WĒTĀ

The Mackenzie Country is a low rainfall region in inland Canterbury that is home to several locally endemic or quasi-endemic wildlife species, including the kakī / black stilt (Himantopus novaezelandiae) and the ngutu pare / wrybill (Anarhynchus frontalis). The region lists 3052 invertebrate species, 28 of which are local endemics (Wakelin et al. 2024). Among the Orthoptera, most notable are the Nationally Endangered robust grasshopper (Sigaus robustus), the Nationally Vulnerable minute grasshopper (Sigaus minutus), and the Nationally Endangered Tekapo ground wētā (Hemiandrus fabella). One of two populations of the Otago lowland grasshopper (Phaulacridium otagoense), listed “At Risk, Declining”, is also found in the Mackenzie District.

Some 123 invertebrate species in the Mackenzie Country are assessed in the New Zealand Threat Classification System (Wakelin et al. 2024). For the majority of these, the main threats are habitat loss caused by the conversion of drylands to farmland or agriculture, with associated irrigation; predation by introduced mammals; and the modification of braided rivers at hand of hydroelectric power generation, or encroachment by lupins.

In 2018 the Department of Conservation built a 0.6 ha (200m long × 30m wide) predator proof fence on Patersons Terrace, Tekapo, designed to exclude all mammals except for feral cats. The fence was built to protect the Nationally Endangered robust grasshopper, making this the first predator exclosure to be erected specifically to the benefit of an invertebrate species. Ongoing monitoring has been unable to detect any increase in grasshopper numbers inside the fence thus far.

A recently published study conducted by Wētā Conservation Charitable Trust trustee and co-founder Maddy Pye has found that the Nationally Endangered Tekapo ground wētā (Hemiandrus fabella) is also present inside the fence, with a population density that is 4.8 times higher than in equivalent habitat outside the fence. The study has also shown that females grow to a larger size inside the fence.

During a total of 252 tracking nights, tracking tunnels recorded 66 mammal tracks, all outside the fence; 65 of these were hedgehogs, one was a mustelid. Given that hedgehogs are known to be voracious wētā predators (Jones et al. 2005; 2013), the higher number of wētā inside an enclosure from which hedgehogs are excluded comes to no surprise.

No wētā surveys were carried out inside and outside the fence before the fence was built. Technically, we cannot prove that the increased number of wētā inside the fence is caused by the removal of predators. The circumstantial evidence however is strong. The habitat inside and outside the fence is virtually the same. Predation of wētā by hedgehogs is proven beyond doubt. A five-fold difference in wētā population density inside and outside the fence is unlikely to be caused by chance. Any other explanation for the higher number of wētā inside the fence just seems quite improbable.

A plea to our readers: if you care about our wētā and other native invertebrates, as well as our native lizards, please do not feed hedgehogs, no matter how cute you think they are. Please treat hedgehogs the same way you would treat possums, stoats, or rats—they are ecological pests.