Study

Translocation and recovery of reptiles after rat eradication, Marotere/Hen and Chicken Islands, Northland, New Zealand

  • Published source details Towns D., Dougherty C. & Cree A. (2001) Raising the prospects for a forgotten fauna: a review of 10 years of conservation effort for New Zealand reptiles. Biological Conservation, 99, 3-16

Summary

The Marotere or Hen and Chicken Islands, are a group of small islands lying off the east coast of Whangarei (North Island, New Zealand). The three largest islands, Lady Alice Island, Whatupuke Island and Coppermine Island are between 75 and 145 hectares in size. They support several endangered endemic reptile species and other native fauna and flora, and also harboured the introduced kiore or Pacific rat Rattus exulans. While the kiore has particular cultural significance to some people, it is a significant predator and competitor of native species of reptiles (skinks, geckos and tuatara Sphenodon punctatus) and seabirds. Tuatara are typical of New Zealand's 'k-selected' fauna i.e. long-lived (up to one hundred years), slow to mature (at least 10 years before they breed), have a low reproductive output (typically 2.5 eggs in a clutch survive to hatching and a female lays eggs only once every two or so years) but with a long breeding life (approx. 50 years). Similarly, native geckos produce only up to two offspring a year but compensate for this low reproductive rate by living up to 40 years or more, if introduced predators are absent.

Rats were therefore eradicated from the islands in order to bolster existing native species populations and to create a safe haven for reptile reintroduction. A prior eradication attempt on Coppermine Island was unsuccessful, (hand-filling of bait stations on foot) so aerial drops of poison bait was used which had the additional benefit of covering areas difficult to access on foot. Kiore were eradicated from Whatupuke Island in 1993, Lady Alice Island in 1994 and Coppermine Island in 1997. At the time there were about 100 tuatara on each of the three largest islands, and also on Hen Island, where Kiore were not eradicated. There had been a long history of tuatara monitoring which has been ongoing since 1968. This monitoring history and the comparison with Hen Island provides a useful insight into the dramatic recovery that reptiles can acheive in the absence of introduced predators.

Reptile translocations: Translocations to Lady Alice were undertaken in 1997/98, Whatupuke in 2000 and Coppermine in 2002, using reptiles taken from rock stacks (lacking rats) and other islands in the same archipelago. Two species of skink, McGregor's skink Cyclodina macgregori and Mokohinau skink Cyclodina 'Mokohinau Island', and one species of gecko, the Pacific gecko Hoplodactylus pacificus were caught and transferred (held in cloth bags) to the now rat-free islands. There were seven transfers with 30-40 individuals involved in each. The last transfer was in January 2005.

The releases of translocated geckos and skinks on to each of the three islands were:

Lady Alice Island - 30 Pacific geckos, 39 McGregors skinks and 30 Mokohinau skinks

Whatupuke Island - 30 McGregors skinks and 30 Mokohinau skinks

Coppermine Island - 30 Mokohinau skinks.

The released skinks and geckos are surveyed to assess the success of the releases. The criteria to gauge success of translocated reptile populations were designed around a 15 year time frame:

Step 1 after 5 years determine that at least some animals have survived.

Step 2 after 10 years confirm that breeding has occurred.

Step 3 after 15 years capture more animals that have been born on the island than were initially released.

Tuatara populations on these three islands were also monitored prior to and after eradication of rats, and also on nearby Hen Island (where rats were not eradicated).

Success of the gecko & skink introductions: To date (2005), Pacific geckos and McGregors skinks have survived on Lady Alice Island, gained weight and bred. The Mokohinau skinks have however disappeared and may have perhaps dispersed away from the release area. In January 2005, another 40 Mokohinau skinks were released on Lady Alice Island, this time onto the beach and not into the deep forest as previously, as from observations on Whatupuke Island, they seem to do well in this type of environment.

On Whatupuke, Mokohinau skinks increased in weight and young were found. However, some of the skinks released were pregnant females and it will be several more years until successful breeding and establishment becomes apparent. The McGregors skinks released also gained weight and have given birth to offspring. Mokohinau skinks released on Coppermine Island have not yet been monitored.

Tuatara populations: Tuatara populations have responded positively to kiore eradication. On Hen Island, where kiore are still present there has been no recruitment to the population in the five sample periods spanning a period of 21 years. In contrast, eight years after eradication of kiore from Lady Alice Island, juveniles comprised between 5% and 40% of the populations.

Determining the true success of such eradications and re-introductions requires long-term planning and monitoring, and may span several generations of conservationists and several terms of government.


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