Gopherus agassizii, the desert tortoise, is a reptile whose home range includes the Mojave Desert, the Sonoran Desert, the Sinaloan thornscrub, and the Sinaloan deciduous forest. (Germano, Bury, Esque, Fritts, & Medica, 1994). These habitats cover parts of California, Nevada, Utah, Arizona, and Mexico. Due to the extreme temperatures of the desert, desert tortoises primarily stay in their burrows during the day and come out at night as their method of thermoregulation (Zimmerman et al., 1994). Compared to the other North American tortoises, the desert tortoise has a larger scope of temperature, moisture, vegetation, and habitat, indicating desert tortoises have high environmental tolerance (Germano et al., 1994). One reason for their ability to tolerate harsh conditions may be the desert tortoises’ low field metabolic rates, allowing them to endure long bouts of food and water shortage (Peterson, 1996a). Another reason for their tolerance is the desert tortoises’ unusually large bladders, which are used in osmoregulation to store waste and water for resorption later on, allowing them to minimize water loss (Peterson, 1996b). One might expect such high environmental tolerance to result in longer life spans; however, desert tortoise populations in areas of the Mojave and Sonoran Deserts have life spans of around 30 years of age, and in general, most desert tortoises do not grow past 50 years of age (Germano, 1992). Demographically speaking, these species have difficulty recovering from population losses—this has been shown in recent years, as their populations across the Mojave Desert have been dwindling because of human encroachment, upper respiratory disease, and loss of habitat, among other reasons (Bury & Corn, 1995). One big factor of the population decline of desert tortoises is climate change: it is predicted that the increased temperatures and droughts that come with climate change will cause the species to move to areas away from their home range in attempts to move to higher elevation, thus causing the tortoises to lose the majority of their natural habitat (Barrows, 2011). Because of the desert tortoise’s status as a vulnerable species on the International Union for Conservation of Nature and Natural Resources (IUCN) Red List of Threatened Species, it is especially important to explore the different conservation methods suitable for restoring and maintaining existing desert tortoise populations (1996). Our paper discusses population models of desert tortoises, factors that cause their population decline, and two conservation methods; based on the research discussed, translocation is currently the best conservation method for protecting and preserving desert tortoises.
Population models that study the desert tortoise all indicate rapid population decline. One indicator of population decline is the size changes of carapace of female desert tortoises. Population growth can be seen by an increase in carapace size throughout its reproductive stages. Demographic analyses report no tortoises changed sizes from yearling to their adult stages (Doak, Kareiva, & Klepteka, 1994). The issue with no change in size reiterates the fact that populations are either stagnant or heading towards a decline in population growth. Another essential indicator of population decline is the effect of adult female population size. Adult female desert tortoises have been known to travel short distances for nesting sites compared to other species (Gibbons, 1986). Their short distance migration allowed for an obtainable data record. Therefore, there are more current reports covering female reproductive rates and life history compared to males (Doak et al., 1994). Reports indicate a strong correlation between reproductive female population size and population growth rate of desert tortoise. Sensitivity analyses report the mean growth and survival rates of all desert tortoise populations. These analyses indicate that the survival increase in reproductive female populations results in overall population growth (Doak et al., 1994). Therefore, conservation efforts should focus on the preservation of adult reproductive female populations since they are essential ecological indicators for population trends among desert tortoise populations.
A major cause of the decline in desert tortoise populations is the habitat destruction made by human interferences such as grazing land conversions and the human introduction of upper respiratory tract disease to desert tortoise populations. Resource competition with livestock such as sheep and cattle reduces the overall fitness of desert tortoise populations (Doak et al, 1994). The ability for desert tortoise survival is drastically reduced as livestock are found to run over them and their nesting sites (Doak et al., 1994). Another threat to desert tortoise populations is the spread of upper respiratory tract disease (URTD) introduced by humans. This disease causes desert tortoises to experience lethargy, hunger, and eventual death (Doak et al., 1994). Reports indicate higher levels of URTD found in adult tortoise populations compared to yearling or juvenile populations (Doak et al., 1994). Therefore, high sensitivity of adult desert tortoise populations to human disturbances illuminates the importance for focus of conservation efforts towards this reproductive stage.
Conservation efforts have been implemented in trying to relieve the endangerment of the Desert Tortoise due to anthropogenic factors, but all vary in their effectiveness and methodology. One current technique that is used to increase conservation of the Desert Tortoise is relocation. Relocation involves the movement of an animal from its home environment to another area within its home range (Berry, 1986). A study by Berry (1986), analyzes the effect relocation had on desert tortoises, which gives insight to how efficient of a conservation method it is. It was found that the relocation of Desert Tortoises from their natural habitat had complex effects on social behavior and movements (Berry, 1986). The researcher further explained that the relocation of the tortoise to an already established resident population area disrupted the present social system of that population, even if the carrying capacity is below its peak (Berry, 1986). This is significant because it shows that relocation may not be beneficial to home species occupying the habitat before the implementation of the new tortoises. Furthermore it was found that relocation had the ability to affect tortoise offspring as their parents were unable to survive in certain cases. Although some were able to survive better than others, the researcher saw an overall decreased survival rate and ultimately the tortoise reproductive rate. In essence, the technique of relocation proves to be largely ineffective to conserving the desert tortoise, especially when analyzing the survival rate of the relocated species in the data provided by Berry.
Another conservation technique employed is translocation. One of the more widespread conservation methods, translocation, has had mixed yet promising results for tortoise populations in the Mojave Desert. Species translocation is a conservation method involving the capture and release of a portion of a population from one location to another. Translocation is implemented with the intent to move a group away from harm or overpopulated areas into areas where they could provide some kind of ecological benefit. Often this is considerably far from the original homeland. The effects of translocation can be observed in a study conducted by Field and colleagues (2007). Desert tortoises were moved from Las Vegas to a detention center where they were detained. Their detention was due to the habitat degradation taking place in their original land. In terms of the Mojave Desert tortoise, success of translocation efforts appear to be dependent on multiple factors, primarily resource abundance in new territories, length of drought seasons, and even the conditions in which the translocated populations were kept in captivity. In their paper “Return to the wild: Translocation as a tool in conservation of the Desert Tortoise (Gopherus agassizii),” Field et al. outline and critique previous translocation experiments and provide support for translocation as a valid conservation strategy through their own experiment.
The federal Endangered Species Act of 1990 outlined a recovery plan involving experimental translocation as a possible translocation strategy (Field et al., 2007). Many biologists encouraged the government to abandon this method, citing previous translocation studies (Berry, 1972,1986; Amant, 1978; Bury et al., 1988), as they concluded that Desert Tortoises placed in captivity were unfit and incompetent when tasked with finding food, shelter, and conserving water resources (experiment survival rates were as low as 0%). However, Field et al. believes these studies lacked validity and didn’t provide enough evidence to discredit translocation as a viable conservation strategy. As a result, Field and colleagues created their own experiment to determine the validity of translocation as a conservation method, studying the movement, habits, and body mass of 32 translocated tortoises split into two groups (a group allowed supplemental water in captivity and a group not allowed supplemental water in captivity). The experiment was conducted with the objective of finding possible confounding variables in previous experiments that may have skewed results, mainly water provisions given to tortoises during captivity.
As the previously mentioned studies involving translocation of desert tortoise populations all obtained subjects from the Desert Tortoise Conservation Center (DTCC), Field and colleagues hypothesized that daily water provisions given to Mojave Desert Tortoises at the DTCC hampered tortoises’ ability to maintain and conserve water levels. Tortoises frequent voiding of diluted urine, inefficient use of shade, and the impaired ability to obtain water rich food were observations cited as evidence for their theories. However, Field and colleagues found that there was no difference in the survival rates of tortoises allowed and not allowed water provisions in captivity. Their observations also led them to conclude that tortoises translocated from captivity were still very capable of burrowing, foraging, and regulating body temperature and water levels when compared to tortoises translocated from the wild (Field et al., 2007). Nearly 80% of the translocated populations are believed to have survived the first season after being relocated and 100% of tortoises survived the second season. Field et al. concluded that previous failures in experimental translocation of Mojave Desert Tortoise populations were due to oversights and confounding variables (like length of drought seasons and placement near predator populations); their findings demonstrate high potential for the long-term success of translocation projects and encourages these methods to be considered valid tools for desert tortoise conservation.
Given the many obstacles standing in the way of the survival of the Desert Tortoise species, it has become imperative that conservation efforts remain persistent in the exploration of new and improving methods that account for and build upon the results of practices already in place. The desert tortoises are endangered due to the impacts that human intervention and climate change have had on their natural habitats. They rely heavily on our support to recover their losses. Of the many conservation efforts currently in practice, relocation and translocation have been two major focal points with conservationists. On account of the poor social adaptations and unforeseen behavioural changes associated with relocation (Berry, 1986), studies conclude that although headed in the right direction, relocation is not ideal. Translocation, however, has yielded more promising results when carried out with careful selectivity towards resource availability within the new habitat (Field et al., 2007). Regarding future efforts, the major questions conservationists need to consider pertain to how to avoid repetition of the same setbacks seen in relocation and translocation. In addition, we need to rethink the areas that we urbanize and set more boundaries so as to not further destroy any more of their habitats. Placement of solar panels, for example, directly affects tortoise populations. We can greatly increase the fitness of these populations if we minimize that kind of harmful interference. This can be achieved with exploration of new protective measures, like the passing of tighter laws and regulations or even the establishment of more reserves. Although we have a ways to go, considerable progress has been made and if continued with the same persistency, the future of the Desert Tortoise looks promising.