1.1 WATER AND WILDLIFE IN THE ARID WEST - AN OVERVIEW
|Many of our western regions are classified as “desert”
habitat. A desert is characterized by low or erratic precipitation levels and
by highly variable temperatures. Western deserts extend from "southeastern
and southern Idaho through
Nevada and Utah, except at higher
elevations, continuing south through southern California and Arizona, and eastward through
central and southern New Mexico"
Deserts exhibit a variety of habitat types, from the relatively homogenous stands of sagebrush in the Great Basin Desert to the highly diverse and structurally rich vegetation observed in the Sonoran Desert. Habitat diversity, with an attendant disparity in species diversity, exists largely due to precipitation patterns and temperature regimes. The structural simplicity of the Great Basin Desert is representative of a short growing season, low precipitation, and varied precipitation patterns. Approximately 60% of the precipitation in the Desert falls as snow. The Sonoran Desert and other southern deserts often exhibit a much greater floral and faunal diversity due to the extended and often year-round growing season. These deserts experience biannual precipitation patterns - with most of the annual precipitation arriving as rain.
Although the Great Basin Desert region supports fewer wildlife species overall than the warmer southern deserts, certain groups of wildlife species (large native ungulates in particular) are more numerous in northern desert habitats. The colder climate’s influence is also expressed through shorter growing seasons and a reduced diversity and availability of insect prey. This is further reflected through less diversity in cold desert passerines (small birds), small mammals, reptiles, and amphibians.
As noted by Jones (1986), desert habitats possess some of the most unusual wildlife in North America. Many of these species are adapted physiologically or morphologically to survive under extreme environmental conditions - including low or infrequent levels of precipitation and highly variable temperatures. A number of small mammals and reptiles require no free water and create their own metabolic “water” sources from forage or prey consumed. Others survive through a variety of conservation strategies such as nocturnal behavior. Some species, such as birds and bats, may be able to travel long distances to obtain water required for drinking or bathing purposes. For example, Townsend’s Big-eared Bats (Corynorhinus townsendii) have been observed foraging in pinyon-juniper habitats in northeastern Nevada up to 25 miles from known water sources (Bradley 1999).
Other desert wildlife species, including cold desert or Great Basin Desert wildlife, require access to free water. For many species, reliable free water is a critical factor to survival and for reproductive success. Many species, without the presence of free water, would be severely limited in range or distribution, would have limited reproductive success (if any), or would simply be unable to exist within a particular geographic area. For example, ungulate species such as the Pronghorn require free, permanent water sources located at less than 5-mile (8-km) intervals (Jones 1986).
Desert riparian habitats (riparian habitats are those areas where vegetation is influenced by the presence of water) and any associated aquatic desert habitats are invaluable to many species of wildlife. The microhabitats represented by small lotic (stream) and lentic (lake, pond, wetland, or seep) waters are mandatory components in the life cycles of all amphibians; are required by many reptiles; and may be critical for riparian obligate or dependent birds and mammals. While invertebrates are rarely considered by land managers or the public at large, habitats associated with water in the desert are also critical to many obligate or dependent species of insects - that in turn help to ensure the survival of other floral or faunal species.
Permanent running waters (lotic systems) support aquatic wildlife including insects, fish, amphibians, and some reptiles. Aquatic species rely on the running water to provide for basic physiological functions such as thermoregulation, water balance, escape cover, and for food sources. Lotic systems also provide food resources (prey) for many other wildlife species such as raptors and other predators, browse and forage for herbivores, insects for insectivorous wildlife, and provides drinking water for a wide variety of terrestrial species.
Permanent standing (lentic) water systems are also very important to a wide variety of desert wildlife. Certain species of lentic fish, such as pupfish, can only survive within the habitats provided by cienagas, springs, bogs, or potholes. Amphibians are totally dependent upon lentic desert waters in the absence of lotic systems. Amphibians and other species may rely upon lentic habitats for one or more of the following needs: reproduction, food, escape cover, and for physiologic processes- such as thermoregulation, water regulation, and developmental stages of young (including the eggs and tadpoles of frogs and toads). As with lotic systems, lentic systems also supply food resources (prey) for other wildlife species such as raptors and other predators, browse and forage for herbivores, insects for insectivorous wildlife, and similarly provides drinking water for a wide variety of terrestrial species.
Temporary lentic waters are also very important for wildlife species, and are common throughout U.S. deserts following summer precipitation events. Water may pool above clay soils that are relatively impervious, may be found in rock depressions, or may be found as temporary ponds among the boulders of canyons and washes. Although these waters may exist only briefly they are important to insects, amphibians (such as toads and salamanders), reptiles, waterfowl and other birds, and even to larger wildlife such as big-horned sheep. These temporary water sources can become especially important during hot summer months.
Table 1.1* summarizes the importance of types of water resources to major vertebrate (wildlife) associations found within desert habitats:
The importance of riparian systems (those areas with vegetation influenced by the presence of water) is further illustrated by the following information condensed from Jones (1986):
Additional points to consider regarding the importance of riparian zones include:
Although natural water sources are critical within desert habitats, many anthropogenic impacts have threatened (and continue to threaten) natural water sources relied upon by western wildlife. It has been estimated that 70-90% of natural riparian ecosystems had already been lost to human activities by 1986 (Ohmart and Anderson 1986). Estimates for the Rocky Mountains/Great Plains region were that 90-95% of our natural riparian systems had been lost. In 1986, over 80% of the remaining riparian habitats on public lands were considered to be in unsatisfactory condition. According to national agency reports there has been little change, despite national directives to properly manage or restore riparian-wetland habitats.
The loss of riparian habitat is a major factor contributing to the decline of native plant and animal populations and to a loss of species diversity throughout the west. Indeed, a majority of North American threatened and endangered species rely upon or require riparian systems for survival. Critical values represented by desert riparian (both lotic and lentic) systems are no exception. It has been estimated that if our riparian ecosystems are completely lost or continue to be reduced, we could lose up to 80% or more of our wildlife species in the western U.S (Ohmart and Anderson 1986).
Many reports and articles have been authored regarding the impacts and loss of riparian and other habitat values in the west. Jones (1986) notes:
With rapid expansion of man into desert habitats, many habitat components crucial to species existence are being altered, especially those important to both man and wildlife. For example, dewatering of perennial streams and springs for domestic and livestock water has drastic effects on wildlife, especially aquatic organisms. Recreation activities have become very popular in deserts near metropolitan areas, and these activities have both direct and indirect effects on wildlife and their habitats (physical disturbance and habitat alteration, respectively). In addition, many regions, especially in the Great Basin Desert, have large coal, oil, and gas deposits. The development of these resources can have significant effects on desert habitats and their faunas…
Irrigation, urbanization, and other developments have greatly reduced natural desert habitats. Artificial urban, and agricultural habitats support fewer species than do native surrounding desert. Other land uses have reduced quality of desert habitats such as off-road vehicles, livestock grazing, and mineral development. Fire prevention and channelization of desert streams and rivers have also reduced natural characteristics of desert ecosystems.
Water developments represent a major human impact within many of our desert ecosystems. Water developments can exert a profound influence on hydrology, plant community values, wildlife populations, and wildlife habitat values. Water developments range from those created specifically for direct human consumption of water resources to those designed to provide water for livestock in areas otherwise lacking free water. Occasionally water development projects are actually designed and carried out to assist native or introduced wildlife populations. Yet in many instances around the west, water developments appear to have been carried out solely for the behalf of domestic livestock- with no apparent thought for the physiological needs or safety of area wildlife.
A discussion of rangeland water developments created largely if not wholly for the benefit of domestic livestock will follow in Section 2.1. Further discussion of water developments designed to assist native or introduced wildlife within western rangeland habitats is found in Section 2.2.
FIGURES & TABLES
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