Rangatira Island is best known as a refuge for rare birds, but it is also home to a spectacular variety of flightless insects, giant spiders, lizards, and seals. Te Papa natural environment curator Colin Miskelly recently spent a month on the island as a volunteer for the Department of Conservation. The team was mainly focussed on the black robin and Chatham petrel recovery programmes, but made time to appreciate some of the lesser-known inhabitants of the island.
Rangatira (South East) Island Nature Reserve is one of the world’s great wildlife sites. Although farmed until the 1960s, it somehow escaped the introduction of rats and cats that destroyed the ecological fabric of the three other large islands in the Chatham Islands group, 800 km east of mainland New Zealand.
The island is best known internationally for its rare birdlife, but locally it is also known for its spiders. Many arachnophobic Chatham Islanders have declined opportunities to visit the island due to the infamous Rangatira spiders.
Rangatira spiders (Dolomedes schauinslandi) are one of New Zealand’s largest spiders (up to 12 cm across), and are over-sized relatives of the familiar nurseryweb spider (Dolomedes minor) of the New Zealand mainland. Like their smaller relative, the female Rangatira spiders make a conspicuous silk nest among foliage, where they guard their young.
These impressive large spiders are endemic to the Chatham Islands, but are now known only from three small islands where rats and mice have never invaded. Although never witnessed, it is assumed that mice hunted the spiders to local extinction on Pitt Island (which has never had rats).
Rangatira spiders are most readily found at night, when they emerge from their daytime lairs to hunt. Their large, bright eye-shine allows them to be seen from up to 20 metres away in a headlight beam, as they seek their prey on tree trunks and the forest floor.
The favourite meal of Rangatira spiders is wētā, which they catch by pouncing on them. While they build substantial silken nursery webs, Rangatira spiders do not use silk to ensnare their prey.
Another large spider on the island that does spin a prey-catching web is the sheetweb spider Cambridgea annulata. Many New Zealanders will be familiar with related sheetweb spiders that are readily seen in mainland forests at night, poised on their characteristic horizontal webs, waiting for prey to fall on the web. Once again, the Chatham Island species is one of the largest members of the genus.
Two species of wētā are extraordinarily abundant on Rangatira Island. Both are super-sensitive to light, and so the night-time forest floor is alive with their hopping forms when a torch or headlamp is shone around. Both are endemic to the Chatham Islands, and both are jumping wētā – though the stocky Talitropsis wētā has a body form approaching that of the much large tree wētā on the New Zealand mainland.
The large, flattened hind-tibiae of the Talitropsis wētā form an effective rear-facing shield when they squeeze head-first into their daytime retreats.
Both the Talitropsis wētā and the more slender Novoplectron serratum are omnivores, but get much of their protein by scavenging. Any bird corpses on the forest floor are covered with a seething mass of wētā at night (until a light is shone on them!), and are reduced to skeletons within a couple of nights.
A diversity of large beetles
Some of the many beetle species on Rangatira join the wētā in scavenging bird corpses, including the large flightless carabid ground-beetle Mecodema alternans. The rodent-free status of Rangatira has provided a refuge for large flightless insects that are now rare or absent from Chatham and Pitt Islands.
The most abundant large beetle on the forest floor and tree trunks at night is a species of darkling beetle, Mimopeus pascoei – a favoured food of the sheetweb spider. Other rarer species that mainly emerge on damp nights include the Chatham Island stag beetle Geodorcus capito and the Chatham Island click beetle Amychus candezei.
The male stag beetles have impressive large mandibles, used to fight other males for access to mates.
The click beetle belies its name – it is a ‘clickless’ click beetle. There are three species in the genus Amychus, each of which is found on islands in different parts of New Zealand (the two other species are found on islands in the outer Marlborough sounds and on the Three Kings Islands).
Perhaps the most intriguing of the large beetles on Rangatira are the Hadramphus weevils found on soft speargrass (coxella, Aciphylla dieffenbachii) and hoho (Chatham Island lancewood, Pseudopanax chathamicus). Their complex story will be the subject of the next blog in this series.
These last three beetle species (the stag beetle, click beetle, and Hadramphus weevil) are all protected under the Wildlife Act 1953 – some of the few New Zealand insects to receive this level of protection.
Endemic or not?
One of the rarer inhabitants of Rangatira (or at least rarely seen) is the giant stick insect Argosarchus horridus – the longest New Zealand insect. Until recently considered an endemic Chatham Island species (as Argosarchus schauinslandi), the stick insect is one of many species collected by Professor Hugo Schauinsland, the director of the Bremen Übersee-Museum when he visited the Chatham Islands in 1897. Recent genetic and morphological comparisons have resulted in the Chatham Island stick insects being included in the widespread New Zealand species Argosarchus horridus.
Another widespread native insect more commonly seen on Rangatira is the magpie moth Nyctemera annulata. This day-active moth feeds on species of Senecio and other closely related daisy species, including introduced ragwort and garden cinerarias. Within the Chatham Islands, the woolly-bear caterpillar larvae of the magpie-moth are mainly found on the endemic Chatham Island groundsel Senecio radiolatus subsp. radiolatus.
A skink and two fur seals
The only lizard known from the Chatham Islands is the Chatham Island skink Oligosoma nigriplantare. Found only on islands lacking rats (i.e. anywhere apart from the main island!), these medium-sized skinks are variable in colour, and are active day or night during warm, fine weather.
Rangatira Island is a major breeding site for New Zealand fur seals Arctocephalus forsteri, with close to 1,000 pups produced annually along the south coast.
Among the more familiar New Zealand fur seals, we saw two bull subantarctic fur seals Arctocephalus tropicalis – rare visitors from the southern Indian Ocean or South Atlantic Ocean. In addition to their pale faces and bellies, short snouts, and distinctive top-knots, subantarctic fur seals have a deep dog-like bark, quite different from the huffing and whinnying of New Zealand fur seals.
With many thanks to the Department of Conservation for the opportunity to assist with Chatham threatened bird recovery programmes and to visit Rangatira Island Nature Reserve.
- The mystery of the giant hoho weevils of Rangatira Island
- Furtive fauna of the Auckland Islands
- Critters of Taumaka (Open Bay Islands)
- Insects of Takapourewa / Stephens Island
- Critters of the Snares Islands
- Critters of the Poor Knights Islands
- Critters of Ohinau Island
- Critters of Titi Island Nature Reserve, Marlborough Sounds
- The petrels of Rangatira Island, Chatham Islands
- The littlest snipe