Rank Method Rank calculation - Biotics v2
Review Date2021-02-02
Change Date2021-02-02
Edition Date2021-02-02
Edition AuthorsTreher (2021)
Threat ImpactHigh - medium
Range Extent20,000-200,000 square km (about 8000-80,000 square miles)
Rank ReasonsYucca brevifolia is endemic to the United States and occurs primarily within the Mojave Desert but also the Great Basin Desert of southeastern California and southern Nevada. It is threatened by fire, drought, climate change, and numerous threats related to habitat loss including off road vehicle use.
Range Extent CommentsYucca brevifolia is endemic to the United States and occurs primarily within the Mojave Desert but also the Great Basin Desert of southeastern California and southern Nevada. This species overlaps in range with Yucca jaegeriana in a small area of Nevada, called the Tikaboo Valley, where the two species hybridize (USFWS 2018, Royer et al. 2016, Lenz 2007). Range extent was calculated using data from iNaturalist (2021), SEINet (2021), and CCH2 (2021).
Threat Impact CommentsYucca brevifolia is threatened by fire, drought, climate change, and numerous threats related to habitat loss. One of the more severe threats to the species is an increase in fire frequency, which causes direct mortality and reduced survivorship over time. A greater presence of non-native annual grasses, which produce a fuel load in the environment, have increased the frequency of fire in the deserts where this species occurs. The plant communities and Joshua Tree itself are not especially tolerant of fire. Native plants, when responding with vigorous growth to an unusually wet winter, also contribute to an increased fuel load and are responsible for some fires in the past. Younger, smaller plants are more vulnerable to mortality than older, larger plants. One cause of mortality post-fire is predation by small mammals seeking a food source (USFWS 2018, Cole et al. 2011). With its primary seed disperser, the Shasta Ground Sloth, extinct, the species ability to shift into suitable habitat is limited (Cole et al. 2011).
Climatic conditions threaten the species in the short and long term, in multiple ways, through increased temperatures, drought, and precipitation timing. While this species is tolerant of drought, young plants including seedlings experience greater mortality and lower germination success during drought, which is increasing in severity currently and into the future due to climate change. Studies are needed to understand the impact of this threat and to document declines. The species range is expected to become more arid, specifically drier and hotter, with climate change. The species habitat is predicted to shrink and move into refugia, northward, and to higher elevations (USFWS 2018).
Habitat loss, including degradation of soil structure loss and loss of plant cover, is caused by development, military activities, grazing animals, renewable energy development, and off road vehicle use. About 2.2% of the species range is in areas with residential and urban development. While plants are largely protected by local ordinances, habitat loss does happen to some degree (USFWS 2018). About 24.6% of the species range in on military lands, but the species is not believed to be impacted, but potentially benefited by management plans and conservation efforts (USFWS 2018). Grazing animals browse plants, including flowers below 2 meters in height, trample young plants, and contribute to a loss of soil structure that can degrade habitat (USFWS 2018, Lybbert and St. Clair 2017). Off road vehicle use can be severe where it occurs, causing mortality to this species and its nurse plants and damage to the seed bank and degradation to the soil structure (USFWS 2018).
An additional threat to the species is damage and mortality from native herbivores (USFWS 2018).