**Abstract**

The Mojave Desert in the southwestern United States is one of the hottest and driest areas of North America. Climate change is likely to exacerbate these conditions. The region is home to many endemic plant species, including 24 federally threatened species. The impact of climate change factors on these sensitive Mojave Desert species is relatively unknown. Here we used a climate change vulnerability assessment to determine which imperiled plants may be most affected by changing climatic conditions. We evaluated the vulnerability of each species under future climate scenarios and calculated scores using metrics such as exposure, sensitivity, niche breadth, and dispersal capability. We found that most listed plant species were vulnerable to climate change, with 21% (N = 5) classified as extremely vulnerable, 25% (N = 6) classified as highly vulnerable, and 42% (N = 10) classified as moderately vulnerable. Contributing factors most frequently associated with vulnerability included various barriers to migration, high habitat specificity, and species sensitivity to changes in hydrological patterns. Many of these species are already threatened by ongoing anthropogenic stressors such as urban growth and associated developments, and these results suggest that climate change will pose additional challenges for conservation and management. Natural resource managers can use the vulnerability ranking and contributing factors identified from these analyses to inform ecological decisions related to threatened plants throughout desert regions.

**Keywords:** threatened species, climate change, Mojave Desert, vulnerability assessment, conservation management, endemic species, biodiversity

## **1. Introduction**

The Mojave Desert occurs across parts of California, Nevada, Utah, and Arizona in the western region of the United States. It is the smallest and driest desert in North America, with annual precipitation varying widely across the region and an average maximum temperature of 36°C in the summer and average minimum temperature of 13°C in the winter [1]. Although it is not the planet's hottest desert, it does hold the record for the highest land surface temperature ever recorded on Earth (56.7°C), registered at Death Valley National Park in 1913 [2]. The region is ecologically and topographically diverse, with elevations ranging from to −85 to 3,633 meters, and consequently hosts a large number of threatened, endangered,

and endemic species. The sunny climate and proximity to national parks and other public lands also attracts human inhabitants. Most of the large cities in the area (e.g., Las Vegas) continue to grow, and anthropogenic developments and recreational activities are increasingly disturbing Mojave Desert ecosystems. Additional stressors on native flora and fauna include invasive species, altered disease dynamics, and increasing wildfire frequency, and populations of many plant and animal species are in decline.

As a patchwork of unique habitats and extreme conditions, the Mojave Desert supports a diverse flora of about three thousand plant species [3]. The number and geological distribution of plants within this landscape is determined by the interaction of many factors such as elevation, precipitation, temperature, soil properties and disturbance. Many Mojave plant species are both regionally endemic, found nowhere else on earth, and locally endemic, restricted to a specific geographic area within the Mojave Desert. Additionally, some species can be defined as rare, having either a narrow geographical range, specialized habitat requirements, or a small population size [4]. Although a species may be considered endemic but not rare, and vice versa, both endemic and/or rare species exhibit attributes which make them more vulnerable to extinction [5]. Consequently, conservation efforts are necessary in some instances to preserve genetic and species diversity as well as ecosystem structure and function [3]. Given the number of endemic and rare species, and their unique habitat requirements, land managers face a daunting task in the Mojave Desert. The United States Endangered Species Act (ESA) of 1973 provided a critical boost to conservation efforts. This legislation was designed to identify and protect plant and animal species facing imminent extinction, and covers species, subspecies, and distinct population segments [6]. The Mojave Desert contains 24 plant species listed as threatened or endangered under the ESA (**Table 1**), many of which are also listed under some level of conservation concern by the International Union for the Conservation of Nature [7].

These 24 listed plants species encompass four states and nine plants families, illustrating the starkly different geographical reaches of the Mojave Desert. They can be further subdivided to exemplify one or more of the three criteria for rarity [4] and/or specific forms of endemism [8]. First, all the listed species have a narrow geographic range, some restricted to soils derived from specific geological formations and some endemic to regional biodiversity hotspots. For example, the seven species from Nevada are all found within the confines of Ash Meadows National Wildlife Refuge and land immediately adjacent (**Figure 1**). This 24,000 acre spring-fed alkaline wetland and alkaline desert upland has the highest concentration of endemic species in the United States [9–10]. Second, all the listed species have highly specialized habitat requirements ranging from eolian dunes of Coachella Valley, California to gypsum outcrops associated with the Harrisburg Member of the Kaibab Formation in northern Mohave County, Arizona. Edaphic endemism (influenced by soils; [8]) is a strong theme running through the Mojave ESA listed species with alkaline-wetland, calcium-carbonate, gypsum, and granite endemism represented. Three listed species from the San Bernardino Mountains in California are restricted to "pebble plains" named for the layer of orange quartzite pebbles that are pushed to the clay soil surface by freezing and thawing [11]. Third, many of the listed species have small or disjointed populations, such as Lane Mountain Milkvetch (*Astragalus jaegerianus*). This species is known from only 4 disjunct populations and is threatened by recreational offhighway vehicle (OHV) use and military activity. Although threats to the 24 ESA listed Mojave Desert plant species are many, climate change remains a ubiquitous concern.


*Climate Change Vulnerability Assessment of Imperiled Plants in the Mojave Desert DOI: http://dx.doi.org/10.5772/intechopen.95783*


*Global conservation status rank according to NatureServe: GX/TX = presumed extinct, GH/TH = possibly extinct, G1/T1 = critically imperiled, G2/T2 = imperiled, G3/T3 = vulnerable, G4/T4 = apparently secure, G5/T5 = secure.*

#### **Table 1.**

*Name, family, and location of Mojave Desert plants listed as threatened (T) or endangered (E) under the United States Endangered Species Act (ESA).*

There has been multi-decadal warming across the Earth's surface, with each of the previous three decades experiencing progressively warmer temperatures than any preceding decade since 1850 [12]. Ecoregions defined by climatic extremes, such as deserts, are particularly vulnerable to climate change. On a global scale, deserts have experienced faster warming and drying over the last 50 years when compared to other regions [13, 14], and this is projected to continue [15]. Desert dwelling species may be directly impacted by thermal or hydric stress, or indirectly via altered habitats, species interactions, or disease dynamics. Several studies have documented negative effects from increasing temperatures on desert wildlife species, including birds [16, 17], mammals [18], invertebrates [19], and reptiles [20]. Studies focused on desert plants have predicted species range shifts in response to climate change [21], and potential changes in vegetation community composition as a result of altered summer precipitation patterns [22]. Climate change impacts are projected to be particularly severe in the southwestern region of the United States, where the Mojave Desert occurs, and resident plants will have to cope with prolonged drought, fewer frost days, warmer temperatures, and an increase in extreme weather events [23–25]. Many species may not be able to survive in such highly transformed environments. Climate change has emerged as one of the greatest threats to biodiversity, with potential to hasten species extinctions, elevating the need to understand how threatened and endangered species may be affected [26]. Management actions have traditionally focused on the establishment of protected areas to conserve habitat and halt species decline, but the efficacy of these strategies is questionable under predicted future climatic regimes [27]. One way to better integrate climate change considerations into management planning for imperiled plant species is to conduct climate change vulnerability assessments (CCVAs).

Vulnerability assessments offer a standardized approach for measuring climate change sensitivity that is efficient, repeatable, and directly comparable among

*Climate Change Vulnerability Assessment of Imperiled Plants in the Mojave Desert DOI: http://dx.doi.org/10.5772/intechopen.95783*

#### **Figure 1.**

*Illustration panel by Tiffany J. Pereira portraying the federally threatened and endangered plants of Ash Meadows National Wildlife Refuge, a vulnerable biodiversity hotspot in Nevada, USA. From left to right:* Centaurium namophilum, Astragalus phoenix, Enceliopsis nudicaulis var. corrugata, Grindelia fraxinipratensis, Ivesia kingii var. eremica*, and* Mentzelia leucophylla*. Not pictured:* Nitrophila mohavensis*.*

different species. Vulnerability can be defined as the degree to which a species is susceptible to climate change, taking into consideration the magnitude of predicted change where the species occurs and the adaptive capacity of the species [28]. Species within an area or ecoregion can be ranked according to their expected sensitivity to changing climatic conditions [29]. A CCVA can be conducted using several different methods, such as a correlative, mechanistic, or trait-based approach, or a combination of several of these types of models. Correlative approaches have frequently been used for plants, but a trait-based approach is best for rapid assessment of a larger number of species [30]. A trait-based method identifies and scores attributes of a species that are relevant to avoiding or tolerating climate change,

such as dispersal ability or sensitivity to temperature or precipitation changes. A species' overall vulnerability to climate change is represented by the sum of these scores. Here we utilize a trait-based approach to assess climate change vulnerability of threatened and endangered plants in the Mojave Desert, and provide suggestions for incorporating these considerations into management actions and conservation planning efforts.
