In the semi-arid south-west of the United States of America (U.S.), there exist rare and lush wetlands that form habitat to majority of the US’s desert most spectacular biota. Basic biological concepts confirm that where there is permanency of adequate water at the surface or even a few feet below the ground, numerous diverse water dependent floral communities can develop. In this part of the US, there is the Sonoran Desert Ecosystem. The ecosystem is one of the few rarest and most exquisite habitation in the continental North America since majority of the original riparian habitats has long dried up and disappeared. The desert riparian ecosystems forms corridors of sufficient moisture and shade which are vital to the survival and establishment of virtually all wildlife species. Despite, most of their natural ecosystem contributions, majority of the riparian lands in the Arizona landscape have become precariously imperiled. For instance in the Sonoran Desert Ecosystem, the riparian areas nurture one of the most threatened and rarest forest types in North America. The forest known as the cotton-wood-willow forests have either been severely lost, degraded or damaged in the recent past. The loss of this forests has accompanied the corresponding forfeiture of the majority of the Arizona Desert wildlife that solely rely on the riparian habitation for survival.
According to Arizona-Sonora Desert Museum, the Sonora Desert Riparian Ecosystems belongs to the larger Sonora Desert Region. More specifically, the region consists of the Sonora Desert itself, the State of Sonora Mexico, and Southern Arizona northwards to the Mogollon Rim. Also included in this ecological zone is the Southeastern California, the Baja Californian Peninsula of Mexico and The Gulf of California (sec. 1). In this region, the riparian ecosystem is diverse in nature and yet share several similarities. For example, all the riparian ecosystem in this area is dependent upon the supplemental underground water from the shallow alluvial aquifers. These riparian ecosystems provide a broad range of ecological services to both the wildlife and the general ecology of this zone. These environmental modifies stabilize river banks, filter water inflow and in the process trap the excess suspended particulate matter. They also filter sediments and provide habitats for several desert animal species. Furthermore, the Sonoran desert riparian ecosystem control and modulate the hydrologic process thus maintaining the water fluxes within the ecosystem. (Patten sec. 1). The prevailing hydrologic and geomorphic processes predominantly control these changes. Water variations in form of floods, fluctuating discharges and seasonal regimes may alter the river channel characteristics. The corresponding changes in the river channel characteristics in turn modifies the extent and diversity of the riparian vegetation. The amount and intensity of the water received mainly from precipitation will to a greater extent enhance the recruitment of the riparian species and recharge the alluvial water table below the ground. The existence and presence of the different geomorphic landscapes usually determine the geographical extent (size) of the riparian zone as well as the vertical depth of the underground water tables. Such determinate features include the river valleys, the size of canyons, and the soil characteristics of the area.
Scientific information concur that differential hydrological designs are the primary driving variables in these riparian ecosystems. The hydrological setup alters both the spatial and temporal extent of the biota in these localities. In the Sonora Desert, along its elevation gradients, the riparian communities’ changes from simple, deciduous forests in one corner to mixed deciduous, coniferous and alpine wetlands in another. Variations in river channel and land terrace characteristics are the major forces causing these changes. The temporal changes in this zone occur when late successional floral communities replace early pioneering plant communities.. For example, when sagebrush and velvet mesquite (Prosopis velutina) plant communities supplants the willow and cottonwood species as the dominant plants. Sediment accumulation along the river wetlands usually stimulates such plant successions. However, in this semi-arid Southwestern part of the U.S, there is an apparent notable similarity in the ecosystem composition. These similarities are attributed to the interplay of similar hydrologic and geomorphic process throughout the area. These processes are attributed to the alteration and establishment of the existence of the Sonoran Desert riparian Ecosystem zones. However, these ecosystems have been significantly altered by human activities. A major factor in the alteration of these areas has been the modification of the natural biotic conditions. These alterations stem from changes brought about by the alterations of the stream flows through dam constructions and excessive withdrawal of ground water. Other contributing factors include animal grazing, land clearance for agriculture, watershed interferences and the introduction of non-native (exotic) wildlife species (Patten sec. 1)
Geographically, the Sonoran Desert Ecosystem is a vast desert territory encompassing an approximate surface area of 260,000 square km (100,387 square mi) (National Park Service sec. 2). The ecosystem has both a latitudinal and longitudinal extent ranges of 25º to 33º North and 105º to 110º West respectively (Daniel, 2003 par. 4). Surrounded by mountain ranges, the region experiences a primarily continental type of climate with pronounced changeability in both diurnal and seasonal temperatures (McGinnies 1981 as cited in Sonoran Desert, 2002 par. 2). These temperatures vary with both elevation and latitude, decreasing as one goes up in altitude or further north in latitude. However, the continental effect on the climate of this region is marginally modified around the Sea of Cortez. Here, atmospheric temperatures fluctuate from 32 to 36 degrees Fahrenheit while the ground level temperatures can reach as high as 100 degrees within 24 hours (Sonora Desertn, 2002, par. 3). Despite being a desert, the Sonoran ecosystem receives most precipitation in the form of rainfall than any other desert landscape in the world. According to Chadwick, the Sonoran Desert Ecosystem receives annual average rainfall of about 10-14 inches. The high rainfall is attributed to its proximity to the western edge of the Pacific Ocean (Daniel, 2003 par.3)
Classified as BWh under the Köppen climate classification system, further information reveals that the weather and climatic patterns of this region vary considerably with a remarkable diurnal temperature and sunshine changes. Much of the region receives sunlight for up to 310 days per year (Dunbier 1968 as cited in Sonoran Desert, 2002 par. 1). The periodic seasonal temperatures range in this area ranges from an approximate average of 52º F in the winter (cold season) to 86º F in the summer (hot season). However, this thermal changes are dynamic and changes from one period to another and from one place in the desert to another. For instance, some desert regions are known to have recorded temperatures to the tune of 32º F at night. However, in some portions of the desert shade temperature can reach as high as 134º F in the shade. Such temperatures have been documented near the tip of Mexico (Daniel 2003 par. 1). The rainfall patterns in this ecosystem reveals a bimodal rainfall regime that is, the area receives rainfall in both the winter and summer seasons. However, this rainfall pattern is highly inconstant and includes periodic downpour from tropical storms (Sonoran Desert, 2002, par. 4).
According to the National Park Service, the Sonoran Desert landscape is unruffled with rocks of sedimentary, metamorphic, and igneous origin. However, the structural composition of these basement rocks vary greatly according to age (sec. 4). It is made up of older sedimentary rocks formed in the Precambrian era around 2 billion years ago to relatively recent igneous outcroppings (ca. 700 A.D.). Ancient historical data reveals that numerous tectonic activities that occurred between 20 and 40 million years ago resulted in the modern day physical features underlying the area. However, the region’s characteristic Basin and Range topography was formed as a result of the plate displacements in the area (National Park Service sec. 4). Together with the bimodal precipitation regime, the mid-continental positioning, and the seasonal variations in temperature, the topographical variation in this area produces the diversity of this ecosystem.
The Sonoran Desert ecosystem is an ecologically rich biome area. It practically represents all the global biome zones. Due to its incredible landscape diversity, the Sonoran Desert Ecosystem captures all the facets of the topo-edaphic and climatic variables and the ecological disturbance regimes that accompany such ecosystem natural establishment designs literally. It is a summation of the course ecological units based on their varied life forms, phenology, and the general physiognomy of the dominant vegetation. In this region, vegetation is an expedient descriptor of the biological communities as it is immobile and integrates all the components of the abiotic site conditions. The floral life forms, therefore, include the phreatophytic desert shrubs, thorn scrubs, and the Madrean Evergreen Woodlands. It further consists of the temperate forests and the interior chaparral vegetation. Briefly, these vegetation types are made up of diverse plant species each with specialized adaptive and productive characteristics. Firstly, in this thirstiest biome, the life form diversity is high representing all the three plant photosynthetic pathways. The C3, C4, and CAM photosynthetic plants in this biome characteristically reveal microphyllous leaf phenology. They include acacias (Acacia spp.), yucca (Yucca spp.), and paloverdes (Cercidium spp.). Secondly, the thorn scrub plant communities are composed primarily of small trees such as cat law acacia and acacia greglii. Thirdly, the grasses in the Sonoran Desert are predominantly the perennial short and mid-grass species. These grass species exhibit the C4 photosynthetic pathway as an adaptive mechanism of water storage. Lastly, the temperate forests, the interior chaparrals, and the Evergreen woodlands occupy the marginal fringes of this ecosystem.
Formerly defined as a BWh climate type, the Sonoran riparian ecosystems support a broad range of biodiversity, acts as natural water filters, cycle nutrients and replenish the ground water supply. In this classification, B stands for a dry climate while BW represents arid climates with annual precipitation usually less than 15 inches (40 cm.). On the other hand, h stands for a dry and hot climate with an average annual temperature of over 65° F. Additionally, these riparian zones receive surface runoff and overflow from which excess sediments, nutrients, and anthropogenic pollutants are filtered out. They also help reduce erosion by controlling the seasonal river floods. However, these wetlands and riparian spaces are vibrant habitats which are fronting rapid loss and dilapidation. These destructions are due to uncontrolled human activities for instance, river damming have caused several hundred mile of riparian corridors to dry up in this area. Furthermore, unlicensed logging, unsustainable mining, and agricultural practices have concurrently drained these wetlands and increased the levels of toxicity chemical build-up and changing nutrient concentrations in the ecosystem.
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