| Summary: | Interactions between coexisting invasive species can cause complications when their populations are managed to protect native biodiversity. The ship rat (Rattus rattus) is a widespread invasive species often targeted for management because of its negative impacts on native wildlife, particularly in otherwise mammal-depauperate ecosystems such as in New Zealand. However, where ship rats are removed, another common, coexisting invasive species, the house mouse (Mus musculus), is often detected more frequently, which may undermine the benefit of the management operation for biodiversity. The aim of this study was to better understand why house mice become more abundant, or potentially also more active and detectable, when released from suppression by ship rats through determining the mechanism involved. The hypothesised mechanisms were: exploitation competition, interference competition and intraguild predation. Focusing on New Zealand, I reviewed diet studies of ship rats and house mice to have a clearer understanding of the resources they may share. I found that whilst some features of their diets differ, ship rats and house mice do show overlap in the range of food items they consume. Therefore they could compete for these shared resources if they were limited. However, in captive experiments I confirmed that ship rats exhibit predatory aggression towards house mice and therefore have potential to directly negatively influence mouse populations regardless of resource availability. In response to the threat of predation by ship rats, house mice exhibited avoidance of caged rats during further captive experiments and this restricted their foraging choices. In the field, the foraging behaviour of mice in podocarp-broadleaf forest was also limited by the risk posed from abundant ship rats, which prevented them from accessing resources. In similar habitat at Pureora Forest Park during a longer term study of mouse populations, mice captured when ship rats were abundant had lower body mass compared to those captured when ship rats were controlled, an effect that was not offset by supplementary feeding. At Pureora, the ship rat control operations did not achieve optimal low ship rat levels, however, despite small mouse sample sizes, both the abundance of mice measured by live-trapping and their activity in tracking tunnels were positively affected. These measures were moderately correlated indicating that activity was related to mouse abundance. However, capture probability varied seasonally and according to rat abundance in unexpected ways, indicating more subtle and complex potential influences of ship rats on the probability of detecting mice. My results indicate that the main mechanism by which ship rats suppress house mice is intraguild predation. This is because though apparently food restricted, house mice did not access resources I provided for them when ship rats were abundant, which rules out exploitation competition. Ship rats appear to view house mice as prey and opportunistically consume them, which differentiates intraguild predation from interference competition as the latter is primarily driven by resource defence. Even if predation events are rare, my research demonstrates that the risk effects of avoiding an abundant opportunistic predator appear to have a strong influence on the abundance and distribution of house mice.
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