Abstract
Among invasive species, mammals contribute disproportionately to global species decline. New Zealand’s invertebrate community is threatened by mouse predation with the potential ‘release’ of mice, through the eradication of top predators such as rats, stoats and cats. Evolving without mammalian predators aside from bats, the terrestrial invertebrates here lack the predator defence mechanisms or lifestyle to maintain their populations under this predation pressure. Mice are known to be the worst predators of ground-dwelling invertebrates, particularly large bodied ones. Though this interaction has been studied within alpine, podocarp and grassland ecosystems, I addressed a gap in the literature and investigate how this unfolds within a beech forest ecosystem. During mast years, beech forests exhibit pronounced predator-prey interactions with large population increases followed by crashes of introduced mammalian predators due to the abundance and subsequent decline in food. My research investigated the impacts of house mice (Mus musculus) on the terrestrial invertebrate community present within the beech forest at the Brook Waimārama Sanctuary, a fenced sanctuary in which all invasive mammals except mice have been eradicated.
A 3.7 ha fence was installed within the sanctuary as a ‘mouse-free’ area, of which mice were eradicated from in February 2024. Using tracking tunnels, I analysed the tracking data of both ground wētā (Hemiandrus sterope), a large-bodied ground-dwelling invertebrate, and mice to understand how mice impact the tracking rates of wētā. I investigated how temperature impacts this relationship, as well as the spatial distributions of both mice and wētā across the study. The results showed a negative, yet insignificant, effect of mice on the tracking rates of wētā within the mouse exclusion area, where wētā tracking rates increased from 5.55% at mouse tracking rates of 70.37%, to 40.64% in the absence of mice. Temperature was significant in shaping the activity of wētā, with warmer conditions increasing activity. Spatial analyses revealed distinct patterns in the distribution of wētā across the study site, suggesting potential use of microhabitat refugia.
Using pitfall traps, I explored the impact of mice on the invertebrate community present within the eradication area, as well as comparing the communities at two more study areas (sanctuary adjacent to the eradication site, and bush outside the sanctuary) under varying introduced mammalian predator regimes. Both the sanctuary adjacent to the eradication site, and bush outside the sanctuary were dominated by small to medium sized invertebrates which changed very little throughout the study. Within the eradication area, numbers of large- bodied invertebrates increased, though there was also a steep decline in small and medium
invertebrates. Analyses revealed a significantly positive relationship between mice and the total abundance of invertebrates, indicating a complex relationship between the invertebrate community and mice presence.
My research highlights the negative implications of mice on terrestrial invertebrates within a beech forest ecosystem. Results indicate a size-specific predation bias, with large-bodied invertebrates suffering most. Results highlight the potential negative implications associated with the ‘release’ of mice given the removal of their predators and illustrate the need for targeted mice control in sensitive and high importance areas for ground-dwelling invertebrates.