Population Of Giant Tortoises Research Articles

New insights into the origin of the Galápagos tortoises with a tip-dated analysis of Testudinidae

ABSTRACT The giant tortoises of the Galápagos Archipelago influenced Darwin’s early thinking on transmutation and now constitute a famous example of island-based speciation and evolutionary dynamics. Questions regarding their evolutionary origin still exist and include whether their giant size evolved before or after the lineage arrived in the Galápagos from the South American continent. Here we present new evidence supporting the ‘before’ hypothesis based on previously reported fossil specimens from the Late Pleistocene of Ecuador. Despite the isolated and fragmentary nature of these tortoise fossils, phylogenetically informative features are preserved. A tip-dated phylogenetic analysis recovers the Ecuador form as the sister taxon to the Galápagos lineage, thus supporting the existence of a closely related, giant tortoise population at approximately the same time and place that the Humboldt Current is thought to have carried the ancestral Galápagos tortoise to the archipelago. The phylogeny also raises the possibility that the same evolution of continental gigantism also facilitated the dispersal of South American tortoises into the Caribbean Basin where they diversified before succumbing to Holocene extinction.

Rewilding giant tortoises engineers plant communities at local to landscape scales

AbstractTrophic rewilding is increasingly being used to promote megafauna reintroductions to island ecosystems, yet ecosystem response to population restoration once megafauna reintroduction occurs remains understudied. In this study of a population of Galapagos giant tortoises reintroduced to an arid island, tortoise exclosures monitored over an 8‐year‐long period revealed that, in response to the presence of tortoises, herbaceous plant cover and numbers of regenerating woody plants decreased, whereas extent of grass cover increased. Vegetation mapping over a 15‐year‐long period across the island indicated a threshold density of 1–2 tortoises per hectare halted incursion of woody plants and triggered a shift in this savannah‐type ecosystem toward more grasses. Restoration of this giant tortoise population has shaped plant communities at both local and landscape scales with cascading effects on many components of biodiversity on the island.

Giant tortoises hunt and consume birds

Tortoises (land turtles) are familiar animals and are generally assumed to be strict herbivores. Their ecological roles are most obvious in giant tortoise species which, due to their size and local abundance, play major roles as keystone species and ecosystem engineers1-3. In the Galápagos and Seychelles islands these species are known to play major roles as the islands' largest herbivores, with exceptionally high biomass and consuming up to 11% of primary production1. In addition they act as ecosystem engineers, dispersing seeds, breaking vegetation and eroding rocks2. However, as slow-moving poikilotherms most people assume their behaviour to be simple. Here we present video evidence of a Seychelles giant tortoise (Aldabrachelys gigantea) attacking a tern chick and pursuing it along a log. Finally the tortoise killed the chick and was observed to eat it. Other tortoises in the same area have been seen making similar attacks, although those were not fully documented. We believe that the exceptional combination of a tree-nesting tern colony with a resident giant tortoise population has created conditions leading to systematic hunting of birds by several individual tortoises; an entirely novel behavioural strategy for any tortoise species. VIDEO ABSTRACT.

Introduction of giant tortoises as a replacement “ecosystem engineer” to facilitate restoration of Santa Fe Island, Galapagos

Re‐establishment of ecosystem engineers via introduction of replacement species is increasingly being proposed to facilitate ecosystem restoration of island ecosystems around the world, yet attendant challenges and risks remain largely unknown. We evaluate the first phase (2015–2020) of a program to restore a population of giant tortoises, ecosystem engineers of terrestrial ecosystems of the Galapagos Islands, to Santa Fe Island during which a total of 551 juvenile and 31 subadult tortoises of the non‐native Española Island species (Chelonoidis hoodensis) were translocated. After 5 years, most (85%) of tortoises released remain alive, exhibiting robust vital rates comparable to those of conspecifics in their native range, having colonized 10% of the island via gradual dispersal from the point‐of‐release. Populations of an arboreal cactus (Opuntia echios)—a keystone species for many organisms on the island and intended beneficiary of tortoise restoration—and pallid land iguanas (Conolophus pallidus)—an endemic, herbivorous reptile considered potentially at risk from the introduction of tortoises, both increased coincident with the arrival of tortoises. Herbivore exclosure experiments revealed that the island's plant community manifested a response to the arrival of tortoises; however, significant engineering of the island's vegetation will require many decades. These results highlight the first stage of what appears to be a successful conservation intervention that can serve as a “proof of concept” for introduction of ecological replacements to other islands in Galapagos, and potentially for other island ecosystems around the world.

Giant tortoise genomes provide insights into longevity and age-related disease

Giant tortoises are among the longest-lived vertebrate animals and, as such, provide an excellent model to study traits like longevity and age-related diseases. However, genomic and molecular evolutionary information on giant tortoises is scarce. Here, we describe a global analysis of the genomes of Lonesome George—the iconic last member of Chelonoidis abingdonii—and the Aldabra giant tortoise (Aldabrachelys gigantea). Comparison of these genomes with those of related species, using both unsupervised and supervised analyses, led us to detect lineage-specific variants affecting DNA repair genes, inflammatory mediators and genes related to cancer development. Our study also hints at specific evolutionary strategies linked to increased lifespan, and expands our understanding of the genomic determinants of ageing. These new genome sequences also provide important resources to help the efforts for restoration of giant tortoise populations.

Giant tortoise habitats under increasing drought conditions on Aldabra Atoll—Ecological indicators to monitor rainfall anomalies and related vegetation activity

Aldabra Atoll has the largest population of giant tortoises (Aldabrachelys gigantea) in the world. As such an important biological resource, it is necessary to understand how the effects of climate change will impact this keystone species; in particular the frequency of drought, which is likely to affect tortoise habitat. To assess whether drought frequency has changed over the last 50 years on Aldabra, we calculated the standardized precipitation index (SPI) to identify drought periods using monthly rainfall data collected during 1969–2013. We found that drought frequency has increased to more than six drought months per year today compared with about two months per year in the 1970s (t=2.884, p=0.006). We used MODIS normalized difference vegetation index (NDVI) as a proxy for vegetation activity, to determine how vegetation has responded to the changing drought frequency between 2000 and 2013. We found that Aldabra’s vegetation is highly responsive to changes in rainfall: anomalies in long-term mean monthly NDVI across Aldabra were found to decrease below the mean during most drought periods and increase above the mean during most non-drought periods. To investigate the response of tortoise habitat to rainfall, we extracted mean NDVI anomalies for three key habitat types. Open mixed scrub and grasslands, the preferred habitat of tortoises, showed the greatest decrease in vegetation activity during drought periods, and the greatest increase in average greenness during non-drought periods. Recent analysis has shown vegetation changes on Aldabra in recent decades. If these changes are caused by decreased precipitation, then the increased frequency of drought could impact the tortoise population, in both the short and long term, by limiting the quality and quantity of forage and/or shade availability within favoured habitats, and by changing the habitat composition across the atoll.

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.

Densities of Ecological Replacement Herbivores Required to Restore Plant Communities: A Case Study of Giant Tortoises on Pinta Island, Galápagos

AbstractLoss of native herbivores and introduction of livestock in many arid and semi‐arid ecosystems around the world has shifted the competitive balance from herbaceous to woody plants, leading to biodiversity loss, reduced plant productivity, and soil erosion. To restore functions of these ecosystems, ecological replacements have been proposed as substitutes for extinct native herbivores. Here we predict how an ecological replacement giant tortoise population (Chelonoidis spp.) would interact with woody plants on Pinta Island in the Galápagos Archipelago, where a small group of replacement tortoises was introduced in 2010 to initiate restoration of the island's plant community. We developed an individual‐based, spatially explicit simulation model that incorporated field‐derived tortoise behavior and tortoise–plant interaction data to test whether tortoise introductions could lead to broad‐scale changes in the plant community and, if so, at what tortoise densities. Tortoises reduced vegetation density in most (81%) 50‐year‐long simulations if the tortoise density was at least 0.7 per hectare, a value well below typical densities. In a smaller proportion of simulations (30%), tortoises increased local vegetation patchiness. Our results suggest that even moderate‐density tortoise populations can reverse woody plant encroachment. Deployment of ecological replacement giant tortoises may therefore be a viable approach for restoring other arid and semi‐arid ecosystems where a native herbivore that previously had strong interactions with the plant community has gone extinct.

Foraging Behavior and Diet Preferences of a Released Population of Giant Tortoises in the Seychelles

Abstract This study evaluates the potential impacts of the release of the giant tortoise, Dipsochelys arnoldi, to vacant habitat within the species' presumed historic range. Five individuals (3 males and 2 females) were released in December 2006 in the isolated Grande Barbe area of Silhouette Island, Seychelles. A comprehensive vegetation survey of all plant species within feeding height of the tortoises in Grande Barbe was conducted. These data, combined with daily feeding observations, were used to calculate feeding rates and diet preferences. Tortoises were observed to have a mean diurnal active time of 257 minutes per day exhibiting a mean feeding rate of 3.86 g of plant material consumed per minute of active time. Individuals were recorded feeding on 18 of 26 recorded plant species, 9 of which were selectively foraged. Species composition of plant species ingested differed between sexes. Males selectively foraged on 6 plant species and females selectively foraged on 7 species. Only 2 plant species we...

Genetic divergence, phylogeography and conservation units of giant tortoises from Santa Cruz and Pinzón, Galápagos Islands

Island radiations can offer challenging systemsfor the implementation of conservation policiesbecause descendent populations may exhibitdifferent levels of adaptive divergence,reproductive isolation, and phylogeneticdistinctiveness. This seems particularly truefor the endangered Galapagos gianttortoises (Geochelone nigra), whichcomprise a lineage that radiated rapidly andconcomitantly with the evolution of thearchipelago. We used mitochondrial DNAsequences and microsatellite markers toinvestigate the genetic structure, and toreconstruct genealogical relationships and thehistory of population colonization of gianttortoises from the Islands of Santa Cruz andPinzon, including samples of a basal taxonfrom the Island of San Cristobal.Populations displayed marked geneticdivergence, contrasting demographic histories,and deep phylogeographic structure. The patternof diversification among populations wasconsistent with geological and biogeographichistory, and to some extent, with adaptive andmorphological divergence. Results stronglyindicate the presence of a minimum of fourconservation units with long-standingevolutionary separation: two in Santa Cruz, onein Pinzon, and one in San Cristobal. Wepropose that these findings be effectivelyintegrated with other existing data by theappropriate environmental agencies to evaluatecurrent conservation efforts and implement newstrategies aimed at protecting the integrityand diversity of giant tortoise populations.

The evolutionary origin of Indian Ocean tortoises ( Dipsochelys)

Today, the only surviving wild population of giant tortoises in the Indian Ocean occurs on the island of Aldabra. However, giant tortoises once inhabited islands throughout the western Indian Ocean. Madagascar, Africa, and India have all been suggested as possible sources of colonization for these islands. To address the origin of Indian Ocean tortoises ( Dipsochelys, formerly Geochelone gigantea), we sequenced the 12S, 16S, and cyt b genes of the mitochondrial DNA. Our phylogenetic analysis shows Dipsochelys to be embedded within the Malagasy lineage, providing evidence that Indian Ocean giant tortoises are derived from a common Malagasy ancestor. This result points to Madagascar as the source of colonization for western Indian Ocean islands by giant tortoises. Tortoises are known to survive long oceanic voyages by floating with ocean currents, and thus, currents flowing northward towards the Aldabra archipelago from the east coast of Madagascar would have provided means for the colonization of western Indian Ocean islands. Additionally, we found an accelerated rate of sequence evolution in the two Malagasy Pyxis species examined. This finding supports previous theories that shorter generation time and smaller body size are related to an increase in mitochondrial DNA substitution rate in vertebrates.

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.

A survey of the Aldabra giant tortoise population introduced on Curieuse Island, Seychelles

The Zoological Society of London carried out a field study in the Republic of the Seychelles to obtain biomedical data from the Aldabra giant tortoise population on Curieuse island and assess the progress of the colony within the context of the Curieuse experiment. The biomedical data will be published in full detail elsewhere, but brief details of the sampling methods are included as an integral part of the handling procedure to which the tortoises were subjected. From evidence gathered in the field it appears that the tortoises have adapted well to the environment, are causing little damage and represent a major tourist attraction.The impact of the tourist industry on the Curieuse experiment was assessed and found to be a source of concern in relation to the successful management of the island.A total of 144 post hatchlings, 1 juvenile and 17 hatchlings were found, distributed in 6 out of 9 designated areas on Curieuse. These represent less than 50% of the total of tortoises re-introduced as part of the Curieuse experiment.There is strong evidence of poaching and theft, probably by local inhabitants.

Clinical and pathological findings related to malnutrition and husbandry in captive giant tortoises (Geochelone species).

A biomedical survey was carried out on the population of giant tortoises housed in some of the major zoological collections in the United Kingdom. Wasting syndrome was diagnosed in individuals which were kept under suboptimal conditions of housing and diet. Clinical manifestations and laboratory and post mortem findings are discussed.

Population processes in a large herbivorous reptile: the giant tortoise of Aldabra atoll.

Physical barriers divide the population of giant tortoises (Geochelone gigantea Schweigger) on Aldabra into several sub-populations of different density, which nevertheless are similar genetically. We measured individual growth rates in each sub-population. Mortality was estimated using data from Bourn and Coe (1979). Reproduction and recruitment were studied using data from previous work (Swingland and Coe 1979) and our own estimates of clutch size, egg weight, and laying frequency from 1975 to 1981.Individual growth rates were strongly dependent only on individual size and sub-population density and not on age or sex. Within a sub-population, the relationship between specific growth rate and size (linear measure) was best fitted by a Gompertz model, except for very young tortoises which grew faster in volume, though not in weight, than expected. Animals at high densities grow slowly to a small size whereas those at low densities grow fast to a large size. At very high density many juveniles remain at a small size without growing or maturing.Mortality of larger (> ca. 5 years old) animals was independent of density, but did depend on size in the highest-density sub-population, as predicted by the Gompertz growth model.Reproduction and recruitment were negatively density-dependent over the whole density range (5 to 35 animals ha-1) studied. Clutch size and laying frequency were strongly influenced by sub-population density, but egg weight was not. Laying frequency varied within sub-populations according to rainfall (presumably via annual food supply).All except one sub-population are seen as stages in the development of the same interactive system. Competition between individuals is nearly, but not purely, of scramble type. The remaining sub-population is either a distinct interactive system in which food supply for very young animals is important, or it is a non-interactive system controlled by the effect of natural enemies on very young animals. This suggests that the equilibrium density and/or dynamics of giant tortoise populations are highly sensitive to mortality factors affecting very young animals.In low density sub-populations the animals are large, have many young, low relative reproductive effort, and a short generation time. In high density sub-populations they are small, have few young, high relative reproductive effort, and a long generation time. This variation is largely phenotypic. It is anomalous with respect to r-K life history theory but is a logical consequence of indeterminate growth combined with size-determined risk and benefit functions and may have contributed to the giant tortoises' success as island colonisers.

The vegetation of Aldabra Atoll: preliminary analysis and explanation of the vegetation map

This paper describes the construction of a vegetation map of Aldabra atoll in the Indian Ocean, at a scale of 1:25000, and discusses the island’s vegetation in terms of its structure and relationships with other vegetation types. The map was based on a classification of 487 rélevés, in each of which the numbers of each species of woody plant and the percentage cover of different species of ground layer vegetation were recorded. Classification was done within a hierarchy of criteria, of which the relative abundance of common woody plants was taken to be the most important, followed by the relative abundances of ground layer species, the other woody plant species, and total woody plant and ground layer vegetation cover. Twenty-six vegetation types were mapped at a 1:25000 scale. The vegetation varied from open ground, with sparse vegetation on bare rock, through a range of scrub forest types, to structured vegetation 10-15 m tall. Changes in vegetation type were spatially correlated with topographical changes, with shelter from the prevailing southeast trade winds, and with the amount of grazing by giant tortoises. Patterns of environmental and biotic change resulted in vegetation heterogeneity at scales ranging from decimetres to kilometres. The nature of these mosaic patterns needs further investigation, but they probably have a strong effect on the vegetation and on the giant tortoise population. Aldabra island has a remarkably species-rich flora for an atoll. This may be due to its being a raised atoll, to its large land area per se , to the land area providing scope for the development of an unusually large freshwater lens, and to the shape and structure of the atoll providing a range of wind-shelter characteristics. The vegetation types are similar to those found on raised reef and other coralline deposits on the East African coast. In earlier times these vegetation types may have been typical of raised reef deposits throughout the world: lack of quantitative recording and especially the extent of human interference on other raised atolls mean that Aldabra’s vegetation is now an isolated example of this set of vegetation types. The effects of a large herbivorous reptile help to set the vegetation structure and make Aldabra unique.

The primary production of aldabra atoll, with reference to habitats used by Giant Tortoises

This paper reports on the seasonal changes of standing crop and production in habitats used by a high-density population of giant tortoises on Aldabra atoll. The study had two main aims: first to investigate the primary production of a raised coral atoll (to our knowledge the first such study) and secondly to provide base data for a study of the interactions of a large reptilian herbivore (the giant tortoise) with its food supply and environment. Environmental heterogeneity made it necessary to measure separately the standing crop and the above-ground net production of different components of the vegetation; these components were usually single species or small groups of species of plants. Measurements of these components were then combined with cover data for the same components in selected places to illustrate the seasonal and spatial variability of primary production on Aldabra. Standing crop biomasses were estimated from harvest samples. Methods for production estimates varied with the component studied, but included harvest difference methods, repeated clipping of the same plots and direct measurement of leaf turnover rates on marked shoots. These methods are compared where appropriate. Net annual above-ground production varied between plant types from 3165 kJ per square metre of plant for ‘tortoise turf’ to 47700 kJ m -2 for Cyperus ligularis , a robust perennial sedge. Total above-ground annual net production of different habitat types (bare ground and rocks between plants being taken into account) varied from 9100 kJ m -2 in a thinly wooded area with high tortoise turf cover (‘open mixed scrub’) to 28200 kJ m -2 in an area of thick scrub forest (‘groves’). The seasonality of production and standing crop also varied considerably between habitats, owing to the role of different components of the ground layer and shrub cover. These results are discussed in terms of the roles of environmental and structural heterogeneity in setting primary production and of the tortoises themselves in their interactions with the vegetation via trampling and grazing. The effect of this heterogeneity on sampling strategies and results is also assessed.

Feeding, digestion and assimilation of a population of giant tortoises (Geochelone gigantea (Sehweigger)) on Aldabra atoll

This paper describes an investigation of food processing by the Giant Tortoise (Geochelone gigantea (Schweigger)) on Aldabra atoll in the Indian Ocean. The dry weight assimilation efficiency of the Aldabran tortoise feeding on tortoise turf was found to be 30 per cent, while energy assimilation was calculated as 34·5 per cent. Weight specific consumption varied with body size according to diet and season. Some hollocellulose was broken down during the passage of food through the gut, but it is not clear whether cellulose breakdown is a regular feature of tortoise digestion. Throughput times for passage of food through the gut show a log-normal distribution with median throughput time of 8·8 days on tortoise turf and 12·0 days on the much coarser coastal grass Sporobolus virginicus (L.). Body composition was determined by the dissection of 8 animals. Gut contents accounted for 15 per cent of live weight, and skin, bone and scute for a further 47 per cent, or 68 per cent of dry weight. The average calorific value of tortoise tissue (excluding gut contents) was calculated as 4274 kJ/kg (fresh weight). Results on consumption and assimilation rates and energy contents were combined with data from earlier studies to produce an animal energy budget for the South East Grande Terre population. Discussion focuses on features of the energy budget of this population, which supports a high standing crop biomass on low rates of consumption and assimilation. Both turnover rates (P/B) and production efficiency (P/A) are below 1 per cent and the population appears to be food-limited. Comparisons are made with African wildlife areas with similar rainfall, dominated by large herbivorous mammals, particularly those dominated by elephant, and with lower density tortoise populations on other Aldabra islands. The interactions of food supply, growth rates, reproductive rates and population density are discussed.

A Comparison of Two Methods of Estimating the Size of a Population of Giant Tortoises on Aldabra

SUMMARY From a sample census of giant tortoises in the south-eastern region of Aldabra atoll, the size of the markable population was estimated as 87 300. A subsequent recapture study of the same population gave an estimate of 68 100. Sampling errors are unlikely to account for the difference, which is significant at the 1% level. After taking into account obvious violations of the underlying assumptions the difference remains. The sample census estimate is thought to be the more reliable. largely based on data from this sample census. The bulk of the tortoise population is in the south-east of Grande Terre (for map see Bourn & Coe 1978) where recapture work was subsequently conducted. An alternative estimate of the size of the population in this area, the Petersen estimate, was obtained from mark-recapture analysis. As it was hoped that the marking programme would form a foundation for a more detailed description of the population dynamics, it was desirable that the basic assumptions of mark-recapture analysis be seen to be satisfied. As the number of tortoises recaptured (221) was large compared with many recapture studies there is an unusual opportunity to test the main assumption underlying the Petersen method, that marked and unmarked animals have the same probability of being caught in the second sample. Seber (1973) observed that it is not always easy to detect departures from this assumption and quoted examples to show that the Petersen estimate may be unreliable even when the assumptions appear to be satisfied. He recommended that 'if the Petersen estimate is to be used extensively for a given species then it should be compared with other methods'. In this paper the Petersen estimate of population size is compared with the sample census estimate, and the advantages and disadvantages of both methods are considered.

Historical records of Indian Ocean giant tortoise populations

Historical records from the seventeenth to the nineteenth centuries indicate that giant tortoises became extinct on many western Indian Ocean islands during this period, and that by 1900 the surviving population on Aldabra was very small. These data are reviewed, the causes of decline and extinction discussed, and the implications for the history of the Aldabra population are considered.