Southern Alps giant wētā (Deinacrida pluvialis) surveys
The Southern Alps giant wētā (Deinacrida pluvialis) is a large, ground-dwelling insect that inhabits the wetter sub-alpine regions of New Zealand’s South Island, about and west of The Main Divide of the Southern Alps, between the Wick Mountains in Fiordland National Park and the Kaimata Range north of the Taramakau River. It is ochre to dark-brown in colour with yellow-brown legs; the abdomen is mainly gray in populations north of Aoraki / Mount Cook. Adults measure up to 55mm in body length (legs and antennae not included) and weigh up to 6.5gm in males, 12gm in females.
The first known specimen was collected by entomologist J.T. Salmon near the western portal of the Homer Tunnel in 1946. More observations were made in the Matukituki River West Branch in 1965 and on the Milford Road down valley of Homer Hut (east of the Homer Tunnel) in 1978. It was not until the 1990s however that the species received some attention. Searches were conducted in the Matukituki River West Branch in 1992 and in Price Basin, Whitcombe River, in 1994. The species was described and named by George Gibbs in 1999.
According to Gibbs, “this wētā normally lives on subalpine to alpine (700-1400 m) slopes and basins in extremely high rainfall areas (up to 7 m annual rainfall) which support well developed vegetation with a community of snow tussock (Chionochloa spp.) and woody shrubs or pure snow tussock. A few individuals have been found at higher (to 1850 m) exposed rocky sites”. Gibbs and a coworker searched the Matukituki River West Branch in 1992; they encountered wētā at a rate of 14 insects per searcher hour on the valley floor, at approx. 900m of elevation. They assumed the species to be safe because of the low predator pressure in the alpine environment. As a consequence of this assumption, no more searches were undertaken, and the species was listed as “Not Threatened” in DOC’s New Zealand Threat Classification System (NZTCS) until 2022.
Southern Alps giant wētā encounter rate during searches in the Matukituki River West Branch, 1992 to 2021 (Hegg 2021).
Herpetologist Tony Jewell was first to raise the alarm after searching the head of the Matukituki River West Branch in 2014. He found few wētā, and clear evidence of predation by rodents. In a report for the Department of Conservation, he wrote:
“I am concerned that the available evidence, both physical and circumstantial, suggests a high level of vulnerability to predatory mammals, and possibly even a Critically Endangered status”, and
“Adults are too bulky to hide away from rats and mice, and are virtually defenceless, lacking speed, flight, effective biting capacity, or any kind of visual or chemical deterrent. Realistically, they are sitting ducks for rodents and stoats and are probably spiralling rapidly towards extinction throughout their range”.
His warning fell on deaf ears.
More recently, our work has confirmed that the Southern Alps giant wētā population in the Matukituki River West Branch is now extinct. Wētā have also disappeared from other sites were they were once recorded, including Rob Roy Stream and Rainbow Stream (Mt Aspiring National Park) and the valleys either side of the Homer Tunnel (Fiordland National Park).
Not only, but if in the 1990s wētā were normally found at elevations between 700m and 1,400m a.s.l. in habitats that support well developed vegetation, with few individuals recorded at higher rocky sites, nowadays they are almost exclusively known from barren, rocky habitats above 1,400m a.s.l. The evidence for a rapid contraction of the species’ geographic range and an upward shift in elevation is overwhelming. The culprit? Mammalian predation. In the alpine environment, mainly stoats, and mice. And while rats don’t reach quite as high, wherever they do reach, the Southern Alps giant wētā has been wiped out by now.
In light of the above, the Southern Alps giant wētā was reclassified from “Not Threatened” to “Threatened, Nationally Endangered” in 2022.
The first stage consists of surveying a number of sites across the species’ geographic range to identify any remaining wētā populations. Among the selected sites is Price Basin at the head of the Whitcombe River, one of two original survey localities, which has not been searched since 1994.
At each site (see map on right), if a wētā population can be found, we intend to implement a long-term monitoring programme consisting of mark-recapture surveys repeated at three-yearly intervals. Surveys are volunteer led and involve teams of four workers marking and resighting wētā over four or five consecutive nights at each location. Tracking tunnels are also run for three nights to monitor rodents. The work is permitted under Authorisation Number 119798-FAU issued by the Department of Conservation under the Wildlife Act 1953, valid until 31 July 2035.
A Southern Alps giant wētā population that was recently discovered in Mt Aspiring National Park and was first surveyed by the Wētā Conservation Charitable Trust in 2021, is now managed by the Southern Lakes Sanctuary since it lies within their boundaries.
Where our surveys indicate that a wētā population is present and that it is low in numbers or that it is declining, we will be working with the Department of Conservation to tackle mammalian predators, if this is not being done already. Because monitoring alone does nothing to help wētā, unless it is followed by action.
