Abstract
The fragmentation of forested environments in Aotearoa me Te Waipounamu/New Zealand has resulted in many small, geographically isolated local populations. These may be subjected to different limiting factors that influence a particular population's growth, distribution, or abundance. Limiting factors can include predation, food availability, and habitat availability. Predation alone is thought to be the primary driver of species decline in most forested areas nationwide. However, the complexity of interactions with other factors can be challenging to unravel. Movement is essential for facilitating population growth and gene flow, provided individuals can successfully establish themselves in these new habitats. Dispersal allows species to adapt to habitat changes, avoid competition, and escape from threats or adverse conditions in their juvenile habitats. Effective dispersal combines the physical act of moving from one place to another with the ability to find suitable resources such as food and shelter and successfully reproduce in these new areas.
I investigated the influence of predation, food availability, and functional habitat connectivity on a locally threatened, isolated insectivorous passerine that is highly sensitive to mammalian predators: the kakaruai (South Island robin; Petroica australis). I studied two populations within the wider Ĺtepoti/Dunedin area: the remnant population persisting at Silver Stream, a reserve where no predator control has occurred since 2016, and the Orokonui Ecosanctuary population, translocated from Silver Stream (and the Silver Peaks) in 2010. Orokonui is a fenced mainland ecosanctuary, which excludes all invasive mammals except mice (Mus musculus). These two populations share the same gene pool but are geographically isolated from one another due to habitat fragmentation.
I firstly investigated the influence of predation and food availability on kakaruai. Initially, this involved a bifactorial experiment comparing interaction effects on nesting success between the two populations. However, upon discovering the impending collapse of the Silver Stream population, I instead compiled and evaluated key vital rates of the Silver Stream and Orokonui populations based on an extensive literature review alongside my own field observations during the 2022-23 breeding season. Only one female was located at Silver Stream during the 2022-23 season. Analysis of breeding behaviours and nesting outcomes at Silver Stream revealed a significantly male-biased sex ratio, likely a direct result of mammalian predators targeting incubating females on nests. My results emphasise the role of mammalian predation as a primary limiting factor and the proximate cause of an imminent local extinction. In contrast, the Orokonui population exhibited significantly different vital rates, including sex ratio, nest survival, and nest site selection, consistent with what would be expected from a predator-free environment.
I then examined whether the Orokonui kakaruai population was experiencing effective dispersal, also known as positive spillover, from inside the predator exclusion area and into surrounding habitats outside the fence. Incorporating data from surveys completed in 2018-2019, I carried out an additional survey season in 2022. I evaluated spillover through three key indicators: (1) detecting the presence of kakaruai at the same sites across multiple years, (2) assessing whether there is an increase in the number of birds within specific sites already occupied, and (3) identifying any newly colonised sites. I provided evidence for all three indicators suggesting spillover from Orokonui and effective dispersal of kakaruai. I found evidence that distance from the ecosanctuary, survey year, habitat type, and functional habitat connectivity all influenced kakaruai presence. The occurrence of kakaruai within the first 1,500 metres of the ecosanctuary increased significantly in the three years between 2019 and 2022. The effective dispersal of juveniles beyond the fence boundaries relies on whether sufficient habitat exists nearby. Functional connectivity, which integrates the quality of habitat with its connectivity to surrounding areas, was critical for kakaruai dispersal.
The major implication that emerges from my research is that the remnant Silver Stream population of kakaruai is in need of immediate conservation intervention due to the impact of mammalian predators. Additionally, my research can be used as a model for investigating effective dispersal at a population level, benefiting those seeking strategies for reinforcing biodiversity beyond ecosanctuary borders and at a landscape scale. My work also provides evidence for positive spillover from an ecosanctuary, provided habitat conditions are appropriate.