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Individual study: Responses of tuatara Sphenodon punctatus to removal of introduced Pacific rats Rattus exulans from the Marotere Islands, New Zealand

Published source details

Towns D.R., Parrish G.R., Tyrrell C.L., Ussher G.T., Cree A., Newman D.G., Whitaker A.H. & Westbrooke I. (2007) Responses of tuatara Sphenodon punctatus to removal of introduced Pacific rats from islands. Conservation Biology, 21, 1021-1031


In New Zealand evidence suggests that, when introduced to islands, Pacific rats Rattus exulans have negative effects on many endemic plants and animals, including the tuatara Sphenodon punctatus. The tuatara is a threatened lizard now occurring only on offshore islands off the main North and South Islands. The tuatara is a long-lived, egg-laying species. In this study the effects of Pacific rats on tuatara was tested by comparing the age structure and body condition of tuatara populations on three of the Marotere Islands before and after removal of rats, and on a fourth island (Taranga ) where rats remained.

Study sites: Three of the largest Marotere Islands (35°54'S, 174°44'E) (Lady Alice, Whatupuke and Coppermine) had Pacific rats removed (see below), whilst Taranga (35°58'S, 174°43'E) a larger island approximately 7 km south of the Marotere group provided a ‘control’ island where rats remained. Other than Pacific rats, no other mammals were present at the time of study. The approximate date of invasion by Pacific rats is estimated (for Lady Alice Island) at about 200 years ago. All supported tuatara populations.

Rat eradication: Rats were eradicated from Whatupuke in 1993 and Lady Alice in 1994 with a single aerial spread of rodenticide (brodifacoum). On Coppermine, an attempted ground-based eradication using bait stations (containing wax block with brodifacoum) begun in September 1992, eradicated most but not all rats. In 1997 they were subsequently eradicated with aerially spread Talon 20 P. There were no identified detrimental effects on tuataras.

Tuatara surveys: Generally, teams of people searched at night with spotlights along trails and other defined areas. Some tuatara captured before 1990 had been permanently marked with unique combinations of toe clips. This practice was discontinued. Unmarked animals caught since 1990 were temporarily marked with white correction fluid or xylene-free silver paint. Survey data were separated into two seasonal periods: spring and early summer samples (September to December; egg-laying period); and late summer and autumn (January to May), when adult males are particularly territorial and visible.

Average snout-vent length was compared for each sex to estimate changes in adult population structure before and after rat eradication. Changes in body condition (weight) over time on each island were comapared. Any juveniles were recorded.

Population density: Estimates of density (based on night searches of marked plots) enabled calculations of population density. Where captures were insufficient for mark-recapture estimates, the minimum number alive (total identified individuals over the sample period) was used as an abundance index.

Recruitment: On Taranga (rats present) no juvenile tuatara were recorded over 21 years. On the three islands with rats removed, there was a significantly higher proportions of juveniles after rat removal, reaching >40% on Lady Alice and Whatupuke. Before rat removal the few juveniles seen were usually near adult size; subsequntly, juveniles were present in a wide range of size classes.

Male weight/length: Male tuatara in autumn showed no significant change in weight after rat removal from Lady Alice; on Coppermine (where in relatively poor condition in the presence of rats) they showed a large increase after rat removal; and on Whatupuke they also increased (average weight  following rat removal 684.2 ± 14.2 g, n = 6) was substantially more than before removal (476.1 ± 19.5 g, n = 14). Average snout-vent length remained the same (before rat removal 243 mm; after 241 mm). Males on Taranga in autumn showed no significant change in weight before and after 1997.

In spring, males on Lady Alice and Coppermine showed significant weight increases following rat removal. There were no samples in spring from Taranga and samples from Whatupuke were insufficient to compare changes for either sex.

Female weight: In autumn, female tuatara weight did not change after rat removal from Lady Alice; on Coppermine (where initially in relatively poor condition) they showed a large increase; and likewise on Whatupuke. On Taranga, the trend  was a decline in after 1997 in the largest females.

Population density and recruitment: Estimated tuatara densities differed among islands and varied between sites on islands. Densities on Coppermine and Whatupuke were similar, ranging between 5 and 26/ha. The highest densities recorded were on the ‘West Bay’ of Lady Alice, where two sites sampled in 2001 and 2005 indicated that population density was stable or increasing. The highest density was estimated as 109–212.5/ha in one of these sites during 2005. Here in 2001 up to 53% of tuatara recorded were juveniles whereas on the same island juveniles formed no more than 8% of ‘South Cove’ samples in 2002. Similarly in 2002, 9 years after rat eradication from Whatupuke, 57% of the sample in one plot was juveniles, but with no juveniles in the second plot. On Coppermine juveniles contributed 0–18% of samples in plots. On Taranga, almost the entire population, (estimated at 44 adults), appeared confined to the main ridge, the area occupied was estimated as 39 ha (tuatara density of approximately 1.1/ha).

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