A Te Papa team recently visited the Snares Islands Nature Reserve, 105 km south-southwest of Stewart Island, where they completed a range of seabird and plant research projects. Here, Colin Miskelly (Curator Terrestrial Vertebrates) describes some of the smaller inhabitants of the Snares Islands.
Watch Science Live: Expedition Snares Island to find out more about our scientific research on the Snares Islands.
The Snares Islands are one of the least modified places in New Zealand. Not only are there few signs of human presence, but no introduced mammal species have ever established on the islands. Rats, mice, stoats, weasels and cats are all predators of large invertebrates elsewhere. As they are all absent from the Snares, the islands have retained a spectacular fauna of large invertebrates.
Perhaps the most unexpected invertebrate present on the Snares is the large terrestrial leech Ornithobdella edentula. Few New Zealanders are aware that we have leeches in the country – let alone large ones like this. Originally known only from the Snares Islands, this leech has more recently been found on Little Solander Island (western Foveaux Strait), and on Kaimohu Island south-west of Stewart Island. While the leeches mainly feed on bird blood (particular from penguins and albatrosses), they have been known to snack on the occasional unsuspecting bird scientist…
Large leaf-veined slugs occur throughout New Zealand, but are far more numerous on islands without rodents. Pseudaneitea huttoni is endemic to the Snares Islands (meaning that it is naturally found only there), and is seen commonly on Olearia tree trunks on damp nights.
One of our tasks while on the Snares Islands was to search for onchidiid slugs along exposed clifftops. Although related to terrestrial slugs and snails, the family Onchidiidae is mainly marine, but a few species have returned to land. It is unknown how many species occur on islands south of New Zealand, and we had been requested to collect specimens for a genetic research project. Despite much searching on the main island, we found onchidiid slugs only on Toru Islet in the Western Chain, 5 km to the south-west of the main island.
The deep peat soil on the Snares Islands is fertilised by the droppings of millions of seabirds. Perhaps as a result, earthworms are abundant, and some species are very large. The endemic Diplotrema haplocystis grows to 30 cm long.
The two species of weta on the Snares Islands are both endemic. The ground weta Hemiandrus subantarcticus belongs to a widespread New Zealand genus, but the jumping weta Insulanoplectron spinosum is placed in its own genus, and so both the species and the genus are endemic to the Snares Islands.
The largest beetle on the Snares Islands is the ground beetle (carabid) Mecodema alternans hudsoni. While this subspecies is endemic to the Snares Islands, the nominate form occurs in Otago and on the Chatham Islands.
Larvae of the endemic scarabaeid (chafer) Prodontria longitarsis are common in the soil on the Snares Islands, and the flightless adults are occasionally seen on vegetation at night. They belong to the same genus as the endangered Cromwell and Alexandra chafer beetles of Central Otago.
Weevils are the most diverse family of organisms on the planet in terms of number of species. This is also the case on the Snares Islands, with ten species recorded, of which the four largest species are illustrated here. The most common of the four is the medium-sized (12-15 mm long) Oclandius vestitus, which is common on Olearia lyalli trunks and branches at night. The similar sized (and similar looking) Catoptes brevicornis australis is much scarcer, and is most readily found on the rare herb Anisotome acutifolia at night.
The two largest weevil species on the Snares Islands each has a fascinating conservation history. Both are flightless, and have life-cycles that are completely dependent on a single plant species, with their larvae feeding on the roots, and the adults emerging at night to browse on the leaves and stems.
The knobbled weevil (Hadramphus stilbocarpae) on the Snares Islands feeds only on the large-leaved punui (Stilbocarpa robusta). The weevil formerly occurred on Big South Cape Island (Taukihepa), off the south-west coast of Stewart Island, but was wiped out there when ship rats invaded in the early 1960s. There and on Bird Island and Solander Island (Foveaux Strait), knobbled weevils feed (or fed) on the related punui species Stilbocarpa lyallii. However, populations found recently in Fiordland (Breaksea Island, Resolution Island and Puysegur Point) feed on the carrot-like Anisotome lyalli. It is unknown why knobbled weevils on the Snares Islands don’t also eat the closely related Anisotome acutifolia.
The rarest weevil on the Snares Islands is Lyperobius nesidiotes, which feeds only on the Snares Island endemic herb Anisotome acutifolia. The plant itself is very rare on the islands, occurring at only two or three sites on the main island. To complicate matters the Lyperobius weevil has never been found on the main island (North East Island), and is considered endemic to the much smaller Broughton Island, where it was found in a patch of Anisotome about the size of a small room. A recent Department of Conservation survey for rare plants on Broughton Island failed to find any Anisotome there, and so it is possible that this large flightless weevil, totally dependent on a single host plant species, has eaten itself to extinction.
Postscript A NIWA research team visited Broughton Island on 4 March 2014 to count Buller’s mollymawks, and found that there are scattered Anisotome plants at at least one site on the island, and so there is still hope for the weevil.
With thanks to the Department of Conservation for permission to visit the Snares Islands Nature Reserve, Pete McClelland for information on leech distribution, and Paul Sagar for the Broughton Island update.