Desert Tortoise Research Articles

Relative abundance and demographic structure of Agassiz’s desert tortoise (Gopherus agassizii) along roads of varying size and traffic volume

Roads are recognized as important contributors to wildlife population declines and are thought to pose greatest risk to vagile species with large home ranges and long generation times. We examined variation in the relative abundance and demographic structure of Agassiz’s desert tortoise (Gopherus agassizii) near roads that varied in traffic volume. We found that the abundance of tortoise sign (scat, tracks, pallets, burrows, and live and dead individuals) varied with traffic volume and distance from the road depending on traffic volume. The relative abundance of tortoise sign was greatest along roads with low traffic volume (<1vehicle/day) compared to roads with intermediate (30–60vehicles/day) and high (320–1100vehicles/day) traffic volumes. Additionally, tortoise sign had lower relative abundances at least 200m from roads with the highest traffic volumes. We found that the frequency of live tortoise encounters decreased with increasing traffic volumes. Tortoise size also correlated significantly with traffic volume, such that tortoises near the highest traffic volume road were smallest. Finally, along the highest traffic road we found greater proportions of juvenile tortoises than along either of the other traffic volume roads. Our results indicate that roads may decrease tortoise populations via several possible mechanisms, including cumulative mortality from vehicle collisions and reduced population growth rates from the loss of larger reproductive animals. Here, we provide evidence that a reptile with a slow life history is susceptible to road presence and that the effect increases with traffic volume.

Nest site characteristics and nesting success of the Western Burrowing Owl in the eastern Mojave Desert

We evaluated nest site selection at two spatial scales (microsite, territory) and reproductive success of Western Burrowing Owls (Athene cunicularia hypugaea) at three spatial scales (microsite, territory, landscape) in the eastern Mojave Desert. We used binary logistic regression within an information-theoretic approach to assess factors influencing nest site choice and nesting success. Microsite-scale variables favored by owls included burrows excavated by desert tortoise (Gopherus agassizii), burrows with a large mound of excavated soil at the entrance, and a greater number of satellite burrows within 5 m of the nest burrow. At the territory scale, owls preferred patches with greater cover of creosote bush (Larrea tridentata) within 50 m of the nest burrow. An interaction between the presence or absence of a calcic soil horizon layer over the top of the burrow (microsite) and the number of burrows within 50 m (territory) influenced nest site choice. Nesting success was influenced by a greater number of burrows within 5 m of the nest burrow. Total cool season precipitation was a predictor of nesting success at the landscape scale. Conservation strategies can rely on management of habitat for favored and productive nesting sites for this declining species.

Mycoplasmal Upper Respiratory Tract Disease Across the Range of the Threatened Mojave Desert Tortoise: Associations with Thermal Regime and Natural Antibodies

Most research of upper respiratory tract disease (mycoplasmal URTD) in the threatened Mojave Desert tortoise (Gopherus agassizii) has worked under the hypothesis that the pathogen, Mycoplasma agassizii, has a relatively consistent and predictable effect on tortoise populations across their natural range. In contrast, we hypothesized that multiple factors influence the prevalence of disease and analyzed biological and environmental variables that vary significantly across the Mojave Desert. We used multiple regression models to analyze associations between mycoplasmal URTD and the genetic structure of 24 tortoise populations, levels of natural antibody (NAb) to M. agassizii in tortoises (one component of the innate immune system), precipitation, and colder thermal regimes. We detected a significant, positive association between mean levels of NAb and seroprevalence to M. agassizii. We hypothesized that NAbs may provide tolerance to mycoplasmal infections and that more tolerant populations may act as host reservoirs of disease. We also detected significant associations between colder winters and mycoplasmal URTD, suggesting that colder winters may depress tortoise immune resistance against M. agassizii or enhance conditions for the growth of M. agassizii.

Are captive tortoises a reservoir for conservation? An assessment of genealogical affiliation of captive Gopherus agassizii to local, wild populations

The conservation of tortoises poses a unique situation because several threatened species are commonly kept as pets within their native ranges. Thus, there is potential for captive populations to be a reservoir for repatriation efforts. We assess the utility of captive populations of the threatened Agassiz’s desert tortoise (Gopherus agassizii) for recovery efforts based on genetic affinity to local areas. We collected samples from 130 captive desert tortoises from three desert communities: two in California (Ridgecrest and Joshua Tree) and the Desert Tortoise Conservation Center (Las Vegas) in Nevada. We tested all samples for 25 short tandem repeats and sequenced 1,109 bp of the mitochondrial genome. We compared captive genotypes to a database of 1,258 Gopherus samples, including 657 wild caught G. agassizii spanning the full range of the species. We conducted population assignment tests to determine the genetic origins of the captive individuals. For our total sample set, only 44 % of captive individuals were assigned to local populations based on genetic units derived from the reference database. One individual from Joshua Tree, California, was identified as being a Morafka’s desert tortoise, G. morafkai, a cryptic species which is not native to the Mojave Desert. Our data suggest that captive desert tortoises kept within the native range of G. agassizii cannot be presumed to have a genealogical affiliation to wild tortoises in their geographic proximity. Precautions should be taken before considering the release of captive tortoises into the wild as a management tool for recovery.

A Strategy for Prioritizing Threats and Recovery Actions for At-Risk Species

Ensuring the persistence of at-risk species depends on implementing conservation actions that ameliorate threats. We developed and implemented a method to quantify the relative importance of threats and to prioritize recovery actions based on their potential to affect risk to Mojave desert tortoises (Gopherus agassizii). We used assessments of threat importance and elasticities of demographic rates from population matrix models to estimate the relative contributions of threats to overall increase in risk to the population. We found that urbanization, human access, military operations, disease, and illegal use of off highway vehicles are the most serious threats to the desert tortoise range-wide. These results suggest that, overall, recovery actions that decrease habitat loss, predation, and crushing will be most effective for recovery; specifically, we found that habitat restoration, topic-specific environmental education, and land acquisition are most likely to result in the greatest decrease in risk to the desert tortoise across its range. In addition, we have developed an application that manages the conceptual model and all supporting information and calculates threat severity and potential effectiveness of recovery actions. Our analytical approach provides an objective process for quantifying threats, prioritizing recovery actions, and developing monitoring metrics for those actions for adaptive management of any at-risk species.

Nest-Guarding by Female Agassiz's Desert Tortoise (Gopherus agassizii)at a Wind-Energy Facility Near Palm Springs, California

Abstract We observed behavior consistent with nest-guarding in Agassiz's desert tortoise (Gopherus agassizii) at two nests in a large wind-energy-generation facility near Palm Springs, California, locally known as the Mesa Wind Farm. As researchers approached the nests, female desert tortoises moved to the entrance of their burrows and positioned themselves sideways, directly over their nests. One female stretched her limbs outward and wedged herself into the burrow (her plastron directly above the nest). Guarding of nests is rarely observed in Agassiz's desert tortoise but can occur as a result of attempted predation on eggs by Gila monsters (Heloderma suspectum) or in direct response to the perceived threat posed by researchers. This is the first report of nest-guarding for G. agassizii in the Sonoran Desert ecosystem of California.

Multiple Factors Affect a Population of Agassiz's Desert Tortoise (Gopherus agassizii) in the Northwestern Mojave Desert

Abstract: Numerous factors have contributed to declines in populations of the federally threatened Agassiz's Desert Tortoise (Gopherus agassizii) and continue to limit recovery. In 2010, we surveyed a low-density population on a military test facility in the northwestern Mojave Desert of California, USA, to evaluate population status and identify potential factors contributing to distribution and low densities. Estimated densities of live tortoises ranged spatially from 1.2/km2 to 15.1/km2. Although only one death of a breeding-age tortoise was recorded for the 4-yr period prior to the survey, remains of 16 juvenile and immature tortoises were found, and most showed signs of predation by Common Ravens (Corvus corax) and mammals. Predation may have limited recruitment of young tortoises into the adult size classes. To evaluate the relative importance of different types of impacts to tortoises, we developed predictive models for spatially explicit densities of tortoise sign and live tortoises using topograp...

Climatic variation affects clutch phenology in Agassiz’s desert tortoise Gopherus agassizii

There is concern about how climate change might affect Agassiz's desert tortoise Gopherus agassizii, a threatened species. We studied the effects of climatic variation on clutch phenology of a population at a wind energy generation facility for 7 field seasons between 1997 and 2011. Using X-radiography, we quantified the following phenophases based on the number of calendar days to each event from 1 January: appearance and disappearance of 65 first and 53 sec- ond clutches, and inter-clutch intervals between first and second clutches. Although third clutches were rare (n = 8), they were observed in 5 of 7 yr and were produced by 8 different females, all of which produced a third clutch only once during the study. Shelled eggs were visible from as early as 11 April to as late as 28 July, and the overall time span that eggs were visible differed among years. After controlling for maternal effects, we observed statistically significant inter-annual vari- ation in all phenophases except for inter-clutch interval. Clutch appearance was late in cool years relative to warm years and especially late after 2 consecutive cool years. Using degree day (DD) methodology, we calculated DD accumulation during the post-hibernation and nesting season for each year. We then used DD estimates for mean date of first clutch appearance to predict the mean date of first clutch appearance in subsequent years with 1 to 10 d accuracy for all but 1 yr (1998) with El Nino conditions. Clutch phenology appears to be correlated with inter-annual variation in climate and may be influenced by climatic events in previous years.

Accumulation of wind-dispersed trash in desert environments

Detrimental effects of plastic debris and other trash have been well-studied in marine and coastal environments, but the extent and severity of the threat to terrestrial ecosystems are largely unknown. I used distance-sampling methods to quantify density of wind-dispersed trash in two protected areas of the Sonoran Desert in southern Arizona. Densities of plastic bags ranged from 5.6/km2 to 35.4/km2, and densities of balloon clusters ranged from 39.2/km2 to 62.7/km2. Density of balloon clusters was greater than the density of desert tortoises, Gopherus morafkai, and western diamond-backed rattlesnakes, Crotalus atrox, in both study areas. Results of this study provide evidence that wind can disperse substantial amounts of trash >2 km into protected natural areas and suggest that accumulation of trash poses a potential threat to desert ecosystems.

Mycoplasma testudineum in Free-ranging Desert Tortoises, Gopherus agassizii

We performed clinico-pathological evaluations of 11 wild Agassiz's desert tortoises (Gopherus agassizii) from a translocation project in the central Mojave Desert, California, USA. Group 1 consisted of nine tortoises that were selected primarily due to serologic status, indicating exposure to Mycoplasma testudineum (seven) or both M. agassizii and M. testudineum (two), and secondarily due to clinical signs of upper respiratory tract disease (URTD). Group 2 consisted of two tortoises that were antibody-negative for Mycoplasma and had no clinical signs of URTD, but did have other signs of illness. Of the Group 1 tortoises, M. testudineum, but not M. agassizii, was amplified by polymerase chain reaction and DNA fingerprinted from two tortoises. Using light microscopy, mild to severe pathologic changes were observed in one or more histologic sections of either one or both nasal cavities of each tortoise in Group 1. Our findings support a causal relationship between M. testudineum and URTD in desert tortoises.

Dispatches

Dispatches

A New Approach to Conservation of the Mojave Desert Tortoise

The Mojave desert tortoise was listed as threatened under the US Endangered Species Act (ESA) because of local population declines and an array of threats. Challenges to the recovery of this species include an incomplete understanding of the threats most responsible for its decline, insufficient information on the effectiveness of management actions, and the intractability of threats across a large geographical range and multiple jurisdictions. Recognition that these challenges require long-term conservation efforts to ensure the species' persistence—with or without the protections of the ESA—necessitates a more structured approach to recovery, including broad stakeholder participation. A conservation-reliant perspective will probably be increasingly relevant for additional species and for adapting land management in the face of climate change by improving regional coordination of management activities, broadening spatial and temporal points of view in management, and increasing the emphasis on addressing...

Spatial and temporal variation in survival of a rare reptile: a 22-year study of Sonoran desert tortoises

Although many species may be vulnerable to changes in climate, forecasting species-level responses can be challenging given the array of physiological, behavioral, and demographic attributes that might be affected. One strategy to improve forecasts is to evaluate how species responded to climatic variation in the past. We used 22 years of capture-recapture data for Sonoran desert tortoises (Gopherus morafkai) collected from 15 locations across their geographic range in Arizona to evaluate how environmental factors affected spatial and temporal variation in survival. Although rates of annual survival were generally high ([Formula: see text] = 0.92), survival of adults decreased with drought severity, especially in portions of their range that were most arid and nearest to cities. In three locations where large numbers of carcasses from marked tortoises were recovered, survival of adults was markedly lower during periods of severe drought ([Formula: see text] = 0.77-0.81) compared to all other periods ([Formula: see text] = 0.93-0.98). Assuming continued levels of dependency of humans on fossil fuels, survival of adult tortoises is predicted to decrease by an average of 3 % during 2035-2060 relative to survival during 1987-2008 in 14 of the 15 populations we studied. This decrease could reduce persistence of tortoise populations, especially in arid portions of their range. Temporal and spatial variation in drought conditions are important determinants of survival in adult desert tortoises.

A quantitative PCR method for assessing the presence of Pasteurella testudinis DNA in nasal lavage samples from the desert tortoise (Gopherus agassizii)

Pasteurella testudinis has been associated with upper respiratory tract disease (URTD) in the threatened desert tortoise (Gopherus agassizii). Our goal was to develop a sensitive and specific qPCR method for detecting DNA from P. testudinis in nasal lavage fluid collected from desert tortoises in the field. Probes for 16S ribosomal RNA and RNA polymerase β-subunit (rpoB) genes were designed. A standard curve generated with DNA extracted from known numbers of bacterial cells determined by flow cytometry revealed a lower detection limit of 50fg/ml (10bacteria/ml). The nasal lavage fluid contained no interfering substances, and the qPCR method did not recognize normal flora DNA. The nasal lavage samples from 20 desert tortoises captured in Clark County, Nevada, USA in 2007 and housed at the Desert Tortoise Conservation Center, were all positive for P. testudinis DNA by qPCR. Another set of 19 lavage samples collected in 2010 from wild desert tortoises in the Mojave Desert were tested and 84% were positive for P. testudinis DNA. Fully validated, this qPCR method will provide a means of determining colonization rate. When used in conjunction with serological methods and clinical evaluations, both infection rate and disease rate can be determined for this potential URTD pathogen. This new assay provides an important tool for managing the threatened populations of the Mojave Desert tortoise.

A trade-off between natural and acquired antibody production in a reptile: implications for long-term resistance to disease

SummaryVertebrate immune systems are understood to be complex and dynamic, with trade-offs among different physiological components (e.g., innate and adaptive immunity) within individuals and among taxonomic lineages. Desert tortoises (Gopherus agassizii) immunised with ovalbumin (OVA) showed a clear trade-off between levels of natural antibodies (NAbs; innate immune function) and the production of acquired antibodies (adaptive immune function). Once initiated, acquired antibody responses included a long-term elevation in antibodies persisting for more than one year. The occurrence of either (a) high levels of NAbs or (b) long-term elevations of acquired antibodies in individual tortoises suggests that long-term humoral resistance to pathogens may be especially important in this species, as well as in other vertebrates with slow metabolic rates, concomitantly slow primary adaptive immune responses, and long life-spans.

Does translocation influence physiological stress in the desert tortoise?

AbstractWildlife translocation is increasingly used to mitigate disturbances to animals or habitat due to human activities, yet little is known about the extent to which translocating animals causes stress. To understand the relationship between physiological stress and translocation, we conducted a multiyear study (2007–2009) using a population of desert tortoises (Gopherus agassizii) near Fort Irwin, California. Blood samples were collected from adult tortoises in three treatment groups (resident, translocated and control) for 1 year prior to and 2 years after translocation. Samples were analyzed by radioimmunoassay for plasma total corticosterone (CORT), a glucocorticoid hormone commonly associated with stress responses in reptiles. CORT values were analyzed in relation to potential covariates (animal sex, date, behavior, treatment, handling time, air temperature, home‐range size, precipitation and annual plant production) among seasons and years. CORT values in males were higher than in females, and values for both varied monthly throughout the activity season and among years. Year and sex were strong predictors of CORT, and translocation explained little in terms of CORT. Based on these results, we conclude that translocation does not elicit a physiological stress response in desert tortoises.

Translocation as a conservation tool for Agassiz's desert tortoises: Survivorship, reproduction, and movements

AbstractWe translocated 120 Agassiz's desert tortoises to 5 sites in Nevada and Utah to evaluate the effects of translocation on tortoise survivorship, reproduction, and habitat use. Translocation sites included several elevations, and extended to sites with vegetation assemblages not typically associated with desert tortoises in order to explore the possibility of moving animals to upper elevation areas. We measured survivorship, reproduction, and movements of translocated and resident animals at each site. Survivorship was not significantly different between translocated and resident animals within and among sites, and survivorship was greater overall during non‐drought years. The number of eggs produced by tortoises was similar for translocated and resident females, but differed among sites. Animals translocated to atypical habitat generally moved until they reached vegetation communities more typical of desert tortoise habitat. Even within typical tortoise habitat, tortoises tended to move greater distances in the first year after translocation than did residents, but their movements in the second or third year after translocation were indistinguishable from those of resident tortoises. Our data show that tortoises translocated into typical Mojave desert scrub habitats perform well; however, the large first‐year movements of translocated tortoises have important management implications. Projects that employ translocations must consider how much area will be needed to contain translocated tortoises and whether roads need fencing to prevent the loss of animals. © 2012 The Wildlife Society.

Comparison of Intraosseous and Peripheral Venous Fluid Dynamics in the Desert Tortoise (Gopherus agassizii)

The efficacy of intraosseous catheterization has not been described previously in the desert tortoise (Gopherus agassizii). The goal of this study was to describe and compare the efficacy of four intraosseous catheter sites (humerus, femur, plastocarapacial junction [bridge], and gular region of the plastron) to jugular catheterization. Five adult tortoises were catheterized in each of the sites at least once. The distribution of a bolus injection of radiopharmaceutical (technetium-99m-diethylenetriaminepentaacidic acid [99mTc -DTPA]) was monitored via gamma camera over 2-min periods at five time intervals over 24 min. Compared to jugular catheterization, the humerus and femur sites provided the next best vascular access, with 84.4 and 61.8% of activity reaching the systemic circulation by 7 min, respectively. The bridge and gular catheter sites were less effective with only 41.9 and 40.8% systemic activity, respectively. Intraosseous catheters were no more technically difficult to place than jugular catheters and were less commonly dislodged, making them a viable option for vascular access in tortoises.

SEROLOGIC AND MOLECULAR EVIDENCE FOR TESTUDINID HERPESVIRUS 2 INFECTION IN WILD AGASSIZ'S DESERT TORTOISES, GOPHERUS AGASSIZII

Following field observations of wild Agassiz's desert tortoises (Gopherus agassizii) with oral lesions similar to those seen in captive tortoises with herpesvirus infection, we measured the prevalence of antibodies to Testudinid herpesvirus (TeHV) 3 in wild populations of desert tortoises in California. The survey revealed 30.9% antibody prevalence. In 2009 and 2010, two wild adult male desert tortoises, with gross lesions consistent with trauma and puncture wounds, respectively, were necropsied. Tortoise 1 was from the central Mojave Desert and tortoise 2 was from the northeastern Mojave Desert. We extracted DNA from the tongue of tortoise 1 and from the tongue and nasal mucosa of tortoise 2. Sequencing of polymerase chain reaction products of the herpesviral DNA-dependent DNA polymerase gene and the UL39 gene respectively showed 100% nucleotide identity with TeHV2, which was previously detected in an ill captive desert tortoise in California. Although several cases of herpesvirus infection have been described in captive desert tortoises, our findings represent the first conclusive molecular evidence of TeHV2 infection in wild desert tortoises. The serologic findings support cross-reactivity between TeHV2 and TeHV3. Further studies to determine the ecology, prevalence, and clinical significance of this virus in tortoise populations are needed.

Nesting Ecology of a Population of Gopherus agassizii at a Utility-Scale Wind Energy Facility in Southern California

We investigated the annual nesting ecology of a population of Desert Tortoises (Gopherus agassizii) inhabiting a utility-scale renewable energy (USRE) facility in southern California and compared our results with populations inhabiting relatively undisturbed sites. In 2000, 15 radio-tracked females produced 29 clutches, and 24 nests were monitored to examine nest-site selection, nest predation, hatching success, date of emergence of hatchlings, and hatchling mass and carapace length. Overall, the nesting ecology of the population inhabiting the USRE facility was very similar to other populations of Desert Tortoises inhabiting relatively undisturbed habitats. Oviposition occurred from 12 May to 8 July, which was similar to other sites. Nest depths (11.1 cm), nest predation (12%), hatchling emergence date (7 August and 29 September), and hatchling morphometrics (i.e., MCL: 44.5 mm; mass: 23 g) were all within ranges reported in other populations. Unlike within other populations, we observed no relationship between hatchling size and either maternal body size or egg width. We found no evidence of females selecting for a particular burrow for oviposition of eggs based on environmental or anthropogenic variables. Most nests were located in or near burrows, and nest depth was greater for nests near the entrance than those deeper in the burrow. Although this study suggests that the nesting ecology of the Desert Tortoise population we studied was not adversely affected by the USRE facility, this relationship is only correlative because our study was not a before-after-control-impact (BACI) study, which would establish a cause and effect relationship. As pointed out in a recent review, BACI studies are critically needed to address the wildlife impacts of utility-scale renewable energy development.