Eldorado valleys are sandy to sandy loam and the broad low relief gentle bajadas are covered with braided shallow sandy washes. The vegetation is diverse and there is an abundance of burrows, including desert tortoise, kangaroo rats (Dipodomys sp.), pocket mice (Perognathus sp. and Chaetodipus sp.), and white-tailed antelope squirrels (Ammospermophilus leucurus). Burrowing Owls often “remodel” desert tortoise and other burrows for their use. Kangaroo rat burrows are typically found at the base of creosote bush (Figure 9) where the shrub canopy offers better predator protection (going into or out of burrow), favorable microclimate, or roots may offer more optimal burrow support. Possibly all three are equally important. Forbs and grasses are typically more associated with the shrub canopy (especially visually evident with creosote bush) than the open spaces between shrubs. Virtually all microbial activity and nitrogen (a scarce nutrient in desert ecosystems) are found beneath the shrub canopy. The soil in the bare areas between shrubs has very little to no nitrogen and microbial activity. The shrub canopy, therefore, is a critical habitat element in the harsh Mojave Desert and provides: a moderating microclimate (shade and reduced temperatures and evaporation), reduced heat loss to cold dry air and clear night skies, and biogeochemical processing and cycling. The shrub canopy provides the primary location for seed-foraging by rodents and birds, and insect foraging by birds and lizards. Mojave litter foraging specialists include lizards (whiptail lizards (Cnemidophorus sp.) and Leconte’s Thrasher. 

Piute Valley contains very large creosote bush clones, indicating that soils and vegetation have been stable over extensive time, possibly thousands of years, (Figure 10). This clone consists of nine individual with a total canopy diameter of 25 ft. Note dense forbs and grass cover in canopy. Creosote bush along with many other plants (including Mojave yucca [Yucca shidigera]) form circular or more typically elliptical shaped clones by the sprouting of new descendents outward from central ancestors, forming new plants as the older inner individuals perish. Therefore, all individuals forming a clone are genetically identical to their original long-deceased single ancestor. Creosote bush clones of this size could be older than 500-1000 years. Although creosote bush is one of the best physiologically adapted plants for extreme arid environments, they still grow very slowly in the harsh Mojave Desert climate.

Upland desert vegetation such as creosote bush and burroweed grow taller, wider, denser, and more vigorous along even very small washes (Figure 11). Similarly, desert vegetation adjacent to roads is typically greener and more vigorous adjacent to roads, because of increased water runoff. Compare creosote bush on the left, adjacent to a dirt road, with more typical shrubs on the right (Figure 12, Empire Wash Road, near the Coast Guard LORAN Station). The invertebrates on shrubs in these microhabitats are more abundant, larger, species rich, and support higher trophic complexity than in the surrounding uplands. In other words, food chains are longer and food webs more extensive, because there are more sap feeding and predatory insects and more spiders in these shrubs where water is more available than in the uplands, supporting greater and more predictable food resources.

Washes are very visible and very apparent in the arid landscape because they are greener (Figure 13) than the surrounding landscape (Figure 14). This broad braided wash complex is in Eldorado Valley with the Bighorn Mountains in the foreground and the Highland Range in the background. The green shrubs in the wash are cheesebush (Hymenoclea salsola), which are very abundant in Mojave dry washes, while the larger brighter green small trees in the background are catclaw acacia (Acacia greggii). Another abundant shrub characterizing Mojave gravel or rocky washes is sweetbush (Bebbia juncea) (Figure 15). This species is straw-like with tiny yellow flowers that are more apparent on close inspection (Figure 16). 

Figure 13	Figure 14
Figure 15	Figure 16

Larger washes in southern Nevada support catclaw, a large shrub or small riparian tree, which represents a critical habitat element for many ecological reasons (Figure 17). The root tangles and associated dense vegetation present under the catclaw canopy represent important microhabitat elements for a wide variety of mammals, birds, lizards, snakes, insects, and spiders (Figure 18, note the extensive network of burrows). Extensive and long-used packrat (Neotoma sp.) middens (nests) are often associated with the spiny tangles at the base of catclaw (Figure 19). This microhabitat, as in creosote bush, provides optimal microclimates, cover from predators and extreme environmental conditions, food resources, and what is rarely appreciated – WATER (see below). Individuals of catclaw can get large, and are usually heavily parasitized by desert mistletoe (Phoradendron californicum) (Figure 20; note the darker reddish clumps of mistletoe). Catclaw, like other legumes, possesses mutualistic bacteria nodules on its roots to transform abundant elemental atmospheric nitrogen into soil nitrate.

An important ecosystem process in the desert, because desert soils are very low in nitrogen compounds and plants cannot directly use atmospheric nitrogen. Mistletoe is capable of photosynthesis, but requires its host (legumes for this species) for water and nutrients. The Phainopepla is a beautiful bird and a forage specialist on mistletoe berries, subsequently providing seeds when its feces land on catclaw branches. However, in many cases the host successfully fights-back the parasitic infection by growing new tissue to cover the germinating seed, preventing development. Other Mojave birds that are fairly restricted to these ephemeral riparian washes include: Black-tailed Gnatcatcher, Verdin, and Black-throated Hummingbird. A diversity of bats also forages extensively over riparian canopy. The tree lizard (Urosaurus ornatus) can be very abundant in association with riparian zones, canyons, and the slopes above Lake Mohave. 

Figure 19

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Riparian zones that possess ephemeral surface water, nevertheless, provide water to the ecological community during drought periods. The roots of many riparian species are either within or in close proximity to the water table (e.g., acacia, mesquite, and cottonwood). Mesquite possesses extremely long tap-roots that may reach a depth of 150 ft in some cases. Herbivorous leaf and sap feeding insects feed on these water nourished leaves and stems, and eventually provide water to ALL higher trophic levels – predator insects and spiders, birds, bats, and lizards. The highest trophic levels (snakes, raptors, and carnivorous mammals) eventually utilize this ground water to the same extent as if it were surface water.

The Mojave ecosystem that contains the highest density and species richness of birds, as well as all other vertebrates is the riparian zone with perennial water flow (Figure 21). The data represent the entire Mojave Desert. Despite the many adaptations of desert vertebrates to aridity and the relatively high species richness of the Southwest desert fauna, riparian habitats with perennial water flows consistently possess the highest biodiversity, even though they represent less than one percent of the arid landscape. 

Riparian zones in eroded or montane habitats form canyons (Figure 22, Eldorado Canyon, note Lake Mohave in background). These rugged landscape elements are important for riparian vegetation, dispersal and travel corridors, rock outcrops, caves, and cliff elements. A large number of birds are associated with canyons in this area, including: Great-horned Owl, Western Screech Owl, White-throated Swift, Cliff Swallow, Poor Will, Say’s Phoebe, and Ash-throated Flycatcher. Mammals frequenting rugged canyons include: desert bighorn sheep, mountain lion, bobcat, ringtail cat, packrats, canyon mouse (Peromyscus crinitus), and a large diversity of bat species. Rock Wrens are restricted to very rocky slopes or boulder fields, and chuckwallas (Sauromalus obesus) to large rock or boulder outcrops. Cliff faces are essential habitat elements for Peregrine Falcons and Canyon Wrens. 

The washes and canyons on the eastern slope of Piute-Eldorado drain into Lake Mohave (Figure 23). Note the dense riparian vegetation, with catclaw in the foreground, while the shoreline consists of dense stands of saltcedar (Tamarix sp.). Saltcedar is an extremely invasive exotic from Asia that has replaced most of the native cottonwood/willow (Populus fremontii, Salix gooddingii, S. exigua, and others) riparian zones in the Southwest. There are a number of reasons for the success of saltcedar: absence of flood pulses (because of dams), disturbance, and cattle favor grazing on native vegetation while avoiding saltcedar. Research has shown that especially birds, but also lizards, mammals, and invertebrates prefer native cottonwood/willow and mesquite vegetation contrasted to saltcedar.
 

The Eldorado Mountains with rugged volcanic topography represent another major landscape in this DWMA (Figure 24). Much of this area approaches 4000 ft. in elevation with peaks over 5000 ft. Although not abundant, desert tortoises occur in mountain terrain throughout their range, typically under 4000 ft in elevation, but have also been found occasionally over 5000 ft. Spirit Mountain (5639 ft) is a peak in the Newberry Mountains just northwest of Laughlin, Nevada (Figure 25). Mountain ranges in Piute-Eldorado are characterized by Mojave montane scrub, with pinyon/juniper woodlands at the highest elevations, while creosote bush is not abundant. Joshua trees (Yucca brevifolia), Mojave yucca, blackbrush (Coleogyne ramosissima), Mormon tea (Ephedra nevadensis and E. viridis), Pima rhatany (Krameria erecta), turpentine broom (Thamnosma montana), shadscale (Atriplex confertifolia), and California buckwheat (Eriogonum fasciculatum) dominate the landscape. Nevada Game and Fish place guzzlers in the mountains to capture and store rain water for wildlife, especially Gambel’s Quail (Figure 26). Desert bighorn sheep, mule deer (Odocoileus hemionus), mountain lions, bobcats, and Golden Eagles are the major megafauna in southern Nevada mountains.

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Figure 26

Cacti are important habitat elements in both valleys and mountains. Birds that particularly nest in the protective spines of cholla include Cactus Wren (the largest wren) and LeConte’s Thrasher, but also Black-throated Sparrow and House Finch. Packrats and other rodents also take advantage of the spiny shelter for their nests. (Figure 27) shows a grove of teddy bear cholla (Opuntia bigelovii), a species that cannot tolerate cold weather and is more typical of the milder winters of the Sonoran Desert. Here it is found at less than 1000 ft elevation near Lake Mohave. 

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High quality Joshua tree woodland is found west of Searchlight at 4200-4400 ft. (Figure 28). The understory shrubs are primarily blackbrush, with only a few scattered, but tall and vigorous creosote bushes. There is more precipitation and humidity at this elevation and lower evaporation rates. The added habitat dimension and litter that Joshua trees provide increase avian, lizard, and invertebrate biodiversity. Scott’s Orioles, Loggerhead Shrikes, and Mockingbirds are characteristic species.  

The trunk and leaf debris of a downed Joshua tree provides excellent habitat elements for lizards and invertebrates (Figure 29). This is particularly important for three lizard species: the nocturnal and arboreal banded gecko (Coleonyx variegatus), a nocturnal litter specialist desert night lizard (Xantusia vigilis), and the litter dwelling Gilbert’s skink (Eumeces gilberti). Out of the four DWMAs, the skink is only found in Piute-Eldorado, while the other two are found in all four. This woodland is diverse, and all three northern Mojave Desert yucca species can be found in close proximity (Figure 30). The Joshua tree is the tallest, most tree-like, and has the shortest leaves. The Mojave yucca has yellowish-green long leaves and may or may not have a “trunk”, which can be up to 8 ft. in height. The banana yucca (Yucca baccata) has bluish-green long leaves and a “trunk” is absent or short. This species is more abundant to the southeast, in Arizona. The two former species are conspicuous and important landscape elements in southern Nevada, and are both abundant and characterize appropriate elevations. Packrat nests are often found at the base of Mojave yucca (Figure 31). Note that this nest is close to a road, not uncommon with packrats (Route 164 near the California border). Packrats require succulent vegetation, and interestingly, are more sensitive to desiccation than the common white lab rat.

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Figure 31