Release reptiles outside of their native range

How is the evidence assessed?
  • Effectiveness
    not assessed
  • Certainty
    not assessed
  • Harms
    not assessed

Study locations

Key messages



  • Abundance (1 study): One replicated study in the US Virgin Islands found that following a translocation of St. Croix ground lizards to a new island, the population grew over a 10-year period.
  • Occupancy/range (2 studies): One replicated, randomized study in the US Virgin Islands found that following a release outside of their native range, St. Croix ground lizards were still present five years later. One randomized study in the US Virgin Islands found that following a release outside of their native range, the area occupied by a population of St. Croix ground lizards increased from the 5th to 7th year following release.
  • Reproductive success (3 studies): Three studies (including two replicated studies) in Maritius, the US Virgin Islands and New Zealand found that following releases outside of their native ranges, there was evidence of reproduction in released populations of Aldabra giant tortoises and Madagascar radiated tortoises, St. Croix ground lizards and Otago skinks after 11 months and 5–7 years.
  • Survival (3 studies): Two studies (including one replicated, before-and-after study) in the Galápagos and New Zealand found that following releases outside of their native ranges, 77% of sterilized Galápagos giant tortoises and 63% of Otago skinks survived for 11 months or one year. One study in the USA found that annual survival of a second group of gopher tortoises translocated to an island was lower for newly released tortoises compared to established individuals from a previous release when the island had been outside of the native range.
  • Condition (1 study): One replicated, before-and-after study in the Galápagos found that sterilized Galápagos giant tortoises translocated outside of their native range as part of an ecological replacement gained weight during the first year following their release as.


About key messages

Key messages provide a descriptive index to studies we have found that test this intervention.

Studies are not directly comparable or of equal value. When making decisions based on this evidence, you should consider factors such as study size, study design, reported metrics and relevance of the study to your situation, rather than simply counting the number of studies that support a particular interpretation.

Supporting evidence from individual studies

  1. A replicated study in 2003 on an island containing forest and scrub in the US Virgin Islands (McNair & Mackay 2005) found that releasing St. Croix ground lizards Ameiva polops outside of their native range on to a newly created island resulted in a population that survived and grew over 10 years after release. Ten years after release, 21 individual lizards were identified on the island (9 adults, 11 juveniles and 1 not aged) and the total population size was estimated at 60. Ten lizards were translocated from Protestant Cay in 1990 and one from Green Cay in 1995 to the dredge spoil islet, Ruth Island (7.5 ha, made in 1965 from the construction of a shipping channel), where the species had not been present before. Lizards were visually surveyed five times from March to May 2003 on 20 randomly chosen 25 x 4 m plots in vegetated parts of the island.

    Study and other actions tested
  2. A study in 1994–2008 on a grassy island in Georgia, USA (Tuberville et al. 2008) found that following translocation to a previously unoccupied island along with provision of starter burrows, adult gopher tortoises Gopherus polyphemus had higher survival than juveniles and translocated tortoises had lower initial survival than established tortoises from a previous translocation to the island.  Initial survival was estimated to be lower for newly released adults (1st year: 75%) compared to established adults (98%), and lower for newly released immature tortoises (1st year: 45%, 2nd year: 79%) compared to established immatures (84%). Twenty-eight of 76 (37%) newly released tortoises were never recaptured. Between 1987–1993, between 25 and 30 unmarked tortoises of unknown origin were released on the previously unoccupied island and not marked until 1994. In 1994, a further 74 tortoises (23 males, 32 females, 19 unsexed immatures) were translocated from a development site in Georgia, USA. Each was permanently marked with unique notches on marginal scutes and PIT tags and provided with starter burrows. Turtles were trapped twice a year by bucket or wire traps placed in front of burrows in autumn and spring from 1994–1998.

    Study and other actions tested
  3. A replicated study in 2006–2013 in grassland on Rodrigues Island, Mauritius (Griffiths et al. 2013) found that that captive-bred Aldabra giant tortoises Aldabrachelys gigantea and Madagascar radiated tortoises Astrochelys radiata, released outside of their native ranges to replace extinct tortoises and provided with supplementary food, bred in the wild. Seven years after captive-bred Aldabra giant and Madagascar radiated tortoises were released, 568 Aldabra and 1,114 radiated tortoises hatched in a private reserve. The authors reported that survival rates had been satisfactory overall. In 2006–2013, captive-bred Aldabra giant tortoises (>480 individuals) and Madagascar radiated tortoises (100 individuals) were introduced as ecological surrogates for extinct Rodrigues giant saddleback tortoise Cylindrapis vosmaeri and Rodrigues domed tortoise Cylindrapis peltastes into a privately-managed 20 ha reserve. Native and endemic vegetation was planted and released tortoises were provided with supplementary food (seasonal fodder, fruit and vegetables) until replanted native vegetation matured. Any hatchlings discovered in the release area were also collected and brought into the nursery facility for up to 4 years before being returned to the release area.

    Study and other actions tested
  4. A replicated, before-and-after study in 2010–2011 in grass and shrubland in the Galápagos Archipelago, Ecuador (Hunter et al. 2013) found that most captive-bred hybrid adult Galápagos giant tortoises released as ecological replacements for an extinct species survived at least one year in the wild and gained weight. At least 30 of 39 (77%) translocated Galápagos giant tortoises survived one year after being released. In one year, tortoises had gained 11 kg each on average, or 22% of their body weight compared to before they were released (weight in 2011: 65 kg; weight in 2010: 54 kg). In total, 39 sterilized adult giant tortoises were introduced to Pinta Island (59 km2) as ecological replacements for the extinct saddlebacked giant tortoise Chelonoidis abingdonii in May 2010. The tortoises had been maintained in captivity for all or most of their lives and were genetic hybrids (13 had domed shells and 26 had saddlebacked-type shells). Tortoises were monitored weekly in May–July 2010 (39 individuals) and up to three times in 2011 (30 individuals) using GPS loggers (20 individuals, 2–6 months of hourly data) or radio transmitters (16 individuals) or satellite GPS transmitters (3 individuals) and visual observation. Tortoises were weighed prior to release (39 individuals) and in June–July 2011 (27 individuals).

    Study and other actions tested
  5. A replicated, randomized study in 2008–2013 in beach-forest on Buck Island, US Virgin Islands (Fitzgerald et al. 2015, same experimental set-up as Angeli et al. 2018) found that St. Croix ground lizards Ameiva polops released outside of their native range and held temporarily in enclosures, survived, bred and dispersed in the 5 years post release. In the first 71 days after translocation, 20 individually-identified St. Croix ground lizards, 32 unidentifiable individuals and one hatchling were observed in release enclosures. Five years later, adult (73% of observations) and juvenile lizards (24% of observations) were observed. Fifty-seven St. Croix ground lizards were translocated to Buck Island (71 ha) in April–May 2008, where they had not previously been present, apart from an unsuccessful translocation attempt in the 1960s. Lizards were marked, toe clipped, and held in enclosures (10 x 10 m) for 71 days after translocations began (7–8 lizards/enclosure, eight enclosures, enclosures removed in July 2008). Lizards were monitored in enclosures using visual surveys (26 x 10-minute surveys) and pitfall traps. Lizards were surveyed after one year (May–June 2009, captured by noosing) and five years (March–May 2013, visual surveys at 61 sites across the island). Invasive predators (rats Rattus rattus and mongoose Herpestes auropunctatus) were eradicated before translocation and vegetation restoration was ongoing.

    Study and other actions tested
  6. A study in 2013–2014 in a man-made rock and shrub habitat in southern South Island, New Zealand (Bogisch et al. 2016) found that 63% of captive-bred Otago skinks Oligosoma otagense released outside of their known native range into a mammalian-predator-free fenced enclosure and provided with supplementary food survived at least 11 months and bred within 15 months. In total, 24 of 30 (80%) captive-bred Otago skinks survived at least three months and 19 of 30 (63%) skinks survived 11 months after release. The authors reported that 12 newborn skinks were observed in the enclosure 15 months after the skinks were released. Thirty captive-bred skinks were released into an oval outdoor enclosure (109 m2 with an 85 cm high wooden fence) in an ecosanctuary in November 2013. The habitat was created to mimic natural Otago skink habitat and included rocky tors planted with native grass and shrubs. Skinks were photographed prior to release to enable individual identification. Skinks were monitored by observation during November 2013–February 2014 and September–October 2014, and by time-lapse photography of the enclosure (pictures were taken every 10 minutes between 0600–2100 h). Skinks were provided with supplementary food of 100 crickets/week but could also feed on invertebrates and small lizards present in the enclosure. The ecosanctuary was surrounded by a predator-proof fence and mammalian predators had almost entirely been eradicated.

    Study and other actions tested
  7. A randomized study in 2013–2015 in mixed forest and scrubland on Buck Island, US Virgin Islands (Angeli et al. 2018, same experimental set-up as Fitzgerald et al. 2015) found that St. Croix ground lizards Ameiva polops released into restored island habitat outside of their native range increased their distribution in the fifth to seventh year after being released. Five years after St. Croix ground lizards were released, they occupied 41% of sites surveyed, six years after release, lizards occupied 60–66% of sites surveyed and seven years after release lizards occupied 74–87% of sites surveyed. Range expansion occurred in adjacent sites progressively further eastwards (see original paper for details). Fifty-seven lizards were introduced to Buck Island, where they had not previously been present, in 2008. Surveys were carried out in 63 sites (1,260 m2 circular sites, at least 80 m apart) across the island five times/season over three days each in May 2013, May 2014, October 2015, May 2015 and October 2015. An additional 192 surveys were carried out in 32 sites in May 2013 and these sites were surveyed twice/day for three consecutive days. Vegetation restoration had been underway for 40 years and invasive predators removed prior to lizards being released.

    Study and other actions tested
Please cite as:

Sainsbury K.A., Morgan W.H., Watson M., Rotem G., Bouskila A., Smith R.K. & Sutherland W.J. (2021) Reptile Conservation: Global Evidence for the Effects of Interventions for reptiles. Conservation Evidence Series Synopsis. University of Cambridge, Cambridge, UK.

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Reptile Conservation

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