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Chelonoidis chathamensis 

Scope: Global
Language: English
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Taxonomy [top]

Kingdom Phylum Class Order Family
Animalia Chordata Reptilia Testudines Testudinidae

Scientific Name: Chelonoidis chathamensis (Van Denburgh, 1907)
Common Name(s):
English San Cristóbal Giant Tortoise, Chatham Island Giant Tortoise
Spanish Tortuga Gigante de San Cristóbal
Synonym(s):
Chelonoidis nigra ssp. chathamensis (Van Denburgh, 1907)
Geochelone elephantopus ssp. chathamensis (Van Denburgh, 1907)
Geochelone nigra ssp. chathamensis (Van Denburgh, 1907)
Testudo chathamensis Van Denburgh, 1907
Taxonomic Source(s): Fritz, U. and Havas, P. 2007. Checklist of chelonians of the world. Vertebrate Zoology 57(2): 149-368.
Taxonomic Notes:

The previous Red List assessments for Galápagos tortoises treated the various allopatric island populations as subspecies of Chelonoidis nigra (now named Chelonoidis niger), as did several authors (Pritchard 1996, Caccone et al. 1999, Beheregaray et al. 2003, Fritz and Havas 2007,TTWG 2007, Rhodin et al. 2008). However, other authors have considered them as full species based on morphology (Bour 1980, Fritts 1983, Ernst and Barbour 1989) and the more recent consensus among researchers (Caccone et al. 2002; Russello et al. 2005, 2007; Poulakakis et al. 2008, 2012, 2015; Chiari et al. 2009) is to treat most as full species based on congruent patterns of mitochondrial and nuclear variation. This elevated species-level taxonomy has been largely accepted by TTWG (2009, 2014) and TEWG (2015) for most, but not all, phylogenetic lineages of Galápagos tortoises. This Red List assessment therefore now treats C. chathamensis as a full species, rather than retaining its previous subspecies ranking from earlier Red List assessments.

Pritchard (1996) restricted the name chathamensis to the now-extirpated tortoise populations of southwestern and central San Cristóbal, but excluded the eastern San Cristóbal population, considering this population to represent a possibly distinct subspecies for which no name was available. For purposes of this Red List Assessment, the eastern San Cristóbal population is considered part of chathamensis until taxonomic clarity emerges.

Testudo wallacei Rothschild, 1902, was described with type locality “Chatham Island?”, but its type locality was designated Jervis Island [Rábida] by Van Denburgh (1914); Rábida tortoises were hypothesized to be an introduced population (Pritchard 1996), later confirmed by genetic analysis (Poulakakis et al. 2012) to represent tortoises from southern Isabela, and the name wallacei is considered a nomen dubium and synonym of C. vicina (Fritz and Havas 2007, TTWG 2014).

Assessment Information [top]

Red List Category & Criteria: Endangered A1abde ver 3.1
Year Published: 2017
Date Assessed: 2017-01-15
Assessor(s): Caccone, A., Cayot, L.J., Gibbs, J.P. & Tapia, W.
Reviewer(s): Rhodin, A.G.J. & van Dijk, P.P.
Contributor(s): IUCN Galapagos Tortoises Red Listing Workshop & Galapagos National Park Directorate
Facilitator/Compiler(s): van Dijk, P.P.
Justification:

The San Cristóbal tortoise population experienced catastrophic decline due to exploitation in historical time (less than three generations ago, at a generation length of 60 years), with recruitment and recovery severely restrained by the impacts of introduced predators, competitors, and vegetation change, from an estimated population size of  24,000 animals historically (J.P. Gibbs unpubl. data), to an estimated 500-700 tortoises in the early 1970s (MacFarland et al. 1974). The population showed a gradual recovery to at least 1,170 animals (juveniles and adults combined) in 2005. A more complete mark/recapture census of this population carried out in 2016 estimated a population of 6,700 tortoises (1,938 tortoises were observed; 44% reproductive adults, resulting in an estimated adult population of ~2,950), representing an overall decline of about 88% and with most threats having largely ceased, qualifying the species as Endangered EN A1abde. The assessment of this taxon in 1996 as the subspecies C. nigra ssp. chathamensis as Vulnerable was based on the very small population size, but did not reflect the historical population decline that the current assessment incorporates. This assessment also incorporates contributions from the international workshop on Galápagos tortoises convened by the Galápagos National Park Directorate in July 2012, as well as the most complete census of tortoises ever carried out on San Cristóbal, in November 2016.

Previously published Red List assessments:

Geographic Range [top]

Range Description:

Chelonoidis chathamensis (sensu stricto) occurred on southwestern and central San Cristóbal [formerly Chatham Island] in the Galápagos Islands of Ecuador; whether the population of northeastern San Cristóbal is part of chathamensis remains to be determined. San Cristóbal has a total surface area of 560 square km, of which about 250 sq.km (45%) could potentially sustain tortoises (J.P. Gibbs unpubl. data).

Countries occurrence:
Native:
Ecuador (Galápagos)
Additional data:
Estimated area of occupancy (AOO) - km2:250
Range Map:Click here to open the map viewer and explore range.

Population [top]

Population:

Based on an estimated average density of one tortoise per hectare of suitable habitat, the pre-human population size was estimated to have been about 24,000 adults (J.P. Gibbs unpubl. data). The population was estimated in the early 1970s at 500-700 individuals (MacFarland et al. 1974). Results of the complete census carried out in November 2016 give a total population estimate of ~6,700 (44% adult, ~2,950; 56% juveniles and hatchlings, ~3,750); this appears to be a rapidly growing and healthy recovering population. Genetic analyses have revealed that the current population is recovering from a severe bottleneck that depleted its genetic diversity (Garrick et al. 2014).

Current Population Trend:Increasing
Additional data:
Number of mature individuals:2950Continuing decline of mature individuals:No
Continuing decline in subpopulations:No

Habitat and Ecology [top]

Habitat and Ecology:

While detailed life history data are not available for the wild populations of Chelonoidis chathamensis, observations on long-term captive animals indicate that the species matures at about 20 years of age and longevity of 100-150 years is not unusual; generation time is thus estimated at ca 60 years. Chelonoidis chathamensis tortoises currently found in the eastern portion of San Cristóbal are large saddlebacked tortoises adapted for browsing on higher-growing vegetation in drier shrubland ecosystems.

Systems:Terrestrial
Generation Length (years):60
Movement patterns:Unknown

Use and Trade [top]

Use and Trade: Tortoises on San Cristóbal were subject to extensive overexploitation for food by sailors and settlers in the 18th and 19th centuries, from which they are beginning to recover, although the population in the southwestern part of the island appears to have been  extirpated by ca 1933 (Pritchard 1996).

Threats [top]

Major Threat(s):

Tortoises on San Cristóbal were subject to extensive overexploitation for food by sailors and settlers in the 18th and 19th centuries, from which they are beginning to recover, although the population in the southwestern part of the island appears to have been  extirpated by ca 1933 (Pritchard 1996). Invasive species impacts have been relatively moderate in San Cristóbal, compared to other islands in Galápagos: feral cats and rats represent a problem, causing hatchling / juvenile tortoise mortality. The goat population on San Cristóbal is relatively low, consequently there is dense, thick vegetation. Feral dogs were eliminated by the resident human population in the 1970s; feral pigs are still present. There are minimal introduced plants in the main tortoise area, and no Solenopsis ants recorded to date. Rabbits were present on some farms but were eliminated a decade ago; they never dispersed into the wild. The growing tortoise population is beginning to disperse back into the western portion of the island; however, some of that habitat is now covered with dense thickets of the invasive introduced species — blackberry (Rubus niveus) and supirosa (Lantana camara) — while other parts have been converted into agricultural land and areas of human habitation. Connections between lowland and upland areas that likely were used historically by tortoises are now largely blocked by the thickets of introduced species, hindering this natural dispersal associated with a growing population. Potential for human-wildlife conflict exists in farmland areas.

Conservation Actions [top]

Conservation Actions:

Legislation and regulations: Chelonoidis chathamensis is protected under Ecuadorian national law. It has been included in Appendix I of CITES since 1975, prohibiting all forms of commercial international trade. Much of San Cristóbal (87%), including the majority of the area of tortoise occurrence, is protected as part of the Galápagos National Park.

Detailed genetic profiling of the animals in the conservation breeding centre is underway; blood samples were collected from 376 tortoises during the 2016 census for subsequent genetic analysis. Genetic analyses to assess the relation between the extant San Cristóbal population and museum specimens are underway. Conservation actions needed are: genetic analyses, habitat assessment in the western highlands where tortoises are beginning to disperse, and control of invasive species.


Citation: Caccone, A., Cayot, L.J., Gibbs, J.P. & Tapia, W. 2017. Chelonoidis chathamensis. The IUCN Red List of Threatened Species 2017: e.T9019A82688009. . Downloaded on 12 December 2017.
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