Species Spotlight: Mojave Desert Tortoise

Gopherus agassizii

Common Name: Mojave Desert Tortoise

Family: Testudinidae

Range: American Southwest (Arizona, California, Nevada, and Utah)

Conservation Status: Critically Endangered

Description:

Gopherus agassizii belongs to a composite of at least two and possibly as many as four species (Murphy et. al., 2011). Tortoises living east and south of the Colorado River are distinct from those located north and west of the river. Murphy et al. explain that considerable studies went towards distinguishing Gopherus morafkai, commonly referred to as the Sonoran Desert Ttortoise from the protected Mojave Desert Ttortoise. Acknowledging the new species reduced the distribution of Gopherus agassizii to 30% of its former range. Aside from genealogy and geography, morphological differences and habitat preferences distinguish the two species. Accordingly, Gopherus agassizii has a significantly wider shell and prefers valleys and alluvial fans, while Gopherus morafkai reside along slopes and rocky hillsides (Murphy et al., 2011).

Slightly flattened front legs enable Mojave Desert tortoises to efficiently dig in desert sands, which they use to construct burrows for shelter and temperature regulation (National Park Service). In the coldest parts of winter desert tortoises hibernate in their burrows, only emerging during occasional warm spells. The rest of the year, desert tortoises will use burrows on cold desert nights to retain heat and for regulation on hot summer days (National Park Service). Desert tortoises may share burrows, but typically live solitary lives (Animals). As keystone species that function as “ecosystem engineers,” desert tortoises provide habitat for other species that cohabitate burrows, including rodents, snakes, lizards, mesocarnivores, and ground birds (Hansen).

Female tortoises only begin reproducing at age 15 and will lay clutches of 6-8 eggs (National Park Service). Young tortoises contend with high mortality rates, with some studies suggesting survival rates of just 2-3% (Hansen). Predation accounts for considerable juvenile tortoise mortality. Ravens along with coyotes and domestic dogs account for most predation of juvenile tortoises (Hansen). Lifestyle and a hard shell protect adults from many potential threats, except for humans. Tortoises that reach adulthood typically live between 30-50 years but can live to age 80 (Animals).

Threats and Conservation:

Tortoises face formidable threats from habitat loss and degradation, perpetuated by agriculture, energy development, and expanded human residence areas (USFWS, 2011). The United States Fish and Wildlife Service (‘Service’) listed the desert tortoise as a threatened species in 1990 (USFWS, 2022). In 1994, the Service designated 6.4 million acres of federal land in the desert tortoise’s range as critical habitat. The Recovery Plan identified six recovery units to attempt to placate ecological and genetic variation (Murphy et al., 2011). The Service maintains site specific plans for fourteen areas designated for the desert tortoise. Plans include discontinuing livestock grazing, installing fencing alongside roads to prevent collisions, reducing off-road travel, placing signs that indicate habitat boundaries, and eliminating raven nests (Hansen). $93 million went towards conservation of the desert tortoise between 1996-2006 (Morris & Owley, 2014).

Agricultural use of desert tortoise habitat depletes the tortoises’ food supplies, particularly through grazing livestock (Berry et al., 2021). Agricultural uses abutted 80% of Mojave Desert tortoise habitat at one point (Berry et al, 2021.). Until 1932’s Taylor Grazing Act, grazing of livestock and use of driveways occurred abundantly and intensely in habitat areas (Berry et al., 2021.). Nevertheless, considerable livestock grazing continued after the Act. Cattle grazing continues through an estimated 17% of critical habitat (Berry et al, 2021.). Climate change and its associated ecological challenges also strain desert tortoises through habitat degradation. Further, the displacement of native forage species of forbs with non-native grasses disrupts tortoises’ natural diets, leading to weight loss, and potentially killing juveniles (Agha, 2020).

Changes in land use, including renewable energy projects, contribute to habitat fragmentation. Moreover, the U.S. Fish and Wildlife Service (‘Service’) indicates that the challenges to the desert tortoise from large-scale energy development include: habitat fragmentation, isolation of desert tortoise conservation areas, and the subsequent possibility of restricted gene flow between these areas (USFWS, 2011). The Service produced a Revised Recovery Plan in 2011 which specifically addresses the need to increase studies on the effect of solar energy projects. Solar projects in desert tortoise habitat increased significantly since the species original listing as endangered—with more expected to go online soon (USFWS, 2011).

Tourism and human travel challenge desert tortoise survival (Berry et al., 2021.). When tortoises existed in greater abundance, human visitors collected live specimens to own as pets. Tortoises can contract a contagious upper respiratory disease from human contact. Given the tortoises’ long lifespan, many uncommitted captors eventually released the animals back to the wild. Upon return, infected tortoises spread the upper respiratory illness, which continues to plague the species today (Jacobson et al., 2014). Despite becoming unlawful in 1939, tortoise collection continues (Berry et al.).

Another human related threat comes from car travel. One study of the Eastern Mojave Recovery Unit suggests that direct motor vehicle impacts likely cause at least 5.3 tortoise mortalities annually. Such a low number of individual deaths might appear trivial; however, due to delayed sexual maturity, high neonatal mortality, and the species’’ small population size, the loss of any adult tortoise is detrimental (Hughson and Darby, 2013).

Status:

The IUCN indicates a decreasing population trend for the Mojave Desert Tortoise, based on ongoing declines in both the quality and area of suitable habitat, disease , and other human related impacts (Berry, et at., 2021). Population and density declines of approximately 90% over three generations (the past 90 years) have been observed and are predicted to continue (Berry, et at., 2021). As such, the species is categorized as Critically Endangered and urgent conservation action is required to halt its unsustainable exploitation (Berry, et at., 2021).

Solutions:

Avoiding land use in desert tortoise habitat provides the most simplistic method to prevent increased strain on the species from habitat loss and degradation (Agha, 2020). U.S. Fish and Wildlife Service monitoring of desert tortoise locations provides an advantageous start-point to avoid infiltrating valuable habitat as developers take on new projects. The strain from solar energy projects poses a formidable hurdle to desert tortoise conservation. Further, given the global impetus to transition to renewable energy, many projects recently undertaken were “fast tracked” despite limited understanding of their impact on wildlife (Morris & Owley, 2014). The necessity for a clean energy transition can seemingly juxtapose environmental concerns with species conservation.

Thus far solar projects in desert tortoise habitat largely rely on translocation as a mitigation strategy (Agha, 2020). “Head starting” offers another mitigation strategy. It involves raising juvenile tortoises in captivity until they grow enough to lower the risk of mortality from predation and other factors (Agha, 2020). Long-term data remains unavailable regarding the effectiveness of translocation and head-starting, though they show promising short-term results (Agha, 2020). Agha et al (2020) propose the use of already disturbed environments, like roof-tops in urban locations and brownfields. Developers could also implement a larger assortment of microgrids, reducing adverse effects at any one site (Bronin, 2010).

Barrier fencing along roads can reduce fatal accidents. Culverts underneath roads can help mitigate the effects of habitat fragmentation (Hughson and Darby, 2013). Barrier fences pose potential problems however, as tortoises may become trapped in fences (USFWS, 2011). Thus, the cost of fencing and culverts—financially, effectively, and aesthetically—make enforcement of reduced traffic speeds, posting warning signs, and increasing public awareness more viable options to reduce the effect of automobile traffic on tortoises (Hughson and Darby, 2013). Hughson et al. (2013) note that drivers respond stronger to notable, strategically (i.e., seasonally) placed road signs as compared to permanent signage.

References:

Agha, Mickey, et al. “Wind, Sun, and Wildlife: Do Wind and Solar Energy Development ‘Short-Circuit’ Conservation in the Western United States?” Environmental Research Letters, vol. 15, no. 7, 2020. <https://doi.org/10.1088/1748-9326/ab8846>

Berry, K.H., Allison, L.J., McLuckie, A.M., Vaughn, M. & Murphy, R.W. 2021. Gopherus agassizii. The IUCN Red List of Threatened Species 2021: e.T97246272A3150871. <https://dx.doi.org/10.2305/IUCN.UK.2021-2.RLTS.T97246272A3150871.en. Accessed on 05 October 2023>

Morris, A. W., & Owley, J. (2014). Mitigating the impacts of the renewable energy gold rush. Minnesota Journal of Law, Science & Technology, 15, 293. <https://scholarship.law.umn.edu/mjlst/vol15/iss1/18>

Jacobson, E., Brown, M., Wendland, L., Brown, D., Klein, P., Christopher, M., & Berry, K. (2014). Mycoplasmosis and upper respiratory tract disease of tortoises: A review and update. The Veterinary Journal, 201(3), 257-264. doi: 10.1016/j.tvjl.2014.05.039

National Park Service, n. d. Desert Tortoise (Gopherus Agassizii). <https://www.nps.gov/moja/learn/nature/desert-tortoise.htm>

Hughson, D.L. & Darby, N. (2013). Desert tortoise road mortality in Mojave National Preserve. California Fish and Game, 99, 222-232.<https://nrm.dfg.ca.gov/FileHandler.ashx?DocumentID=78259>

Murphy R, Berry K, Edwards T, Leviton A, Lathrop A, Riedle J (2011) The dazed and confused identity of Agassiz’s land tortoise, Gopherus agassizii (Testudines: Testudinidae) with the description of a new species and its consequences for conservation. ZooKeys 113: 39-71. <https://doi.org/10.3897/zookeys.113.1353>

Bronin, S. C. (2010). Curbing energy sprawl with microgrids. Connecticut Law Review, 43(2), 547. <https://scholarship.law.cornell.edu/cgi/viewcontent.cgi?article=2831&context=facpub>

United States, Congress, Hansen, James V, and Robert Bennet. Endangered Species: Research Strategy and Long-Term Monitoring Needed for the Mojave Desert Tortoise Recovery Program, 03 ed., GAO, U.S. General Accounting Office, 2002, pp. 1–58. 23.

U.S. Fish and Wildlife Service. (2011). 2011 revised recovery plan for the Mojave desert tortoise (Gopherus agassizii) (pp. 1-246). U.S. Fish and Wildlife Service, Pacific Southwest Region.  

U.S. Fish and Wildlife Service. 2022. Range-wide Monitoring of the Mojave Desert Tortoise (Gopherus agassizii): 2020 Annual Reporting. Report by the Desert Tortoise Recovery Office, U.S. Fish and Wildlife Service, Reno, Nevada. <https://www.fws.gov/sites/default/files/documents/USFWS.2022%20report.%20Rangewide%20monitoring%20report%202020.pdf>