Mycoplasmosis in free-ranging desert tortoises in Utah and Arizona

Upper respiratory tract disease (URTD) has been associated with major losses of free-ranging desert tortoises (Gopherus agassizii) in the southwestern United States. This prompted a clinical examination of 63 free-ranging desert tortoises for signs of URTD and sampling for Mycoplasma agassizii, the causative agent of URTD. Tortoises were sampled from three sites in the eastern Mojave Desert (1992-93), and from three sites in the Sonoran Desert (1992-94). Plasma samples were tested for antibodies to M. agassizii using an enzyme-linked immunosorbent assay (ELISA). Nasal aspirate samples from 12 Sonoran tortoises were tested using polymerase chain reaction (PCR) test directed at the 16S rRNA gene of M. agassizii. Nasal aspirate samples from all tortoises were cultured for M. agassizii. In the Mojave Desert, nine tortoises had clinical signs of URTD and eight were seropositive for M. agassizii. In the Sonoran Desert, there were no clinical signs of URTD, but two tortoises were seropositive, and two tortoises had positive PCR results.     

Pathology of upper respiratory tract disease of gopher tortoises in Florida

Between August 1993 and September 1995, 24 gopher tortoises (Gopherus polyphemus) were received for pathological evaluations from various locations in Florida (USA). All tortoises were examined for clinical signs of upper respiratory tract disease (URTD) including nasal and ocular discharge, palpebral edema, and conjunctivitis. Of the 24 tortoises, 10 had current or previously observed clinical signs of URTD and 14 did not. A blood sample was drawn for detection of anti-mycoplasma antibodies by ELISA, and nasal lavage samples were collected for culture and detection of Mycoplasma agassizii gene sequences by polymerase chain reaction (PCR). Of the 14 clinically healthy tortoises, eight were sero-, culture- and PCR-negative, and six were seropositive for antibodies against M. agassizii. Of those six, five were culture- and/or PCR-positive for M. agassizii, and one was culture- and PCR-negative. Of the 10 ill tortoises, nine were seropositive by the ELISA and one was in the suspect range. Nine of the ill tortoises, including the suspect tortoise, were culture- and/or PCR-positive for M. agassizii, and one was culture- and PCR-negative. For histologic evaluation and discussion, the eight sero-, culture-, and PCR-negative tortoises were designated URTD-negative, and the other 16 were classified as URTD-positive. Histologic evaluation of the upper respiratory tract (URT) indicated the presence of mild to severe inflammatory, hyperplastic, or dysplastic changes in 14 URTD-positive tortoises. Seven of eight URTD-negative tortoises had normal appearing nasal cavities; one had mild inflammatory changes. Transmission electron microscopy revealed an organism consistent with Mycoplasma spp. on the nasal mucosal surface of tortoises with clinical signs and lesions of URTD. Additionally, gram-negative bacteria were isolated more frequently from the nasal cavities of URTD-positive tortoises than URTD-negative tortoises. Because clinical signs of URTD were never observed in six of the URTD-positive tortoises, we also conclude that subclinical URTD can occur in gopher tortoises.     

Western blot can distinguish natural and acquired antibodies to Mycoplasma agassizii in the desert tortoise (Gopherus agassizii)

Mycoplasma agassizi has been identified as a cause of upper respiratory tract disease (URTD) in the threatened Mojave population of the desert tortoise (Gopherus agassizii), and anti-M. agassizii antibodies have been found by ELISA in as many as 15% of these animals across their geographic range. Here we report that a cohort of 16 egg-reared desert tortoises never exposed to M. agassizii had ELISA antibody titers to this organism that overlapped with titers obtained from some M. agassizii-infected tortoises. These natural antibodies were predominantly of the IgM class. Western blots of plasma from these non-infected tortoises produced a characteristic banding pattern against M. agassizii antigens. A group of 38 wild-caught desert tortoises was tested by ELISA, and although some of these tortoises had antibody titers significantly higher than the non-infected tortoises, there was considerable overlap at the lower titer levels. However, Western blot analysis revealed distinct banding patterns that could readily distinguish between the non-infected tortoises and tortoises with acquired antibodies, regardless of ELISA antibody titers. We conclude that desert tortoises have natural antibodies to M. agassizii that can compromise the determination of infection status by ELISA. However, the Western blot technique can distinguish between natural and acquired antibody patterns and can be used to confirm the diagnosis of M. agassizii infections in the desert tortoise.     

Detection and characterization of Mycoplasma spp. and Salmonella spp. in free-living European tortoises (Testudo hermanni, Testudo graeca, and Testudo marginata)

Free-living and captive chelonians might suffer from upper respiratory tract disease (URTD), a pathology primarily caused by Mycoplasma agassizii. Wild tortoises can also be an important reservoir of Salmonella spp., which are commensal in the host reptile but are potential zoonotic agents. Between July 2009 and June 2010, we screened free-living European tortoises (spur-thighed tortoises Testudo graeca, Hermann's tortoises Testudo hermanni, marginated tortoises Testudo marginata) temporarily housed in a wildlife center in Italy. We molecularly characterized 13 Mycoplasma isolates detected in all Testudo spp. studied, and three PCR-positive animals showed typical URTD clinical signs at the time of sampling. Three Salmonella enterica serotypes (Abony, Potsdam, Granlo), already related to reptile-associated human infections, were also identified. These results highlight the potential role played by wildlife recovery centers in the spread and transmission of pathogens among wild chelonians and to humans.     

Clinical disease and laboratory abnormalities in free-ranging desert tortoises in California (1990-1995)

Desert tortoise (Gopherus agassizii) populations have experienced precipitous declines resulting from the cumulative impact of habitat loss and human and disease-related mortality. Diagnosis of disease in live, free-ranging tortoises is facilitated by evaluation of clinical signs and laboratory test results but may be complicated by seasonal and environmental effects. The goals of this study were: 1) to describe and monitor clinical and laboratory signs of disease in adult, free-ranging desert tortoises at three sites in the Mojave Desert of California (USA) between October 1990 and October 1995; 2) to evaluate associations between clinical signs and hematologic, biochemical, serologic, and microbiologic test results; 3) to characterize disease patterns by site, season, and sex; and 4) to assess the utility of diagnostic tests in predicting morbidity and mortality. Venous blood samples were obtained four times per year from tortoises of both sexes at the Desert Tortoise Research Natural Area (DTNA), Goffs/Fenner Valley, and Ivanpah Valley. Tortoises were given a physical examination, and clinical abnormalities were graded by type and severity. Of 108 tortoises, 68.5% had clinical signs of upper respiratory tract disease consistent with mycoplasmosis at least once during the study period. In addition, 48.1% developed moderate to severe shell lesions consistent with cutaneous dyskeratosis. Ulcerated or plaque-like oral lesions were noted on single occasions in 23% of tortoises at Goffs and 6% of tortoises at Ivanpah. Tortoises with oral lesions were significantly more likely than tortoises without lesions to have positive nasal cultures for Mycoplasma agassizii (P = 0.001) and to be dehydrated (P = 0.0007). Nine tortoises had marked azotemia (blood urea nitrogen [BUN] > 100 mg/dl) or persistent azotemia (BUN 63-76 mg/dl); four of these died, three of which had necropsy confirmation of urinary tract disease. Laboratory tests had low sensitivity but high specificity in assessing morbidity and mortality; there was marked discrepancy between serologic and culture results for M. agassizii. Compared with tortoises at other sites, tortoises at DTNA were more likely to be seropositive for M. agassizii. Tortoises at Goffs were significantly more likely to have moderate to severe shell disease, oral lesions, positive nasal cultures for M. agassizii, and increased plasma aspartate aminotransferase activity. The severe disease prevalence in Goffs tortoises likely contributed to the population decline that occurred during and subsequent to this study.     

Mycoplasma and herpesvirus PCR detection in tortoises with rhinitis-stomatitis complex in Spain

Chelonid herpesvirus (ChHV) and mycoplasmal infections cause similar clinical signs in terrestrial tortoises and may be the most important causative agents of rhinitis-stomatitis complex, a common disease in captive tortoises worldwide. Currently, diagnosis of ChHV and Mycoplasma spp. infections is most often based on serologic testing. However, serologic results only detect past exposure, and the specificity of these tests can be reduced due to antigenic cross-reactions with other pathogens. Molecular-based techniques could help to define the causative agent and to better manage infected tortoises. Using polymerase chain reaction, we analyzed 63 tortoises (59 spur-thighed tortoise, Testudo graeca; three Greek tortoise, Testudo ibera; and one Russian tortoise, Agryonemys horsfieldii) with clinical signs of rhinitis-stomatitis complex to identify the causative agent. Molecular evidence of ChHV type I (24%), type II (3%), and Mycoplasma agassizii (6%) infections, as well as coinfection of Mycoplasma-ChHV and both types of ChHV, were detected. Both ChHV and M. agassizii are considered pathogenic in captive tortoises and both are a threat to wild populations. However, neither agent was detected from most of the symptomatic tortoises we evaluated, indicating that other agents could be involved in the rhinitis-stomatitis complex.     

Western blot can distinguish natural and acquired antibodies to Mycoplasma agassizii in the desert tortoise (Gopherus agassizii)

Mycoplasma agassizi has been identified as a cause of upper respiratory tract disease (URTD) in the threatened Mojave population of the desert tortoise (Gopherus agassizii), and anti-M. agassizii antibodies have been found by ELISA in as many as 15% of these animals across their geographic range. Here we report that a cohort of 16 egg-reared desert tortoises never exposed to M. agassizii had ELISA antibody titers to this organism that overlapped with titers obtained from some M. agassizii-infected tortoises. These natural antibodies were predominantly of the IgM class. Western blots of plasma from these non-infected tortoises produced a characteristic banding pattern against M. agassizii antigens. A group of 38 wild-caught desert tortoises was tested by ELISA, and although some of these tortoises had antibody titers significantly higher than the non-infected tortoises, there was considerable overlap at the lower titer levels. However, Western blot analysis revealed distinct banding patterns that could readily distinguish between the non-infected tortoises and tortoises with acquired antibodies, regardless of ELISA antibody titers. We conclude that desert tortoises have natural antibodies to M. agassizii that can compromise the determination of infection status by ELISA. However, the Western blot technique can distinguish between natural and acquired antibody patterns and can be used to confirm the diagnosis of M. agassizii infections in the desert tortoise.     

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.     

Reference intervals and physiologic alterations in hematologic and biochemical values of free-ranging desert tortoises in the Mojave Desert

Desert tortoise (Gopherus agassizii) populations have experienced precipitous declines resulting from the cumulative impact of habitat loss, and human and disease-related mortality. Evaluation of hematologic and biochemical responses of desert tortoises to physiologic and environmental factors can facilitate the assessment of stress and disease in tortoises and contribute to management decisions and population recovery. The goal of this study was to obtain and analyze clinical laboratory data from free-ranging desert tortoises at three sites in the Mojave Desert (California, USA) between October 1990 and October 1995, to establish reference intervals, and to develop guidelines for the interpretation of laboratory data under a variety of environmental and physiologic conditions. Body weight, carapace length, and venous blood samples for a complete blood count and clinical chemistry profile were obtained from 98 clinically healthy adult desert tortoises of both sexes at the Desert Tortoise Research Natural area (western Mojave), Goffs (eastern Mojave) and Ivanpah Valley (northeastern Mojave). Samples were obtained four times per year, in winter (February/March), spring (May/June), summer (July/August), and fall (October). Years of near-, above- and below-average rainfall were represented in the 5 yr period. Minimum, maximum and median values, and central 95 percentiles were used as reference intervals and measures of central tendency for tortoises at each site and/or season. Data were analyzed using repeated measures analysis of variance for significant (P < 0.01) variation on the basis of sex, site, season, and interactions between these variables. Significant sex differences were observed for packed cell volume, hemoglobin concentration, aspartate transaminase activity, and cholesterol, triglyceride, calcium, and phosphorus concentrations. Marked seasonal variation was observed in most parameters in conjunction with reproductive cycle, hibernation, or seasonal rainfall. Year-to-year differences and long-term alterations primarily reflected winter rainfall amounts. Site differences were minimal, and largely reflected geographic differences in precipitation patterns, such that results from these studies can be applied to other tortoise populations in environments with known rainfall and forage availability patterns.     

Guidelines for the field evaluation of desert tortoise health and disease

Field evaluation of free-ranging wildlife requires the systematic documentation of a variety of environmental conditions and individual parameters of health and disease, particularly in the case of rare or endangered species. In addition, defined criteria are needed for the humane salvage of ill or dying animals. The purpose of this paper is to describe, in detail, the preparation, procedures, and protocols we developed and tested for the field evaluation of wild desert tortoises (Gopherus agassizii). These guidelines describe: preparations for the field, including developing familiarity with tortoise behavior and ecology, and preparation of standardized data sheets; journal notes to document background data on weather conditions, temperature, rainfall, locality, and historic and recent human activities; procedures to prevent the spread of disease and parasites; data sheets for live tortoises to record tortoise identifiation, location, sex, body measurements and activity; health profile forms for documenting and grading physical abnormalities of tortoise posture and movements, general condition (e.g., lethargy, cachexia), external parasites, and clinical abnormalities associated with shell and upper respiratory diseases; permanent photographic records for the retrospective analysis of progression and regression of upper respiratory and eye diseases, analysis of shell lesions and evaluation of growth and age; and indications and methods for salvaging ill or dying tortoises for necropsy evaluation. These guidelines, tested on 5,000 to 20,000 tortoises over a 10 to 27 yr period, were designed to maximize acquisition of data for demographic, ecological, health and disease research projects; to reduce handling and stress of individual animals; to avoid spread of infectious disease; to promote high quality and consistent data sets; and to reduce the duration and number of field trips. The field methods are adapted for desert tortoise life cycle, behavior, anatomy, physiology, and pertinent disease; however the model is applicable to other species of reptiles. Comprehesive databases of clinical signs of disease and health are crucial to research endeavors and essential to decisions on captive release, epidemiology of disease, translocation of wild tortoises, breeding programs, and euthanasia.     

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.     

Potential Facilitation Between a Commensal and a Pathogenic Microbe in a Wildlife Disease

We assessed the potential for microbial interactions influencing a well-documented host–pathogen system. Mycoplasma agassizii is the known etiological agent of upper respiratory tract disease in Mojave desert tortoises (Gopherus agassizii), but disease in wild animals is extremely heterogeneous. For example, a much larger proportion of animals harbor M. agassizii than those that develop disease. With the availability of a new quantitative PCR assay for a microbe that had previously been implicated in disease, Pasteurella testudinis, we tested 389 previously collected samples of nasal microbes from tortoise populations across the Mojave desert. We showed that P. testudinis is a common commensal microbe. However, we did find that its presence was associated with higher levels of M. agassizii among the tortoises positive for this pathogen. The best predictor of P. testudinis prevalence in tortoise populations was average size of tortoises, suggesting that older populations have higher levels of P. testudinis. The prevalence of co-infection in populations was associated with the prevalence of URTD, providing additional evidence for an indirect interaction between the two microbes and inflammatory disease. We showed that URTD, like many chronic, polymicrobial diseases involving mucosal surfaces, shows patterns of a polymicrobial etiology.

     

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

We investigate a cornucopia of problems associated with the identity of the desert tortoise, Gopherus agassizii (Cooper). The date of publication is found to be 1861, rather than 1863. Only one of the three original cotypes exists, and it is designated as the lectotype of the species. Another cotype is found to have been destroyed in the 1906 San Francisco earthquake and subsequent fire. The third is lost. The lectotype is genetically confirmed to be from California, and not Arizona, USA as sometimes reported. Maternally, the holotype of Gopherus lepidocephalus (Ottley & Velázques Solis. 1989) from the Cape Region of Baja California Sur, Mexico is also from the Mojavian population of the desert tortoise, and not from Tiburon Island, Sonora, Mexico as previously proposed. A suite of characters serve to diagnose tortoises west and north of the Colorado River, the Mojavian population, from those east and south of the river in Arizona, USA, and Sonora and Sinaloa, Mexico, the Sonoran population. Species recognition is warranted and because Gopherus lepidocephalus is from the Mojavian population, no names are available for the Sonoran species. Thus, a new species, Gopherus morafkaisp. n., is named and this action reduces the distribution of Gopherus agassizii to only 30% of its former range. This reduction has important implications for the conservation and protection of Gopherus agassizii, which may deserve a higher level of protection.     

Detection of a novel Chlamydia species in captive spur-thighed tortoises (Testudo graeca) in southeastern Spain and proposal of Candidatus Chlamydia testudinis

The spur-thighed tortoise (Testudo graeca) is an endangered Mediterranean tortoise that lives in North Africa, Southern Europe and Southwest Asia. In the wake of recent legislation making their keeping as domestic animals illegal, many of these animals have been returned to wildlife recovery centers in Spain. In the present study, a population of such tortoises showing signs of ocular disease and nasal discharge was examined for the presence of Chlamydia spp. Cloacal, conjunctival and/or choanal swabs were collected from 58 animals. Using a real-time PCR specific for the family Chlamydiaceae, 57/58 animals tested positive in at least one sample. While only a few samples proved positive for C. pecorum, sequencing of the 16S rRNA gene revealed a sequence identical to previously published sequences from specimens of German and Polish tortoises. Whole-genome sequences obtained from two conjunctival swab samples, as well as ANIb, TETRA values and a scheme based on 9 taxonomic marker genes revealed that the strain present in the Spanish tortoises represented a new yet non-classified species, with C. pecorum being its closest relative. We propose to designate the new species Candidatus Chlamydia testudinis.     

Physical and biochemical abnormalities associated with prolonged entrapment in a desert tortoise

A desert tortoise (Gopherus agassizii) was trapped underground without food or water for nearly 11 mo near Yucca Mountain, Nevada (USA). Physical abnormalities included weight loss, sunken eyes, and muscle atrophy. Biochemical abnormalities determined from blood sampling included marked azotemia and hyperosmolality, which were attributed largely to accumulation and retention of nitrogenous wastes. Moderate hypercholesterolemia, hypophosphatemia, and increased aspartate transaminase activity, and mild hyperchloremia, hypocalcemia, hyperbilirubinemia and anemia also were observed, compared with results obtained from other tortoises sampled at the same time. The lack of, or only mild, alterations in most laboratory data exemplified the high degree of physiological adaptation tortoises can undergo when deprived of food and water for a prolonged period.     

Sighting frequency decreases over two decades in three populations of Testudo hermanni from central Italy

Sighting frequency of tortoises (Testudo hermanni), standardized by the number of person-hours of field effort, was studied between 1992 and 2013 at three different study areas in central Italy. Although the frequency of tortoise sightings fluctuated substantially across years and among study areas, there was a significant declining trend in all the three areas, with GLM analyses showing also a significant interaction between study area and sampling year. The decreasing trend was higher in Castel Fusano than in the other two sites, with yearly frequency of sightings being independent of search effort in all of the study areas. These trends may indicate population declines, because no other explanations seem plausible. Total rainfall from previous autumn to the study period was positively correlated to tortoise sightings in only one locality. The yearly frequency of sightings of juvenile tortoises did not vary significantly among study areas and across years, thus indicating that simple differences in detectability cannot explain the observed patterns. Summer fires and forest overgrowing seem to explain the tortoise decline in two study areas (Castel Fusano and Oriolo) whereas we were not able to identify any specific reason for tortoise decline in the third study area (Manziana).     

Histomorphometric studies of dermal bone in the desert tortoise, Gopherus agassizii.

Dermal bone biopsies were collected from the periphery of the carapaces of adult desert tortoises (Gopherus agassizii) from grazed and ungrazed habitats near the Arizona/Utah border (USA). Quantitative bone histomorphometry was performed on these biopsies as well as on dermal bone biopsies collected from captive juvenile desert tortoises. Except for mild osteomalacia, carapaces of adult desert tortoises from the grazed habitat were relatively normal. No signs of osteopenia were observed. Based on the low numbers of osteoblasts and osteoclasts in dermal bone of both populations of adult desert tortoises, it appears that the peripheral carapace is relatively inert with very low levels of dermal bone turnover. Bone cells and osteoid were more common in dermal bone biopsies from the carapace and plastron of captive juvenile desert tortoises than in adult desert tortoises. However, the great variability in the incidence of bone cells among individuals and the difficulty in collecting juvenile desert tortoises in the field limit the usefulness of dermal bone biopsies from animals of this age group. Based on these results, we propose that dermal bone of the peripheral carapace is a poor sample site for evaluating the effects of dietary or environmental conditions on calcified tissues in desert tortoises.     

The Dominance of Introduced Plant Species in the Diets of Migratory Galapagos Tortoises Increases with Elevation on a Human‐Occupied Island

The distribution of resources and food selection are fundamental to the ecology, life history, physiology, population dynamics, and conservation of animals. Introduced plants are changing foraging dynamics of herbivores in many ecosystems often with unknown consequences. Galapagos tortoises, like many herbivores, undertake migrations along elevation gradients driven by variability in vegetation productivity which take them into upland areas dominated by introduced plants. We sought to characterize diet composition of two species of Galapagos tortoises, focussing on how the role of introduced forage species changes over space and the implications for tortoise conservation. We quantified the distribution of tortoises with elevation using GPS telemetry. Along the elevation gradient, we quantified the abundance of introduced and native plant species, estimated diet composition by recording foods consumed by tortoises, and assessed tortoise physical condition from body weights and blood parameter values. Tortoises ranged between 0 and 429 m in elevation over which they consumed at least 64 plant species from 26 families, 44 percent of which were introduced species. Cover of introduced species and the proportion of introduced species in tortoise diets increased with elevation. Introduced species were positively selected for by tortoises at all elevations. Tortoise physical condition was either consistent or increased with elevation at the least biologically productive season on Galapagos. Santa Cruz tortoises are generalist herbivores that have adapted their feeding behavior to consume many introduced plant species that has likely made a positive contribution to tortoise nutrition. Some transformed habitats that contain an abundance of introduced forage species are compatible with tortoise conservation.     

The rise and fall of the Aldabran giant tortoise population.

At the end of the 19th century, after prolonged and extensive harvesting, indigenous giant tortoises had been eliminated from all islands in the Indian Ocean, except Aldabra atoll, where only a few survived. With greatly reduced levels of exploitation during the 20th century, the population recovered to a revised estimated total of 129,000 in 1973-1974, when the first sample census was conducted. A repeat census in 1997 revealed a highly significant reduction in numbers over the past 24 years to an estimated total of 100,000. The great majority of tortoises are still found at relatively high density in south-eastern Grande Terre, where the number of animals has declined by more than one-third. In contrast, low-density subpopulations on Malabar and Picard have almost doubled in size, but they represent less than 5% of the total population. Corroborative evidence for the crash in the Grande Terre subpopulation comes from two independent observations: a significant increase in tortoise mortality; and a significant decline in tortoise counts on long-term population monitoring transects. These population changes are attributed to natural population regulatory mechanisms, exacerbated by low rainfall years in the period 1980-1997, including two consecutive years of below average rainfall in 1995-1996 and 1996-1997.     

Fine-scale analysis reveals cryptic landscape genetic structure in desert tortoises

Characterizing the effects of landscape features on genetic variation is essential for understanding how landscapes shape patterns of gene flow and spatial genetic structure of populations. Most landscape genetics studies have focused on patterns of gene flow at a regional scale. However, the genetic structure of populations at a local scale may be influenced by a unique suite of landscape variables that have little bearing on connectivity patterns observed at broader spatial scales. We investigated fine-scale spatial patterns of genetic variation and gene flow in relation to features of the landscape in desert tortoise (Gopherus agassizii), using 859 tortoises genotyped at 16 microsatellite loci with associated data on geographic location, sex, elevation, slope, and soil type, and spatial relationship to putative barriers (power lines, roads). We used spatially explicit and non-explicit Bayesian clustering algorithms to partition the sample into discrete clusters, and characterize the relationships between genetic distance and ecological variables to identify factors with the greatest influence on gene flow at a local scale. Desert tortoises exhibit weak genetic structure at a local scale, and we identified two subpopulations across the study area. Although genetic differentiation between the subpopulations was low, our landscape genetic analysis identified both natural (slope) and anthropogenic (roads) landscape variables that have significantly influenced gene flow within this local population. We show that desert tortoise movements at a local scale are influenced by features of the landscape, and that these features are different than those that influence gene flow at larger scales. Our findings are important for desert tortoise conservation and management, particularly in light of recent translocation efforts in the region. More generally, our results indicate that recent landscape changes can affect gene flow at a local scale and that their effects can be detected almost immediately.