Saved from extinction? Establishment and dispersal of Mercury Islands tusked weta, Motuweta isolata, following translocation onto mammal-free islands

Abstract

The Mercury Islands tusked weta, Motuweta isolata (Orthoptera: Anostostomatidae), survived only on 13 ha Ahu or Middle Island, a mammal-free island in the Mercury Group, New Zealand. Between 2000 and 2009, 567 individuals were translocated in nine releases to six nearby islands from which mammals had been removed. These translocations occurred to reduce the chance of accidental extinction of the Middle Island population of only a few hundred adults and to contribute to the restoration of the other islands. All translocated insects originated from the captive-bred progeny of one male and two females collected from Middle Island between 1998 and 2001. Their establishment on Double and Red Mercury Islands, after their releases in 2000 and 2001 respectively, was confirmed by searching plots, and by using footprint tracking tunnels on Red Mercury Island between 2008 and 2012. Tracking tunnels provided better data and proved more cost effective than searching plots for detecting large tusked weta. Tracking tunnels demonstrated that the population on Red Mercury expanded outwards from the release sites by 50–100 m each year between 2009 and 2012. These weta are now estimated to be present over more than half the Island. Tusked weta have also survived on Stanley, Korapuki and Ohinau Islands after releases in 2007, but they remain within 100 m of the release sites. No confirmed progeny of the weta released on Cuvier Island in 2008 and 2011 were detected. No tusked weta were detected on Middle Island using tracking tunnels on eight occasions between 2009 and 2012, suggesting this species is likely to be locally extinct. Despite possible failure on one island, these translocations have resulted in a significant conservation success outcome.