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Biological Resources
Biodiversity and Ecoregions
Flying over Nevada's numerous mountain ranges or speeding across shrub-covered basins on the Loneliest Road in America (Highway 50), travelers will undoubtedly miss one of the state's most notable features - the enormous variety of wildlife and habitats that grace the state.
The corrugated topography and dramatic elevation changes gives rise to many distinctive climate and vegetation zones, from salt desert scrub surrounding dry lakebeds (playas) to alpine tundra with persistent snowfields. In between, lies a rich diversity of shrub, woodland, forest, grassland, and riparian zones. Botanists have found over 2800 different native plants in Nevada, 139 of which occur nowhere else.
Though predominantly arid to semi-arid, the moister, higher elevation climate zones capture sufficient amounts of snow and rain to feed numerous rivers, creeks, lakes, wetlands, and springs. Many unique native fishes, freshwater snails, birds, amphibians, and insects inhabit these widely distributed aquatic resources. Overall, Nevada hosts well over 3800 plant and animal species and some of the most biologically diverse ecoregions in North America.
Nevada is inhabited by a large number species and subspecies (i.e., taxa) that are unique to Nevada (i.e., endemic). With 309 kinds of plants and animals found in the state and nowhere else, Nevada ranks sixth in the nation for the number of endemic animal and plant species (Figure 3-1). As scientists continue to study the state's biological resources, the number of taxa will change. In particular, the number of endemic aquatic and terrestrial invertebrates is certain to increase.
Why does a predominantly arid state harbor so much biodiversity? Basically, the complex history of regional climate swings that occurred over recent millennia propelled a series of changes in the distribution and abundance of water and vegetation, as well as landform features.
As new and diverse habitat conditions formed, animals and plants were migrating, adapting, and evolving in order to survive. An important condition for species evolution is isolation. The basin and range topography, fluctuations in large ancient lakes, and vegetation zones shifting with climate changes resulted in populations of terrestrial and aquatic species becoming separated and isolated.
With 314 named mountain ranges and 232 (hydrographic) basins, the basin and range topography is the state's most prominent feature. Mountain ranges are mostly tilted fault-bounded blocks, five to 15 miles wide, with many extending more than 50 miles. Peaks and ridges typically rise 1,000 to 5,000 feet above the floors of the intervening basins, and occupy roughly 40 to 50 percent of the total land area. The basins are filled with rock and soil eroded over millions of years. Very coarse to fine grained sedimentary layers make up the valley fill deposits, which range in thickness from several hundred feet to more than 2 miles (Fiero, 1986). Elevations of larger valley bottoms vary from 500 feet above mean sea level to 6,800 feet. Twenty-five mountain ranges have at least one peak over 10,000 feet (Grayson, 1993). Nevada climbing enthusiasts report summiting 42 peaks higher than 11,000 feet (Strickland, 2002).
Periodically during the past 10,000 years, many northern Nevada basins were filled with freshwater lakes. The inundated valleys separated populations of the same species and created new habitat conditions. The largest
prehistoric lake within the state was Lake Lahontan, which at its peak inundated about 8,600 square miles in the Humboldt, Truckee, Black Rock, Carson, and Walker basins. Remnant features of the wetter, cooler periods, the last of which ended 4,000 years ago, are found in desert "sinks" distributed throughout northern Nevada (Grayson, 1993). Relict landform features include terminal lakes, playas, and wetland complexes ringed by ancient wave cut terraces. Pyramid and Walker lakes are the lone survivors of ancient Lake Lahontan. These rare relict lakes are fed by snowmelt from the Sierra Nevada Range, which ironically captures so much moisture from Pacific storms that an enormous rain shadow is cast across the state.
The relatively recent and rapid climate transition from wetter and cooler conditions to drier and warmer brought about region-wide changes in the distribution and abundance of plant species and community types. Conifer forests withdrew into the mountains, replaced by pinyon and juniper woodlands and expanding shrubs and grasses. As water bodies receded and groundwater recharge declined, wetlands and riparian zones contracted. Ultimately, the climate changes and the highly segmented landscape provided new, unique, and isolated habitats in which aquatic and terrestrial species adapted and evolved. Thus, the ecosystems in which we live are the recent product of a dynamic period in the state's natural history. Nevada consists of four major ecosystem units, or ecoregions - the Great Basin, Mojave Desert, Columbia Plateau, and Sierra Nevada (Figure 3-2).  Most of the state's population resides in the Great Basin and Mojave Desert ecoregions.
The Great Basin covers about 48 million acres (68% of the state). Roughly two-thirds of the ecoregion falls within Nevada's borders, with the remainder in Utah and California. Of 110 ecoregions in North America, the Great Basin ranks fifth in total species richness and second in diversity of imperiled species (The Nature Conservancy, 2000). Valley bottoms in the Great Basin sit at higher elevations and more northerly latitudes than the Mojave Desert; thus, the climate is cooler, moister, and vegetation grows thicker.
Salt tolerant shrubs and playas prevail in the lower valleys. Expanses of sagebrush and other shrub communities cover most of the higher valleys and slopes, occasionally mixed with grasses, especially at higher elevations. Pinyon and juniper, or pygmy conifer, woodlands occupy large portions of lower elevation mountain slopes and ranges. Conifer and hardwood forest occur in widely dispersed patches. Major rivers are limited to the northern and western
extremities. Numerous perennial and ephemeral creeks drain higher elevation ranges. Thousands of springs dot valleys throughout the Great Basin. Almost all precipitation falls during winter, with temperatures cold enough to bring more snow than rain. Warm springs and hot summers hasten snowmelt from the mountains and quickly evaporate the moisture in upland soils. Gradually, stream flow dwindles to a low flow or dry state by late summer.
The Mojave Desert covers the state's southern tip, and extends into California, Utah, and Arizona. Compared to the Great Basin, Mojavean valleys are broader and mountain ranges fewer. Vegetation is widely spaced on the hot, dry valley floors and slopes. Cacti and Mojave yucca are abundant at lower elevations, cohabitating with white bursage. Higher desert vegetation zones are identified
by blackbrush, creosote bush, and shadscale. Joshua trees and perennial grasses occur in higher shrub-dominated valleys. Mid-level mountain elevations support pinyon and juniper in several ranges. Forested mountain areas of pine and fir have a limited, high elevation distribution (Utah State University, 1996).
On average, less than five inches of rain falls in the winter and during the summer monsoon season, but higher elevations often receive several feet of snow. Extensive water bearing carbonate rock formations contribute flow to some perennial stream reaches and numerous springs, a number of which are inhabited by rare fishes and snails.
The Colorado River flows through the eastern portion of the ecoregion. Other important streams are the Amargosa, Muddy, Virgin, Meadow Valley, and White rivers. Desert tortoise, Amargosa toad, Mojave yucca, and Joshua trees are distinctive life forms in this ecoregion. Over 1.4 million people inhabit Las Vegas Valley, which lies centrally in the Nevada portion of the Mojave Ecoregion. Urban development, outdoor recreation, military uses, and large reservoirs are major land uses.
The southern portion of the Columbia Plateau ecoregion stretches across northern Nevada, and extends into Idaho, Oregon, Washington. In Nevada, landforms are a mix of basin and range and volcanic plateau features, with inclusions of low lying alkaline lakebeds in the westernmost portion. A variety of sagebrush and perennial grass, or sagebrush steppe, communities prevail as the dominant vegetation type. Salt desert scrub and pinyon woodlands are scarce in the cooler climate, which favors juniper woodlands and mountain mahogany.
Rocky Mountain type subalpine conifer and aspen forest patches occur at higher elevations of the volcanic highlands and mountain ranges. Higher average annual precipitation sustains many small perennial streams that flow northward to tributaries of the Snake River in Idaho. The valleys are semi-arid, although irrigated pastures makes up a greater portion of the vegetative cover than elsewhere in Nevada. Livestock grazing, irrigated pasture, big game habitat management, and hunting and fishing are major land uses. Towns are small and remote, sustained by the agricultural- and outdoor recreation-based economy.
In contrast, fast growing cities and towns are clustered along the margin of the Sierra Nevada ecoregion. Moderately well forested, the steep granite slopes along the eastern edge of the Sierra Nevada Range is the source of numerous mountain streams. The Eastern Sierra and Carson Range watersheds feed Lake Tahoe and three major rivers that yield a substantial amount of water for farming, urban and industrial development, water-based recreation, and desert lakes and wetlands. Eastern Sierran mixedconifer forest and mountain shrub communities are accustomed to a milder climate pattern and thus have a limited presence eastward. Only small patches of Sierran plant communities occupy the most favorable locations in adjacent mountains of the Great Basin ecoregion. Commercial logging, ranching, and forest/range wildlife habitats are being replaced by urban and suburban development, outdoor recreation facilities and trails, and tourism along the eastern Sierra Front.
A few important generalizations can be made about the ecoregions in Nevada. Compared to the Columbia Plateau and Sierra Nevada ecoregions, natural plant communities of the Great Basin and Mojave Desert ecoregions appear to be less resilient and slower to recover from intensive land use and natural disturbance.
Dispersal of noxious weeds and cheatgrass appears to be a more significant problem in the Great Basin and Columbia Plateau ecoregions than the others, although red brome continues to invade Mojavean shrub communities. A majority of the more than 2.3 million acres that burned during 1999 and 2000 were from wildfires located in the Great Basin ecoregion. The environmental and habitat impacts of urbanization are most evident in biologically diverse areas of the Sierra Nevada, western Great Basin, and Mojave Desert ecoregions. However, in all ecoregions, intensive agricultural, mining, past logging, and outdoor recreation land uses as well as uniform suppression of fires have, to varying degrees, contributed to widespread, significant ecological changes in rangeland, forest, aquatic, and riparian zones.
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