Ecoregion Spotlight: Sonoran Ecoregion

By Mike Giusti

 Sonoran Eco RegionThe Desert Province is comprised of six ecoregions, Colorado, Mono, Mojave, Southeastern Great Basin, Southern California Mountains and Valleys and the Sonoran.  This article is focusing on one of the smaller of these ecoregions, the Sonoran.  The Sonoran Ecoregion is located in southeastern California and is bordered on its eastern side by the Colorado River. The ecoregion extends approximately 60 miles to the north, south and west of Blythe.

Although not very large in size, this ecoregion supports a wide variety of habitats and species.  As described in the Manual of California Vegetation (MCV), there are twelve macrogroups or habitat types within the Sonoran ecoregion.  These are California Annual and Perennial Sonoran CWHR TypesGrasslands, Great Basin Saltbush Scrub, Inter-Mountain Dry Shrubland and Grassland, Mojave-Sonoran Semi-Desert Scrub, North American Pacific Coastal Salt Marsh, North American Warm Semi-Desert Cliff, Scree and Rock Vegetation, Southwestern North American Riparian Flooded and Swamp Forest, Warm Semi-Desert/Mediterranean Alkali-Saline Wetland, Western North American Freshwater Marsh, North American Warm-Desert Xero-Riparian, and Western North American Warm Temperate Ruderal Flooded & Swamp Forest.

Photo credit: Magdelena Rodriguez Creosote desert scrub habitat

Photo credit: Magdelena Rodriguez
Creosote desert scrub habitat

For the SWAP update, we focused on how three indicator species – desert tortoise, Mojave ground squirrel and Nelson’s desert bighorn sheep – use the different macrogroups or habitats, and how  these associations could help inform the selection of the target habitats for the Mojave, Colorado and Sonoran ecoregions.  For the Sonoran ecoregion, the target we selected was the Mojave-Sonoran Semi-Desert Scrub, commonly referred to as creosote desert scrub.  While this is the target for the Sonoran Ecoregion, our planning efforts are applicable to most of the other ecoregions in the Desert Province.

Likewise, targets selected for the Mojave ecoregion (alkali desert scrub) and the Colorado ecoregion (microphyll riparian woodland/desert wash) can be applied to the Mojave with minor modifications. The creosote desert scrub is an upland desert scrub found on hill slopes and alluvial fans throughout the arid Southwest where winter temperatures are not as cold as in the Great Basin Desert and summer temperatures are very hot.  The Mojave has frost and occasional winter snows whereas the Sonoran rarely has any frost. Also, the warmer Sonoran desert tends to have more summer rain, and more distinctive emergent arborescent species, such as saguaro, and ocotillo; the Mojave is cooler with fewer large cacti and large thorny trees,  and hosts species including the prominent Joshua trees and other Yucca species.

Creosotebush (Larrea tridentata) characterizes this macrogroup as the name indicates. Ocotillo, Joshua-tree, saguaro are also diagnostic species of the macrogroup, but are localized within the distribution of the macrogroup forming a sub-classification in the MCV hierarchy.  Other widespread diagnostic shrubs include brittlebush (Encelia farinosa), and Burrobush (Ambrosia dumosa), among many others.   The perennial desert grasses such as big galletta (Pleuraphis rigida), and desert needle grass (Stipa speciosa) also are considered part of this macrogroup.

There are several Species of Greatest Conservation Need (SGCN) associated with the Sonoran Ecoregion and the Desert Province.  In addition to the desert tortoise, Mojave ground squirrel and bighorn sheep there are Large-billed savannah sparrow, Couch’s spadefoot, tricolored blackbird, short-eared owl, long-eared owl, burrowing owl, Swainson’s hawk, northern harrier, yellow warbler, willow flycatcher, loggerhead shrike, vermillion flycatcher, least Bell’s vireo, pronghorn antelope, pallid bat, western mastiff bat, California leaf-nosed bat, Mojave River vole, American badger, red diamond rattlesnake, flat-tail horned lizard, California legless lizard, Mojave fringe-toed lizard, golden eagles and Gila monsters.  Although the gila monster is very rare, they are occasionally found in the California desert.

Photo credit: Magdelena Rodriguez Desert Tortoise

Photo credit: Magdelena Rodriguez
Desert Tortoise

The creosote desert scrub macrogroup is usually found mixed with other habitat types although it may appear they are in distinct areas.   The soils are such that they provide suitable burrowing habitat for desert tortoise and Mojave ground squirrels and with adequate rainfall, they can develop lush vegetation that supports many of the other species.  Even the dry grasses during the summer can provide valuable nutrition for many of the species.

Photo credit: Phil Leitner Mojave Ground Squirrel

Photo credit: Phil Leitner
Mojave Ground Squirrel

There are several major threats that can affect the Mojave-Sonoran Semi-Desert Scrub   macrogroup. The use of off-road vehicles is a significant issue.  However, the biggest threat to the habitat is renewable energy.  While there are many benefits from renewable energy as an adaptation strategy for reducing green-house gases and therefore favorable for reducing the effects of climate change, the loss of tens of thousands of acres of habitat will alter the landscape of the Sonoran Ecoregion and could significantly affect the SGCN’s occurring here, many of which are endemic to the province.

Photo credit: Milo Rivera Bighorn Sheep

Photo credit: Milo Rivera
Bighorn Sheep

In addition to removal of habitat, there is a high likelihood that the loss of connectivity would result in disruption of gene flow for many species. Desert vegetation is also known for its slow recovery after disturbance often taking 100 years or more to recover the structure, function and composition present pre-disturbance. Consequently, the impacts from the loss of habitat due to rapid development of renewable energy are expected to be multifaceted and long lasting.

Under SWAP 2015, three main strategies were developed to address the potential impacts resulting from development of renewable energy in the desert province. These strategies propose developing a regional conservation strategy, establishing co-management partnership for monitoring, management and advocacy for resource conservation, and providing training for department staff, other resource management agencies staff and stakeholders on best practices for designing renewable energy projects in ways that minimize potential impacts while enhancing the overall viability of the ecological health in the desert ecoregion.