Tortoise Populations Research Articles

Effect of nest location on gopher tortoise nest survival

ABSTRACTGopher tortoise (Gopherus polyphemus) populations are declining range‐wide primarily because of habitat loss, fragmentation, and degradation, but low recruitment due to nest predation also may be contributing to this decline. The relationship of nest location and nest survival is unknown. Therefore, our objective was to determine whether nest location influenced depredation risk and potential recruitment success. Between 30 May 2013 and 3 September 2014, we constructed and monitored 105 artificial gopher tortoise nests (45 in 2013 and 60 in 2014) with 35 nests at each of 3 classifications of nest sites: within active burrow aprons (i.e., the mound of sand at the burrow entrance), inactive burrow aprons, and open sandy sites. Using the artificial nests, we investigated the influence of nest site classification, distance to road, and prescribed fire frequency (<1 year and 1 year since burn) on gopher tortoise nest survival. Additionally, we monitored 21 natural nests at burrow aprons to assess predation patterns as compared to artificial nests. Seventy‐two (57%) of 126 nests were fully depredated during the study, and predation rates at artificial and natural nests were similar. We identified predators at 48 nests using trail cameras; the nine‐banded armadillo (Dasypus novemcinctus) was the primary nest predator (69%). Nests in open sandy areas away from burrows exhibited greater daily survival rates (DSR; 0.991 ± 0.005 SE) compared to nests in burrow aprons (active burrow aprons: 0.967 ± 0.018; inactive burrow aprons: 0.949 ± 0.027). Additionally, nests at locations burned the year of the study had a lower chance of survival (DSR: 0.959 ± 0.022) than nests at locations burned the previous year (DSR: 0.991 ± 0.005). Management practices that maintain open sandy areas (e.g., prescribed fire) away from burrows may increase tortoise nest survival. Fire return intervals of >1 year that are implemented frequently enough to maintain open sandy areas would be beneficial to nesting tortoises. © 2016 The Wildlife Society.

The dependence of Hyalomma aegyptium on its tortoise host Testudo graeca in Algeria.

Hyalomma aegyptium (Linnaeus, 1758) (Ixodida: Ixodidae) has recently been confirmed as a carrier of numerous pathogenic, including zoonotic, agents. Four environmentally distinct regions of Algeria, located between the humid coastal zone and the arid Saharan Atlas range, were selected in order to compare differences in tick abundance among localities, and the correlations between tick abundance and host population characteristics and other environmental conditions. Sampling was carried out during May and early June in 2010-2012. A total of 1832 H. aegyptium were removed from 201 tortoises. Adult ticks accounted for 52% of the collection. In the pre-adult stages, larvae were dominant. Data on prevalence, intensity (mean ± standard deviation, range) and abundance of tick infestation were calculated for each locality. Locally, prevalences reached 100%. The sex ratio was biased in favour of males (4.2). Intensities of infestation differed significantly among the localities studied for all developmental stages of the tick. The intensity of infestation by adult ticks was positively correlated to the size of the tortoise and with tortoise population density in the habitat. However, findings for immature tick stages were independent of both variables. No significant correlations between infestation intensities and the climatic parameters tested were found. Immature ticks were observed to prefer the front parts of their tortoise hosts, whereas the majority of adults were attached to the rear parts.

Host contact and shedding patterns clarify variation in pathogen exposure and transmission in threatened tortoiseGopherus agassizii: implications for disease modelling and management

Most directly transmitted infections require some form of close contact between infectious and susceptible hosts to spread. Often disease models assume contacts are equal and use mean field estimates of transmission probability for all interactions with infectious hosts. Such methods may inaccurately describe transmission when interactions differ substantially in their ability to cause infection. Understanding this variation in transmission risk may be critical to properly model and manage some infectious diseases. In this study, we investigate how varying exposure and transmission may be key to understanding disease dynamics in the threatened desert tortoise Gopherus agassizii. We created heterogeneity in Mycoplasma agassizii exposure (the putative bacterial agent of a respiratory disease) by varying the duration of interactions between naturally infected and uninfected captive desert tortoises. Using qPCR, we identified new infections and compared models of transmission probability as a function of contact duration and pathogen load. We then examined the contact patterns of a wild tortoise population using proximity loggers to identify heterogeneity in contact duration. The top-ranked model predicting M.agassizii transmission included a dose term defined as the product of the number of days in proximity to an infected host and the infection level of that host. Models predicted low transmission probability for short interactions, unless the infectious host had a high load of M.agassizii: such hosts were predicted to transmit infection at higher rates with any amount of contact. We observed predominantly short-lived interactions in a free-ranging tortoise population and thus, expect transmission patterns in this population to vary considerably with the frequency and duration of high infection levels. Mean field models may misrepresent natural transmission patterns in this and other populations depending on the distribution of high-risk contact and shedding events. Rapid outbreaks in generally solitary species may result from changes to their naturally low-risk contact patterns or due to increases in the frequency of severe infections or super-shedding events - population characteristics that should be further investigated to develop effective management strategies.

The desert tortoise trichotomy: Mexico hosts a third, new sister-species of tortoise in the Gopherus morafkai-G. agassizii group.

Desert tortoises (Testudines; Testudinidae; Gopherus agassizii group) have an extensive distribution throughout the Mojave, Colorado, and Sonoran desert regions. Not surprisingly, they exhibit a tremendous amount of ecological, behavioral, morphological and genetic variation. Gopherus agassizii was considered a single species for almost 150 years but recently the species was split into the nominate form and Morafka’s desert tortoise, Gopherus morafkai, the latter occurring south and east of the Colorado River. Whereas a large body of literature focuses on tortoises in the United States, a dearth of investigations exists for Mexican animals. Notwithstanding, Mexican populations of desert tortoises in the southern part of the range of Gopherus morafkai are distinct, particularly where the tortoises occur in tropical thornscrub and tropical deciduous forest. Recent studies have shed light on the ecology, morphology and genetics of these southern ‘desert’ tortoises. All evidence warrants recognition of this clade as a distinctive taxon and herein we describe it as Gopherus evgoodei sp. n. The description of the new species significantly reduces and limits the distribution of Gopherus morafkai to desertscrub habitat only. By contrast, Gopherus evgoodei sp. n. occurs in thornscrub and tropical deciduous forests only and this leaves it with the smallest range of the three sister species. We present conservation implications for the newly described Gopherus evgoodei, which already faces impending threats.

Modeling Agassiz's desert tortoise population response to anthropogenic stressors

Agassiz's desert tortoise (Gopherus agassizii) populations are exposed to a variety of anthropogenic threats, which vary in nature, distribution, severity, and frequency. Tortoise management in conservation areas can be compromised when the relative importance of these threats is not well understood. We used HexSim to develop simulation models for desert tortoise populations occupying 2 study areas in the western-central (Superior Cronese in California, USA) and the eastern (Gold Butte-Pakoon in Nevada and Arizona, USA) Mojave Desert, each with a distinct set of site-specific threats. We developed threats models that were parameterized from published information, and conducted independent simulations of threats at varying levels of severity for each study area. Modeled tortoise populations in both study areas were subjected to simulations of threats associated with human presence and subsidized predators. Additional simulated threats in the Superior Cronese model included disease and habitat degradation on land in-holdings, whereas tortoise populations in the Gold Butte-Pakoon model were further exposed to simulations of wildfire, livestock grazing, and feral burros. We used our 2 study area-specific simulation models to rank the threats' relative importance to desert tortoise population viability. Threats more widely distributed in time and space within the modeled conservation areas significantly limited tortoise population growth more than threats that were patchily distributed or temporally dynamic. Our use of a spatially explicit population model allowed us to evaluate and prioritize the effects of threats over site-specific, dynamic, simulated landscapes, which differed from previous modeling efforts for desert tortoises. Our threat prioritization will inform and improve ongoing management efforts attempting to increase desert tortoise population viability by altering anthropogenic disturbance regimes. © 2016 The Wildlife Society.

Wide variation in carapacial scute patterns in a natural population of speckled tortoises, Homopus signatus

The arrangement of scutes on the carapaces of extant chelonians is very similar among species, but intraspecific deviations from typical scute patterns are common. Because intraspecific variation may relate to inbreeding depression, unfavourable egg incubation conditions and the presence of environmental pollutants, investigations of carapacial scute patterns in natural populations can enhance insight into the ecologies and survival challenges of chelonians. A population of the tortoise Homopus signatus, inhabiting an arid range with substantial annual rainfall variation, was sampled to record carapacial scute patterns. The typical pattern of H. signatus consisted of five vertebral scutes, four pairs of costal scutes, a cervical and a supracaudal scute, and 12 pairs of marginal scutes. Although it was expected that size classes (i.e. representing tortoises born in different years) and sexes would have different percentages deviations from the typical scute pattern as a result of different egg incubation conditions, percentages were similar among size classes and sexes. A high percentage of 44% (25% if counts of the relatively variable marginal scutes were excluded) of all tortoises had deviant carapacial scute patterns. Since the study population was situated in a relatively pristine area and supernumerary carapacial scutes appear related to egg incubation conditions in other chelonians, the wide variation in carapacial scute patterns in wild H. signatus may be the result of frequently challenging incubation conditions in the species’ harsh environment.

Effect of Supplemental Heat in Captive African Leopard Tortoises (Stigmochelys pardalis) and Spurred Tortoises (CENTROCHELYS sulcata) on Growth Rate and Carapacial Scute Pyramiding

Carapacial scute pyramiding (CSP) is common in captive-raised turtles and tortoises. Several dietary and environmental hypotheses have been proposed to explain this phenomenon, but few have been scientifically investigated. The effect of increased heat exposure on CSP and growth was examined in juvenile African leopard (Stigmochelys pardalis) and spurred (Centrochelys sulcata) tortoises. Monthly measurements of individual tortoise surface temperature, length, height, width, weight, and pyramid height of treatment (heat) and control (no heat) groups for 2 years were higher in the treatment groups. Allometric comparison further revealed increased pyramiding in the treatment group. Humidity and diet did not differ between treatment and control groups. The results of this research investigation indicate that growth rate and CSP appear to be directly related and both increase with excess nocturnal heat. The significance of the atypical carapace growth and subsequent consequences for naturally occurring tortoise populations, especially in the context of global climate change, is unknown but requires further investigation.

A comparison of line-transect distance sampling methods for estimating gopher tortoise population densities

Line-transect distance sampling (LTDS) is increasingly used to estimate gopher tortoise (Gopherus polyphemus) densities. The process requires detecting tortoise burrows, and then determining occupancy. We compared 3 LTDS approaches that differ in how burrow detection and occupancy data are integrated, and 2 search strategies. Surveys were conducted at Avon Park Air Force Range in South-central Florida, USA, from April through October 2009. These include 1) LTDS using data from occupied burrows only; 2) LTDS with burrows treated as “clusters” of size 1 or 0 depending on occupancy; 3) LTDS to estimate burrow densities, with occupancy modeling to estimate occupancy rates; and 4) 2 search strategies: standard versus expanded searches. The LTDS method produced reliable burrow density estimates, and tortoise densities were estimated most precisely and with least effort using data from occupied burrows only. However, any method could be biased if burrow occupancy is uncertain. Indeed, in the Florida scrub and pine flatwoods habitats, video-camera scoping was hindered by groundwater or obstructions, resulting in undetermined occupancy for 45% of burrows. Improving scope technologies and surveying during the dry season has improved subsequent results for xeric habitats; however, research is needed to improve detectability of tortoises in mesic habitats with flooded burrows. If occupancy is uncertain, we recommend collecting data on all burrows, irrespective of occupancy, so that burrow densities can be estimated even if occupancy cannot. We do not recommend expanding the search area with sigmoid search paths adjacent to transects owing to potential violations of assumptions, nor use of occupancy modeling, because model requirements may lead to unacceptable exclusion of data. © 2015 The Wildlife Society.

Persistence of distinctive morphotypes in the native range of the CITES-listed Aldabra giant tortoise.

Understanding the extent of morphological variation in the wild population of Aldabra giant tortoises is important for conservation, as morphological variation in captive populations has been interpreted as evidence for lingering genes from extinct tortoise lineages. If true, this could impact reintroduction programmes in the region. The population of giant tortoises on Aldabra Atoll is subdivided and distributed around several islands. Although pronounced morphological variation was recorded in the late 1960s, it was thought to be a temporary phenomenon. Early researchers also raised concerns over the future of the population, which was perceived to have exceeded its carrying capacity. We analyzed monthly monitoring data from 12 transects spanning a recent 15‐year period (1998–2012) during which animals from four subpopulations were counted, measured, and sexed. In addition, we analyzed survival data from individuals first tagged during the early 1970s. The population is stable with no sign of significant decline. Subpopulations differ in density, but these differences are mostly due to differences in the prevailing vegetation type. However, subpopulations differ greatly in both the size of animals and the degree of sexual dimorphism. Comparisons with historical data reveal that phenotypic differences among the subpopulations of tortoises on Aldabra have been apparent for the last 50 years with no sign of diminishing. We conclude that the giant tortoise population on Aldabra is subject to varying ecological selection pressures, giving rise to stable morphotypes in discrete subpopulations. We suggest therefore that (1) the presence of morphological differences among captive Aldabra tortoises does not alone provide convincing evidence of genes from other extinct species; and (2) Aldabra serves as an important example of how conservation and management in situ can add to the scientific value of populations and perhaps enable them to better adapt to future ecological pressures.

Biogeography of Parasitic Nematode Communities in the Galápagos Giant Tortoise: Implications for Conservation Management.

The Galápagos giant tortoise is an icon of the unique, endemic biodiversity of Galápagos, but little is known of its parasitic fauna. We assessed the diversity of parasitic nematode communities and their spatial distributions within four wild tortoise populations comprising three species across three Galápagos islands, and consider their implication for Galápagos tortoise conservation programmes. Coprological examinations revealed nematode eggs to be common, with more than 80% of tortoises infected within each wild population. Faecal samples from tortoises within captive breeding centres on Santa Cruz, Isabela and San Cristobal islands also were examined. Five different nematode egg types were identified: oxyuroid, ascarid, trichurid and two types of strongyle. Sequencing of the 18S small-subunit ribosomal RNA gene from adult nematodes passed with faeces identified novel sequences indicative of rhabditid and ascaridid species. In the wild, the composition of nematode communities varied according to tortoise species, which co-varied with island, but nematode diversity and abundance were reduced or altered in captive-reared animals. Evolutionary and ecological factors are likely responsible for the variation in nematode distributions in the wild. This possible species/island-parasite co-evolution has not been considered previously for Galápagos tortoises. We recommend that conservation efforts, such as the current Galápagos tortoise captive breeding/rearing and release programme, be managed with respect to parasite biogeography and host-parasite co-evolutionary processes in addition to the biogeography of the host.

Recommendation for Gopher Tortoise Burrow Buffer to Avoid Collapse from Heavy Equipment

Abstract Gopher tortoises (Gopherus polyphemus) occur in open-canopy pine habitat on well-drained soils in the southeastern United States, where they construct burrows that offer protection from thermal extremes, fire, and predators. Gopher tortoise populations have declined over the past 50 y, primarily as a result of habitat loss and degradation. Southeastern pine forests require active management with prescribed fire, mechanical thinning, or removal of hardwoods to maintain suitable habitat for gopher tortoises. In addition, many pine forests in the Southeast that support gopher tortoise populations are managed for multiple uses including intensive silviculture. Heavy equipment associated with these activities used in proximity to gopher tortoise burrows can cause them to collapse, potentially causing harm to tortoises or other imperiled organisms that use their burrows. Hence, there is a need for practical guidelines for use of heavy equipment for timber harvest, management, and other activities around gopher tortoise burrows to minimize risk to tortoises. We conducted a field study to determine the distance at which heavy equipment caused gopher tortoise burrows to collapse using a feller buncher, rubber-tire front-end loader, and an agricultural tractor with a tree-mower attachment in sandy clay loam (15 burrows) and undifferentiated deep sand (15 burrows) soils at a site in southwestern Georgia. All burrows were confirmed to be unoccupied by tortoises or other vertebrate commensal species using a camera scope before collapse. The greatest mean distance to collapse across all vehicles tested in sandy clay loam and undifferentiated deep sand was 2.19 ± 0.56 m and the maximum distance to collapse was 3 m. Given the variation in collapse distance, we recommend a buffer that extends 4 m in radius from the entrance of the gopher tortoise burrow to minimize risk of collapse from heavy equipment.

Severe mortality of a population of threatened Agassiz’s desert tortoises: the American badger as a potential predator

In the Mojave Desert of the southwestern United States, adult Agassiz's desert tor- toises Gopherus agassizii typically experience high survival, but population declines associated with anthropogenic impacts led to their listing as a threatened Species under the US Endangered Species Act in 1990. Predation of adult tortoises is not often considered a significant threat as they are adapted to deter most predation attempts. Despite these adaptations, some populations have experienced elevated mortality attributed to predators, suggesting that predation pressure may occasionally increase. During the tortoise activity seasons of 2012 and 2013, we observed unsus- tainably high mortality in 1 of 4 populations of adult desert tortoises (22 and 84%, respectively) in the western Mojave Desert in the vicinity of Barstow, CA. Photographic evidence from trail cam- eras and examination of carcass condition suggest that American badgers Taxidea taxus —a sometimes cited but unconfirmed predator of adult tortoises — may have been responsible for some of the mortality observed. We discuss the American badger as a plausible predator of a local tortoise population, but recommend further investigation into these events and the impacts such mortality can have on tortoise persistence.

Kinship, inbreeding and fine-scale spatial structure influence gut microbiota in a hindgut-fermenting tortoise.

Herbivorous vertebrates rely on complex communities of mutualistic gut bacteria to facilitate the digestion of celluloses and hemicelluloses. Gut microbes are often convergent based on diet and gut morphology across a phylogenetically diverse group of mammals. However, little is known about microbial communities of herbivorous hindgut-fermenting reptiles. Here, we investigate how factors at the individual level might constrain the composition of gut microbes in an obligate herbivorous reptile. Using multiplexed 16S rRNA gene sequencing, we characterized the faecal microbial community of a population of gopher tortoises (Gopherus polyphemus) and examined how age, genetic diversity, spatial structure and kinship influence differences among individuals. We recovered phylotypes associated with known cellulolytic function, including candidate phylum Termite Group 3, suggesting their importance for gopher tortoise digestion. Although host genetic structure did not explain variation in microbial composition and community structure, we found that fine-scale spatial structure, inbreeding, degree of relatedness and possibly ontogeny shaped patterns of diversity in faecal microbiomes of gopher tortoises. Our findings corroborate widespread convergence of faecal-associated microbes based on gut morphology and diet and demonstrate the role of spatial and demographic structure in driving differentiation of gut microbiota in natural populations.

Functional relationships reveal keystone effects of the gopher tortoise on vertebrate diversity in a longleaf pine savanna

Keystone species are important drivers of diversity patterns in many ecosystems. Their effects on ecological processes are fundamental to understanding community dynamics, making them attractive conservation targets for ecosystem management. However, many studies assume keystone effects are constant. By developing functional relationships of species’ effects and assessing how they vary with context, we can design more efficient conservation strategies to maintain keystone impacts. The threatened gopher tortoise (Gopherus polyphemus) is presumed to be a keystone species promoting biodiversity in endangered longleaf pine ecosystems of the Southeastern Coastal Plain, USA. Although many commensals use tortoise burrows, their putative keystone influence on emergent diversity patterns lacks critical evaluation. We quantified the functional relationship between tortoise burrow density and non-volant vertebrate diversity in a longleaf pine savanna, located in central Florida. Tortoise burrow density had a positive effect on vertebrate diversity and evenness but did not affect species richness. This relationship was robust across fire disturbance regimes and was the primary factor explaining diversity at the local scale. Our results demonstrate keystone effects of the gopher tortoise through an ecosystem engineering mechanism. Continued gopher tortoise population declines will have large, negative impacts on vertebrate diversity in this biodiversity hotspot. Therefore, maintaining gopher tortoise populations is critical to effectively conserve dependent species and the function of endangered longleaf pine ecosystems. We show that developing a functional understanding of keystone relationships (not a binomial categorization) can lead to important insights into community processes.

Not putting all their eggs in one basket: bet-hedging despite extraordinary annual reproductive output of desert tortoises

Bet-hedging theory makes the counter-intuitive prediction that, if juvenile survival is low and unpredictable, organisms should consistently reduce short-term reproductive output to minimize the risk of reproductive failure in the long-term. We investigated the long-term reproductive output of an Agassiz's desert tortoise (Gopherus agassizii) population and conformance to a bet-hedging strategy of reproduction in an unpredictable but comparatively productive environment. Most females reproduced every year, even during periods of low precipitation and poor germination of food plants, and the mean percentage of reproducing females did not differ significantly on an annual basis. Although mean annual egg production (clutch size × clutch frequency) differed significantly among years, mean clutch size and mean clutch frequency remained relatively constant. During an El Nino year, mean annual egg production and mean annual clutch frequency were the highest ever reported for this species. Annual egg production was positively influenced by maternal body size but clutch size and clutch frequency were not. Our long-term results confirm earlier conclusions based on short-term research that desert tortoises have a bet-hedging strategy of producing small clutches almost every year. The risk of long-term reproductive failure is minimized in unpredictable environments, both through time by annually producing multiple small clutches over a long reproductive lifespan, even in years of low resource availability, and through space by depositing multiple annual clutches in different locations. The extraordinary annual reproductive output of this population appears to be the result of a typically high but unpredictable biomass of annual food plants at the site relative to tortoise habitat in dryer regions. Under the comparatively productive but unpredictable conditions, tortoises conform to predictions of a bet-hedging strategy of reproduction with relatively small but consistent clutch sizes. Published 2015. This article is a U.S. Government work and is in the public domain in the USA, Biological Journal of the Linnean Society, 2015, 115, 399–410.

Multiscale Burrow Site Selection of Gopher Tortoises (Gopherus polyphemus) in Coastal Sand Dune Habitat

Lau, A. and Dodd, C.K., Jr., 2015. Multiscale burrow site selection of gopher tortoises (Gopherus polyphemus) in coastal sand dune habitat.The gopher tortoise (Gopherus polyphemus) is a keystone species in upland sandhill ecosystems of the SE United States, and its habitat requirements have been well documented. Few studies have been conducted on populations that occur in coastal sand dunes. Because of close proximately to the ocean and highly fragmented linear habitat, coastal populations of gopher tortoises are affected by unique landscape factors that are not observed in upland contiguous populations. In this study, burrow site selection of gopher tortoises in a coastal sand dune site was quantitatively modeled. Significant biological, environmental, and anthropogenic factors that may influence burrow site selection at fine and coarse spatial scales were identified. Land cover type, distance to edge, soil resistance, percentage of herbaceous cover, slope angle, and number of tortoise burrows have significant influences on burrow site selection probability. Factors that influence burrow locations in coastal populations thus might differ in importance from those that influence burrow locations in more spatially contiguous upland populations. Our results indicate that coastal gopher tortoise populations require site-specific management that focuses on the limited availability of optimal burrow construction sites.

A plant invasion affects condition but not density or population structure of a vulnerable reptile

By altering ecosystem structure and function, invasions by nonnative plants have the potential to alter the quantity and quality of habitat for animals. We examined effects of buffelgrass (Pennisetum ciliare), a nonnative grass that is increasing in distribution markedly throughout the southwestern United States and northern Mexico, on demographic characteristics and condition of Sonoran desert tortoises (Gopherus morafkai). In 2010 and 2011, we established 50 4-ha plots that spanned the gradient of buffelgrass cover in areas with environmental features characteristic of high-quality habitat for tortoises in southern Arizona. We surveyed each plot four times per year to characterize density, population structure, and condition of tortoises. We detected tortoises on 45 of 50 plots (90 %) and ≥1 tortoise during 114 of 200 surveys (57 %). We used a hierarchical model to estimate density of tortoises while accounting for imperfect detection rates. Density of tortoises averaged 0.35 individuals/ha (SE = 0.04) and did not vary appreciably with the amount of buffelgrass cover; similarly, age and sex structure of tortoise populations did not vary with buffelgrass cover. Condition of adult tortoises, however, averaged 10 % lower in areas where cover of buffelgrass was high (>15 %) relative to areas where buffelgrass was absent or cover was low (<1 %). Although the demographic characteristics we measured on this long-lived species did not vary with current levels of buffelgrass cover, reduced condition of tortoises in areas invaded by this nonnative grass could manifest as population-level effects over longer time periods, especially because buffelgrass is predicted to expand its current distribution.

Desert tortoises (Gopherus agassizii) are selective herbivores that track the flowering phenology of their preferred food plants.

Previous studies of desert tortoise foraging ecology in the western Mojave Desert suggest that these animals are selective herbivores, which alter their diet according to the temporal availability of preferred food plants. These studies, however, did not estimate availability of potential food plants by taking into account the spatial and temporal variability in ephemeral plant abundance that occurs within the spring season. In this study, we observed 18 free-ranging adult tortoises take 35,388 bites during the spring foraging season. We also estimated the relative abundance of potential food plants by stratifying our sampling across different phenological periods of the 3-month long spring season and by different habitats and microhabitats. This methodology allowed us to conduct statistical tests comparing tortoise diet against plant abundance. Our results show that tortoises choose food plants non-randomly throughout the foraging season, a finding that corroborates the hypothesis that desert tortoises rely on key plants during different phenological periods of spring. Moreover, tortoises only consumed plants in a succulent state until the last few weeks of spring, at which time most annuals and herbaceous perennials had dried and most tortoises had ceased foraging. Many species of food plants—including several frequently eaten species—were not detected in our plant surveys, yet tortoises located these rare plants in their home ranges. Over 50% of bites consumed were in the group of undetected species. Interestingly, tortoises focused heavily on several leguminous species, which could be nutritious foods owing to their presumably high nitrogen contents. We suggest that herbaceous perennials, which were rare on our study area but represented ~30% of tortoise diet, may be important in sustaining tortoise populations during droughts when native annuals are absent. These findings highlight the vulnerability of desert tortoises to climate change if such changes alter the availability of their preferred food plants.

Naturally rare versus newly rare: demographic inferences on two timescales inform conservation of Galápagos giant tortoises.

Long-term population history can influence the genetic effects of recent bottlenecks. Therefore, for threatened or endangered species, an understanding of the past is relevant when formulating conservation strategies. Levels of variation at neutral markers have been useful for estimating local effective population sizes (Ne) and inferring whether population sizes increased or decreased over time. Furthermore, analyses of genotypic, allelic frequency, and phylogenetic information can potentially be used to separate historical from recent demographic changes. For 15 populations of Galápagos giant tortoises (Chelonoidis sp.), we used 12 microsatellite loci and DNA sequences from the mitochondrial control region and a nuclear intron, to reconstruct demographic history on shallow (past ∽100 generations, ∽2500 years) and deep (pre-Holocene, >10 thousand years ago) timescales. At the deep timescale, three populations showed strong signals of growth, but with different magnitudes and timing, indicating different underlying causes. Furthermore, estimated historical Ne of populations across the archipelago showed no correlation with island age or size, underscoring the complexity of predicting demographic history a priori. At the shallow timescale, all populations carried some signature of a genetic bottleneck, and for 12 populations, point estimates of contemporary Ne were very small (i.e., < 50). On the basis of the comparison of these genetic estimates with published census size data, Ne generally represented ∽0.16 of the census size. However, the variance in this ratio across populations was considerable. Overall, our data suggest that idiosyncratic and geographically localized forces shaped the demographic history of tortoise populations. Furthermore, from a conservation perspective, the separation of demographic events occurring on shallow versus deep timescales permits the identification of naturally rare versus newly rare populations; this distinction should facilitate prioritization of management action.

Thesis Abstract Morphological and phylogeographic analysis of Brazilian tortoises (Testudinidae).

Thesis Abstract Morphological and phylogeographic analysis of Brazilian tortoises (Testudinidae).