Te Papa vertebrate curator Dr Colin Miskelly recently spent 12 days on Ohinau Island (east of Whitianga, Coromandel Peninsula) as part of a Te Papa seabird research team. This blog reports on the species of reptiles that he found on the island.
Ohinau is a 43 ha forested island owned by Ngati Hei, and lying about 7 km south of other islands in the Mercury Island group. The island formerly held populations of Pacific rats (kiore), mice and rabbits, but has been free of introduced mammals since these were eradicated by Ngati Hei and the Department of Conservation (DOC) in 2005. No lizard species have been translocated to Ohinau, and so the six species that we found all survived the presence of rats, or have colonised naturally subsequently.
The most conspicuous lizard species on Ohinau Island is the shore skink (Oligosoma smithi), which is abundant at the top of the boulder beach where we landed. This is a sun-loving, day-active species that is abundant on most rodent-free northern islands, and present but scarcer on the adjacent mainland. It is variable in colouration, but most animals are blackish. We mainly saw them between the high tide line and the vegetation edge, but a few were seen in open areas further back among vegetation.
Another dark skink found along the shore was the nocturnal Suter’s skink (O. suteri, also known as the egg-laying skink or diving skink). This slender, sinuous species was mainly seen at night, lower on the shore than the shore skinks. A few were also seen sun-basking, and many glimpses of dark skinks scuttling for cover among the boulders could not be assigned to species.
Suter’s skink was apparently a new record for Ohinau Island, though they are known from the nearby Flat Rock and Black Rocks. It is unknown whether they were present (undetected) on Ohinau all along, or whether they are a recent colonist. Suter’s skinks live among tidewrack, and readily dive into tidepools to escape predators, and so are more suited to dispersal between islands than most New Zealand lizards. Like most of our lizards, they can be abundant in the absence of rodents (and are so on many northern offshore islands). They still hang on at a few mainland sites between Cape Reinga and the Coromandel Peninsula.
The boulder beach habitat where shore skinks and Suter’s skinks live is dynamic. Spring tides and big seas over several days during our visit re-arranged the boulder beach with much thumping and grinding. We presume that the lizards moved further up the beach, away from the wave action.
The two other skink species on Ohinau were harder to find. The day-active moko skinks (O. moco) were mainly seen in the lighthouse clearing at the summit, with one large animal seen near the shoreline. Moko skinks are another species typical of northern offshore islands, but also found at a few mainland sites.
Copper skinks (O. aeneum) were found mainly under the forest, foraging among leaf litter in the daytime, or hiding under rocks and decaying vegetation. We did not see any night-active skinks under the forest.
Two gecko species were present on Ohinau Island, both of them nocturnal. The large Duvaucel’s gecko (Hoplodactylus duvaucelii) was abundant around our camp site, but only a single animal was seen elsewhere on the island, suggesting that they had expanded from a refugium along the boulder beach following rat eradication. Duvaucel’s gecko is New Zealand’s largest living lizard species, but the largest animals that we saw were much smaller than those on the Poor Knights Islands and the Hen and Chicken Islands further north.
The smaller common gecko (Woodworthia maculata) was found in scattered locations around the main island, on two offshore stacks accessible at low tide, and on nearby Ohinauiti Island. We did not see any active under forest at night, but were not specifically looking for them.
The apparent absence of large, nocturnal forest-dwelling skinks on Ohinau Island is almost certainly due to predation to extirpation by Pacific rats. Three such species occur within the Mercury Island group: robust skink (O. alani), Whitaker’s skink (O. whitakeri) and marbled skink (O. oliveri). The latter species also occurs on Old Man Rock (3 km north-west of Ohinau) and on the Alderman Islands (30 km to the south-east). Marbled skinks and tuatara (Sphenodon punctatus) are obvious candidates for reintroduction to Ohinau Island.
With thanks to Ngati Hei for permission to visit Ohinau, and DOC for logistic support.