Restoring Existing Tortoise Populations to Historical Numbers
The long-term goal of the Giant Tortoise Restoration Initiative is to restore all tortoise populations to their historical distribution and numbers. When the Galapagos National Park was established in 1959, there were 11 surviving tortoise species. Some were on a path to extinction. Others, while vulnerable or threatened, could survive without human intervention.
Given that the historical population throughout the Archipelago was between 200,000-300,000 tortoises, and the current population is 10-15% of that, this recovery will be a long, slow process. The primary method to rebuild these populations as quickly as possible is by collecting eggs and/or hatchlings from natural nests and rearing the young in captivity, by-passing the years of highest mortality before releasing them back into the wild.
- As of the end of 2017, more than 7,000 juvenile tortoises returned to their island of origin – including Española, Isabela, Pinzón, San Cristóbal, Santa Cruz, and Santiago.
- The first tortoises repatriated to Española Island reached sexual maturity and began nesting in the early 1990s. The population has been increasing naturally on the island since that time.
- Black rats, introduced to Pinzón Island in the late 1800s, were eradicated in 2012. Natural recruitment of juvenile tortoises into the wild population has since begun.
- The first-ever complete census of the San Cristóbal tortoise population, completed in November 2016, resulted in a population estimate of 6,700 animals, with many juveniles and subadults (tortoises less than 20-25 years old).
- The Eastern Santa Cruz Tortoise was determined to be a separate species from the larger tortoise population on the western side of Santa Cruz. The publication officially naming the new species Chelonoidis donfausti came out in 2015.
- Goats and donkeys were eliminated from northern Isabela and Santiago Islands in 2006; pigs were also eliminated from Santiago (they were not present on northern Isabela).
- Goats were eliminated from Pinta Island in 2006.
Española (Chelonoidis hoodensis). The Española tortoise population was the closest to extinction in 1959. Through a 50-year breeding program, the population is healthy and recovering. However, the result of decades of goats on the island (eradicated in 1978) and the absence of significant numbers of tortoises resulted in changes to the vegetation that negatively impact the recovery of the tortoise population – primarily the vast thickets of woody vegetation that prohibit movement of the tortoises. GTRI personnel have been studying the inter-relationships among tortoises, cactus and woody vegetation to determine the best strategy for improving tortoise habitat so that the population can continue to expand in both distribution and number.
Pinzón (C. duncanensis). The Pinzón tortoise population was also on the road to extinction due to 100% predation of tortoise eggs/hatchlings by introduced black rats beginning in the late 19th century. Repatriations began in 1970, and the population is doing well. In December 2012, the Galapagos National Park Directorate led a successful campaign to eradicate the introduced black rats. In 2014, tortoise hatchlings were observed on the island for the first time in a century. Rearing young tortoises in captivity will continue to help the population increase more rapidly.
San Cristóbal (C. chathamensis). When the GTRI was launched, the San Cristóbal tortoise population was one of the least well-known. A breeding center was established there in 2004 to build up the population, but it was not known at the time whether that was a necessary tool for that population. An island-wide census was finally carried out by the GTRI team in November 2016. Nearly 2,000 tortoises were marked, and the population was estimated at 6,700. The high number of juveniles and sub-adults indicated a healthy, growing population. Future work on San Cristóbal will focus on the expanding range of the tortoises into the farmlands of the western half of the island.
Santa Cruz – Western (C. porteri) and Eastern (C. donfaustoi). Until recently, the giant tortoises of Santa Cruz Island were considered a single species. In 2015, the Eastern Santa Cruz Tortoise was designated a separate species. Both species are Critically Endangered. The western species has a large population but is still much smaller than the historical population. It is threatened by feral pigs, which dig up tortoise nests and eat the eggs and/or hatchlings), and human-tortoise conflicts. The eastern species is endangered in part because of its small population. Knowledge of this species is limited. The introduced Solenopsis ant, which attacks tortoise hatchlings in nests and as they emerge, is present in nesting zones of both species. A complete census is planned of the eastern species to determine number, range, threats, and health of the population. To increase population numbers more rapidly, eggs and/or hatchlings are collected from natural nests and reared in the Tortoise Center, to be released at 4-5 years old.
Santiago (C. darwini). The Santiago tortoise is also Critically Endangered, in large part due to exploitation in past centuries. Due to the ease of removing female tortoises (closer to the coast and lighter), exploitation by whalers and other mariners skewed this population to mostly males, which is a severe limit to population growth. To rebuild the population more quickly, eggs and/or hatchlings are collected from natural nests and reared in the Tortoise Center and then released at 4-5 years old. A more recent threat is the expansion of the introduced blackberry following the eradication of pigs, donkeys, and goats. These thickets are impenetrable, hindering tortoise use of the habitat and potentially blocking migrations. Further study and habitat management is needed.
Southern Isabela – Cerro Azul Volcano (C. vicina) and Sierra Negra Volcano (C. guntheri). The Sierra Negra tortoise population was historically the largest in the Archipelago, estimated at over 70,000 animals. Fewer than 1,000 remain. The historical Cerro Azul population was estimated at 18,000 animals, with fewer than 3,000 remaining. These volcanoes both have numerous subpopulations of tortoises that do not appear to intermix in current times. Poaching, while not a problem for most Galapagos giant tortoise species, is ongoing, albeit at a limited level, on southern Isabela. Both species have breeding groups at the Tortoise Center in Puerto Villamil. Offspring are released on a regular basis. Much more research is needed to determine the complex genetics of these two species and to provide better population estimates. Work is also needed to reduce and eventually eliminate poaching.
Northern Isabela – Alcedo Volcano (C. vandenburghi), Darwin Volcano (C. microphyes) and Wolf Volcano (C. becki). The three tortoise species on the northern volcanoes of Isabela are generally healthier populations than most of the other species. The tortoises on Alcedo and Wolf are considered Vulnerable, while those on Darwin are designated as Threatened, due to its small population size as well as a lack of knowledge of this species. The population on Wolf Volcano has an odd mixture of tortoises from several of the other islands, mixed in with the native population. These are the result of humans moving tortoises during the time of the whalers. Some of the tortoises have interbred, producing offspring with partial ancestry from different islands. This occurrence has provided a treasure trove of tortoises that can be used to repopulate islands where tortoises are extinct. A breeding program to restore tortoises to Floreana Island was initiated in 2017 using tortoises collected from Wolf Volcano.
Fernandina (C. phantasticus). The status of the Fernandina Island tortoise is unknown. A single tortoise, the only known Fernandina tortoise in the world, was collected during the California Academy of Sciences expedition in April 1906. While thought to be extinct due to volcanic eruptions in past centuries, there have been anecdotal observations indicating that there may indeed still be a very few left on the island. A search will be conducted using both helicopter and ground crews to make a final determination.
All photos © GTRI.
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