|Scientific Name:||Chelonoidis porteri (Rothschild, 1903)|
Chelonoidis nigra ssp. nigrita (Duméril & Bibron, 1835)
Chelonoidis nigra ssp. porteri (Rothschild, 1903)
Chelonoidis porteri (Rothschild, 1903)
Geochelone elephantopus ssp. nigrita (Duméril & Bibron, 1835)
Geochelone elephantopus ssp. porteri (Rothschild, 1903)
Geochelone nigra ssp. nigrita (Duméril & Bibron, 1835)
Geochelone nigra ssp. porteri (Rothschild, 1903)
Testudo nigrita Duméril & Bibron, 1835
Testudo porteri Rothschild, 1903
|Taxonomic Source(s):||Fritz, U. and Havas, P. 2007. Checklist of chelonians of the world. Vertebrate Zoology 57(2): 149-368.|
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. porteri as a full species, rather than retaining its previous subspecies ranking from earlier Red List assessments.
|Red List Category & Criteria:||Critically Endangered A2bde ver 3.1|
|Assessor(s):||Cayot, L.J., Gibbs, J.P., Tapia, W. & Caccone, A.|
|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. & Rhodin, A.G.J.|
The historical population estimate is 35,000 animals for the western Santa Cruz population (within and surrounding the current Tortoise Reserve, or “La Reserva”) (J.P. Gibbs unpubl. data), before the extensive exploitation in the 19th century (less than three generations ago, at generation length of 60 years). This population is currently estimated at about 3,400 animals (Gibbs, unpubl. data, 2010), indicating a three-generation decline rate of over 90%, and its ongoing recovery remains compromised by the impacts of introduced predators (pigs, black rats, fire ants), invasive vegetation, agricultural land use, barriers to migratory routes, and occasional human-related mortality (consumption, roadkill). For the Western Santa Cruz tortoise population, a historical decline of over 90% is realistic, and the population qualifies as Critically Endangered, CR A2bde. "Chelonoidis nigra ssp. porteri" was previously assessed in the 1996 IUCN Red List as Endangered C2a (<2,500 adults, in decline, fragmented, no subpopulation larger than 250 adults), but at that time also included the eastern Santa Crux population at Cerro Fatal, now separated as the newly described species C. donfaustoi (Poulakakis et al. 2015). This assessment also incorporates knowledge contributions from the international workshop on Galápagos tortoises convened by the Galápagos National Park Directorate in July 2012.
|Previously published Red List assessments:|
Chelonoidis porteri occurs on the southwestern slopes of Santa Cruz Island [formerly Indefatigable Island] in the Galápagos Islands of Ecuador. Santa Cruz has a total surface area of 980 sq. km, of which approximately 141 sq. km currently supports the western Santa Cruz tortoise and 500 sq. km. likely could sustain a fully recovered population.
|Range Map:||Click here to open the map viewer and explore range.|
Based on an estimated average density of one tortoise per hectare of suitable habitat, the pre-human population size of the population of Chelonoidis porteri was estimated as 35,000 adult animals (J.P. Gibbs unpubl. data). Following historical depletion, the population was estimated at 2,000-3,000 animals in the early 1970s (MacFarland et al. 1974) and 3,400 in 2010 (J.P. Gibbs unpubl. data). Based on these estimates, the Western Santa Cruz Tortoise population currently comprises less than 10 percent of its original size, with a population decrease of >90%. Recruitment and population growth continue to be impacted by depredation of nests by pigs, but the work of the Galápagos National Park protecting nests over the last several decades is beginning to show some success. While small tortoises are difficult to find, many young adult tortoises (perhaps 20-30 years old) are beginning to show up in farmlands and alongside roads where they have not been seen in decades. A more complete survey of this apparently expanding population is needed.
|Current Population Trend:||Increasing|
|Habitat and Ecology:||Chelonoidis porteri is a large tortoise with a dome-shaped carapace; it feeds predominantly on low-growing vegetation. As occurs in other Galápagos tortoise taxa, first reproduction likely occurs after 20 years of age, and a generation time of 60 years is estimated. Higher elevation areas are dominated by males and some females, middle elevation areas by males, females and some juveniles, and lower elevation areas by females and juveniles. Adult tortoises migrate up and down the elevation gradient, with movements timed to short, rainfall-driven bursts in lowland plant productivity. Nests occur between 6-149 m above sea level, in the hottest, driest and least productive part of the tortoise range. Nest temperature and egg survival declines with increasing elevation whereas hatchling survival and growth are highest at intermediate elevations. Hatchlings disperse between 100-600 m from their nests before becoming sedentary in ranges typically <0.04 ha (Blake et al. 2012, unpubl. data).|
|Generation Length (years):||60|
|Movement patterns:||Altitudinal Migrant|
|Use and Trade:||Chelonoidis porteri was historically overexploited for consumption by settlers and sailors (Pritchard 1996).|
|Major Threat(s):||As well as the severe declines suffered as a result of historical overexploitation for consumption by sailors and later settlers, the Chelonoidis porteri population has also been impacted by the effects of introduced vegetation, introduced goats, donkeys and other herbivores competing with it for food and altering vegetation dynamics, and introduced rats, cats, dogs, pigs, and ants preying on its eggs, hatchlings, and juveniles. Much of the upper sector of the species’ current range has been converted to agriculture and pastureland. While pastures may provide tortoises with nutritious forage, human encroachment, particularly where tortoise migration routes are disrupted by fences and roads, remains a cause for concern (Blake et al. 2015). As the population expands into new areas, there are more tortoises entering roadways and there have been several instances of tortoises being hit by cars. A potential future threat could arise due to current demands by some local stakeholders to open more trails into the park on the western side of Santa Cruz (for example an alternative trail to Tortuga Bay and a possible trail from the highlands to the coast near Eden Island). These efforts could cause serious problems for this tortoise population – both in terms of an increase of tortoises on the roads and greater access of humans to tortoise areas.|
Legislation and regulation: Chelonoidis porteri is protected under Ecuadorian national law. It has been included in Appendix I of CITES since 1975, prohibiting all forms of commercial international trade. Most (84%) of Santa Cruz Island, including much of the native range of C. porteri, is protected as part of the Galápagos National Park. However, tortoises migrate into the highlands, which is now predominantly an agriculture zone, and sometimes spend months there. Progress has been made with the eradication of goats and donkeys on Santa Cruz, and preventing future increases in these populations is vital. Pigs, the predator that causes the most destruction of tortoise nests, are regularly controlled in the nesting zones, but the population remains a threat. Park rangers also protect nests with fencing to prevent access by pigs. Recent evidence of fire ants (Solenopsis) preying on tortoise hatchlings is cause for concern. Barriers to tortoise migration need to be reduced while controlling the remaining livestock, by engaging farmers in the use of raised barbed wire and other barrier alternatives to closed fencing. In recent years, as the tortoise population grows, more tortoises are showing up on roads and in farms where they were previously absent; this has resulted in some tortoises being hit by cars. Research needs include understanding the ecological impacts of migrations and human impacts on these migrations; understanding how tortoises contribute to dispersal of invasive plants; developing control of Solenopsis in tortoise nesting zones, and determining if there are critical areas for habitat management, particularly removal of invasive woody vegetation. Socioeconomic research on how best to mediate human-tortoise conflict is also needed. The resilience of C. porteri in the face of future environmental change will be maximized by ensuring the quality and connectivity of foraging and nesting areas both within and beyond the current and possible future elevational range occupied by the tortoises.
|Citation:||Cayot, L.J., Gibbs, J.P., Tapia, W. & Caccone, A. 2017. Chelonoidis porteri. The IUCN Red List of Threatened Species 2017: e.T9026A82777132.Downloaded on 26 May 2018.|
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