Estimating the impact of 'Predator Free' Wellington on Wellington tree wētā (Hemideina crassidens)
Authors:   , Hosnell Jarrod 
Published: February 2025

<p><strong>The Wellington tree wētā (Hemideina crassidens) is a large-bodied nocturnal insect native to Aotearoa New Zealand. H. crassidens can inhabit environments with invasive mammals, therefore they provide an opportunity to study responses to mammalian predator presence. By comparing populations within a 'Predator Free' area and a 'Predator Present' area, it is possible to estimate how the removal of invasive mammal species influences certain endemic species. Populations of H. crassidens were examined with a before-after control-impact (BACI) study design within Wellington City, Aotearoa New Zealand. Eight suburbs had H. crassidens observations recorded between 2017 and 2024, where complete rat elimination had occurred in two suburbs during 2020 under the 'Predator Free' Wellington initiative. H. crassidens populations were observed to increase significantly in rat eliminated areas compared to the rest of Wellington. A more in-depth analysis of H. crassidens populations was done within the Wellington suburbs of Miramar and Kaori. This study addressed how morphology, abundance, and sex ratio of H. crassidens were influenced by native regenerating forests, and mature pine habitats within Miramar and Karori. In addition, Spatially Explicit Capture-Recapture (SECR) was applied to estimate the true population density, detection rate, spatial parameters, and total population number within each study site. SECR is rarely performed on insects, hence the study is a test to determine if H. crassidens can effectively have their density determined with such a method.</strong></p><p>The main question I asked is “does ‘Predator Free’ Status influence the populations of H. crassidens, or is habitat type a stronger influence? To answer this, the population abundance, body size, and sex were compared within two suburbs of Wellington, 'Predator Free' Miramar, and 'Predator Present' Karori using a two-by-two crossed design. Population abundance was significantly influenced by 'Predator Free' status and habitat type, with the greatest number of wētā observed within Karori’s 'Predator Present' native forest and the fewest in the Karori pine plantation site. Body length was observed to be larger on average in Karori habitats, while male head size was larger within mature pine plantation forests regardless of ‘Predator Free’ status. Many H. crassidens were found within the same gallery for weeks or months on end which resulted in non-independent observation counts between sessions. There was difficulty in recapturing H. crassidens after they disperse, however a true population density estimate was achieved. Native regenerating forests contained the higher density estimates; with the largest density observed within Karori. Total population estimates aligned with habitat area, which suggests that H. crassidens populations are likely influenced by multiple factors and predator-control isn’t the largest influence. It is likely that for future experiments using SECR with artificial wētā galleries, the galleries need to be closer together (<5m) or have multiple on the same tree at varying heights to increase the number of “spatial recaptures”. The results of this experiment can therefore guide future studies for monitoring tree wētā, particularly where information on movement is central to the monitoring.</p>