Katie Predick, Conservation Research and Education Department
Earth Day is a time to celebrate the fascinating, biodiverse, resilient nature all around us. And while it’s essential to appreciate and celebrate the unique nature of the Sonoran Desert and beyond, every Earth Day that passes without significant climate or environmental action brings us closer to the uncertain, disruptive future impacted by continuing climate change. How will climate change impact the animal and plant residents of the Sonoran Desert? Katie Predick, Desert Museum scientist, shares what ecosystem changes likely lie ahead as the region continues to heat up and dry out.
The southwestern United States is historically the hottest and driest part of the country—and is expected to become still hotter and drier due to climate change. Plants and animals in the Sonoran Desert are adapted to endure the region’s harsh climate but many live on the edge of physiological limits for water and temperature stress. Thus, slight changes to climate can have outsized impacts on desert species.
As a result of climate change, the southwest is already experiencing hotter maximum summer temperatures and warmer fall days. Temperatures currently considered unusually high will soon become commonplace. While total annual rainfall is declining, storms have become larger and more intense. These longer periods of drought interspersed with high intensity rainstorms will paradoxically increase both droughts and floods. Streamflow and groundwater recharge are expected to decline because of reduced snowfall and increased evaporation due to hotter temperatures.
If these climate changes continue, the abundance and biodiversity of desert species will also change. Woody plants common to the Sonoran Desert, like mesquite and palo verde, are expected to decline. The iconic saguaro will likely become less abundant under drier conditions and regeneration will decline without woody “nurse plants.” The decline of native vegetation can provide drought-tolerant invasive grasses the opportunity to establish and spread. The range of invasive grasses, such as buffelgrass, is limited by freezing temperatures. As temperatures warm these species may spread to higher latitudes and elevations.
As vegetation changes, so must the animals that rely upon it. When native woody vegetation is replaced by invasive grasses, animals reliant on shrubby landscapes are supplanted by animals partial to grassland habitat. Riparian habitat is particularly vulnerable and will degrade as water availability declines and drought intolerant vegetation (like cottonwood and willow) diminishes. Desert fish, many of which are already endangered, may not survive in warmer water.
Ectotherms, such as reptiles, rely on moving among habitats to manage their body heat and will have to alter their behavior to survive higher temperatures. Eggs and pupae will be even more susceptible to heat stress because they are unable to move themselves into a cooler spot. The sex of some desert reptiles, such as the threatened desert tortoise, is determined by egg incubation temperature. Eggs incubated at higher temperatures produce more female young and population sex ratios may become unbalanced. However, populations should persist as long as some male turtles continue to hatch.
Birds and mammals are susceptible to heat stress and dehydration at high temperatures, particularly animals with limited water storage capability (such as small or young birds). Entire populations of a species may not survive when multiple risk factors exist. For example, pronghorn populations will likely be lost from vulnerable areas due to climate impacts on their food sources, gestation timing, and lactation success.
Phenological changes, or “timing mismatches” can impact many parts of the food chain by changing plant growth, flower bloom timing, insect activity, animal hibernation, and animal migration. Events that historically occurred simultaneously may no longer be “in sync” as species react differently to changing climatic conditions. Forty years of data collected at the Desert Museum and Saguaro National Park, among other studies, show that desert shrubs and flowers are blooming earlier, which could impact food sources for migratory hummingbirds.
As climate changes, the boundaries of the entire Sonoran Desert ecosystem may expand north and east as the southwest becomes warmer and more arid. Desert inhabitants will shift, as some species persist, and others leave or arrive. In a changed climate, familiar elements of the desert landscape will combine with new components to create a novel desert ecosystem unlike anything we observe today.
What can we do to preserve the desert’s diversity and resiliency in the face of change? We can help to slow the pace of change by supporting community- and individual-scale efforts to shift to greener energy, food, goods and infrastructure. This will give us, and the ecosystems in which we live, more time to adapt. Conservationists and ecologists are also experimenting with techniques to help plants and animals adapt, for example, by selecting for resilient varieties, assisting their migration to more suitable climates, or conditioning them to the stresses of future climate. Helping people adapt to these stresses is also the focus of a tremendous amount of creative innovation. For a dose of hope, explore the programs and projects at the University of Arizona’s Arizona Institute for Resilient Environments and Societies (AIRES).
For more background on climate adaptation in the Sonoran Desert, read our 2018 issue of Sonorensis.