Establishing an Agassiz’s Desert Tortoise monitoring program within the Coachella Valley multiple species habitat conservation plan area: Final report to the Coachella Valley conservation commission on work performed near the Orocopia Mountains

Abstract

In support of the goals of the Coachella Valley Multiple Species Habitat Conservation Plan and Natural Community Conservation Plan (CVMSHCP/NCCP), a population of Agassiz’s desert tortoises ( Gopherus agassizii ) was marked and studied to establish a desert tortoise monitoring program near the Orocopia Mountains beginning in early 2017 and ending in the summer of 2018, following the epic drought of 2012‒2016. This effort compliments a similar effort in the nearby mouth of Cottonwood Canyon in 2015‒2016. Surveys were performed to locate tortoises, tortoise burrows, and tortoise remains at the eastern end of the CVMSHCP area north of the Orocopia Mountains and south of Interstate 10 in Riverside County, California. Although the area is considered Critical Habitat for the recovery of tortoise populations, it was heavily impacted by military training activities in the early 1940s and continues to be impacted by off-highway vehicle use. Data were collected from all live and dead tortoise specimens encountered. Only 22 live tortoises were found during transects covering approximately 21 km 2 of habitat surveyed. The sex ratio of live adult tortoises was strongly biased toward males and the sex ratio of recently (4‒5 years) dead carcasses during the long drought was strongly biased toward females. High female mortality may have resulted from the interaction of drought (including increased predation) and the reproductive strategy of tortoises. We located only one new live tortoise in the drought year of 2018 when there was no germination of winter annual food plants. A subsample of nine tortoises was outfitted with radio transmitters, and females (n = 4) were X-radiographed at approximately 10-day intervals from April–July. Mean clutch size was about 4 eggs as is typical for tortoises in this region. Additional tortoises were located opportunistically in and around the Santa Rosa Mountains (located in the southern end of the CVMSHCP area), and these tortoises were also marked for future identification. Blood samples were taken from adult tortoises and scute clips were taken from a subset of juveniles for ongoing studies to determine genetic diversity and relationships of desert tortoises within the CVMSHCP/NCCP area and beyond. The low tortoise density and high adult female mortality observed by us and others in the area may compromise the long-term viability of the population, especially given published predictions of the negative effects of future droughts on tortoises in the region.