Setting an ecological baseline
by Sophie S. Parker | By definition, desert ecosystems receive little rainfall − less than 10 inches (25 centimeters) annually. For much of the year, the only locally and naturally available sources of surface water in the desert are seeps and springs − places where groundwater comes to the Earth’s surface. If you are a naturalist who happens upon a desert spring on a warm day, you are fortunate indeed. You may encounter green, cool shade, aquatic organisms such as pupfish and spring snails, the right conditions for the growth of rare plants, and the potential to spot signs of wide-ranging mammals such as bighorn sheep.
Wet islands of life surrounded by vast arid expanses, desert springs are globally recognized biodiversity hotspots that support both regionally important and locally endemic species. For example, the two areas of highest local endemism in North America include Cuatro Ciénegas Biosphere Reserve in the State of Coahuila, Mexico, and Ash Meadows National Wildlife Refuge in the Mojave Desert of Nevada. Both are desert spring complexes.
Many desert springs are threatened by unsustainable groundwater pumping and water diversions. Due to the severity of this and other threats, desert springs were characterized in the scientific literature as “endangered” by William Shepard in 1993, and were recognized as being “among the most threatened ecosystems throughout the world” by Astrid Kodric-Brown and James Brown in 2007.
Due to surface water scarcity, all springs in the Mojave Desert are ecologically important. However, springs across the Mojave differ in the timing and permanence of their flow and in the plant and animal communities that they support. Some Mojave Desert springs are perennial, fed by extensive underground aquifers. These springs may be vulnerable to groundwater extraction that occurs anywhere in the aquifers supplying their water. In contrast, local springs are hydrologically isolated (“perched”), and dependent on precipitation. They may naturally dry up on a seasonal basis or be more variable in their flow.
Discriminating between perennially flowing regional springs (sensitive to groundwater pumping) and local intermittent springs (that may be less vulnerable to large-scale groundwater extraction) is of critical importance to desert land managers.
The Bureau of Land Management (BLM) manages over 10 million acres of public lands in the California deserts. These lands are governed by the Desert Renewable Energy Conservation Plan Land Use Plan Amendment (DRECP LUPA) adopted in 2016. The LUPA defines land uses and management criteria, including groundwater withdrawal rules to protect springs and other groundwater-dependent resources. There are 1,120 mapped springs in the Mojave Desert of California, and at least 300 are located on BLM land. While this may seem like a large number, spring-fed habitats are small features in a vast landscape, like stars in the night sky. Furthermore, on-the-ground surveys can be difficult to conduct in the remote locations where springs are found. This has led to a lack of information about the physical and ecological characteristics of desert springs. It has also made it difficult for land managers to prioritize groundwater protection and appropriately limit groundwater extraction.
To implement management strategies that will support the long-term sustainability of desert springs and their associated groundwater-dependent ecosystems, our public land management agencies require additional information about springs themselves and the species they support. Out of this need grew the Mojave Springs Research Project, which I have the privilege to lead. We are focused on studying the relationships between the physical and biological characteristics of springs in the Mojave Desert. Our goals are to gather information on a variety of springs, better understand the ecology of these springs, and develop easy-to-measure indicators that can be used for low cost, long-term monitoring.
We used a 2016 survey of 312 field-visited springs conducted by hydrogeologist Andy Zdon, now of Partner Engineering and Science, Inc., to select a subset of fifty-five springs for our study. These are located primarily on BLM land in the California Mojave Desert. Andy has conducted additional on-site measurements of the physical conditions of this subset of springs, and has collected water samples for the study of stable isotopes and other chemical and hydrological parameters. This should provide us with information about the origin of the water at each spring. Dr. Naomi Fraga, from the California Botanic Garden, has conducted botanical surveys around each of the fifty-five springs to record the presence of all plant species, with a special focus on wetland obligate and rare species.
Dr. Maura Palacios Mejia from UCLA has completed environmental DNA (eDNA) analyses at four of the springs – Ahn, Big Morongo, Bonanza, and Hummingbird – to detect obligate freshwater arthropod, mollusk, and fish species, and describe the biodiversity of springs.
Spatial scientist Brian Cohen, with The Nature Conservancy (TNC), is analyzing remotely sensed infrared and multispectral imagery to identify and map ground-water dependent vegetation at a higher level of accuracy and completeness than currently exists. His future objective is to verify where plants possess increased humidity to determine if differences exist in the imagery from perennial vs. intermittent springs. Bill Christian and Stephanie Dashiell, also of TNC, have provided policy analysis and partner outreach for this project.
While most of our fieldwork has been completed, analyses are still underway. Our initial findings are preliminary but already intriguing. Andy’s hydrology work establishes that the source of spring water can be ascertained using groundwater temperature, stable isotope analysis, and analysis of tritium and radiocarbon age. This information can then be used to determine if a spring is in hydraulic connection with a regional aquifer system (and therefore susceptible to impacts from regional groundwater pumping), or if it is a local spring influenced by conditions in the immediate watershed. Naomi’s botanical surveys have documented a total of 570 plant taxa at the fifty-five springs. Given that about 2,200 vascular plant taxa are known to occur within the Mojave and Sonoran deserts of California, this means that more than a quarter of all vascular plants found in the California deserts are found at this subset of springs. This is remarkable given the diminutive footprint of these groundwater-dependent ecosystems across the landscape. Maura’s eDNA work has demonstrated that each of the four springs that she sampled contained a unique biological community, and that many species observed in the field and with camera traps – including Bighorn Sheep, Bobcat, Kitfox, Loggerhead Shrike, and Red-spotted Toad − could also be detected using eDNA techniques on samples of water and sediment.
Our next step will be to look at the data collectively across spring sites and determine what factors of a spring’s physical metrics are predictive of the plants and animals observed there. This will get us closer to understanding the diversity among Mojave Spring ecosystems. Outcomes of our project will include reports of analyses from isotopic, botanical, eDNA, and remote sensing work; photographs of species and habitats from botanical surveys; presentations of methods and results provided at various local meetings, national gatherings, and international conferences; manuscripts for publication in the peer-reviewed, scientific literature; and management recommendations for land managers. We look forward to sharing many of these at www.scienceforconservation.org.
Dr. Sophie S. Parker is a Lead Scientist with the California chapter of The Nature Conservancy. She has provided science leadership and support for the Conservancy’s work in southern California for the past twelve years, and she leads the Energy Program’s science team in the Mojave and Sonoran deserts.
Kodric-Brown A, Brown JH (2007) Native fishes, exotic mammals, and the conservation of desert springs. Front Ecol Environ 5: 549–553.
Shepard WD (1993) Desert springs – both rare and endangered. Aquatic Conservation: Marine and Freshwater Ecosystems 3: 351–359.