Investigating the impact of potential increased competition with common brushtail possums (Trichosurus vulpecula) in the decline of bush rats (Rattus fuscipes) in Booderee National Park

Abstract

Environmental change influences the composition and abundance of species within ecosystems. Changes in ecosystem dynamics can lead to disruptions to species interactions, resulting in unforeseen consequences, and risk of species loss. Competition is one such interaction that plays an important role in the maintenance and structuring of ecosystems but is often overlooked in ecosystem change and conservation research. Changes in ecosystem dynamics can arise via multiple pathways (e.g., novel disturbances and/or species introductions). However, an underappreciated pathway is through well-intended conservation efforts, such as the removal of invasive predators, that can result in unintended negative outcomes. In Booderee National Park, located on the south coast of New South Wales, Australia, there has been an effective and ongoing fox-baiting program for more than 20 years. This program has been linked to population increases of the common brushtail possum Trichosurus vulpecula, a native species. During the same period, a significant decline of native small mammals, including the bush rat Rattus fuscipes, has been detected. The objective of this thesis is to investigate whether increased competition with the common brushtail possum has contributed to the decline of bush rats in Booderee National Park. To achieve this, I used multiple methods to quantify the association between the two species and attempt to characterise the mechanism(s) of competition underpinning this association. In doing so I addressed the following aims: (1) quantify changes in the co-occurrence of co-occurrence of common brushtail possums and bush rats over time, (2) document evidence of competitive interactions through experimental manipulation of supplementary food sources, and (3) determine if there is a significant overlap in the diets of the species. To tackle the first aim (Chapter 3), I analysed 17 years of mammal trapping to examine the co-occurrence of bush rats and common brushtail possums. The model revealed a negative association between the two species' abundances, however time since fire had a greater influence on bush rat populations. For the second aim (Chapter 4), I used camera traps to study competitive interactions, finding that heavy possum presence led to reduced bush rat foraging activity and shorter bait consumption periods, indicating temporal avoidance by bush rats. Addressing the third aim (Chapter 5), I employed DNA metabarcoding of faecal samples to assess diet overlap between possums and bush rats. Results showed both species are generalists with overlapping diets, though bush rats exhibited a wider diet range. This flexibility likely aids bush rats in selecting outside of possums' preferred food items, mitigating competition effects. These findings collectively suggest that possums are contributing to the decline of bush rats in Booderee National Park through a negative, primarily competitive, interaction. This association has arisen from the growth of possum populations following the successful fox baiting program in the park. Without proper management, possum populations can be expected to continue to grow, and this negative interaction will continue to be detrimental to bush rats. My study demonstrates that conservation actions, such as invasive predator control, can have cascading negative impacts on ecological community composition. The change in the abundance and population growth patterns of a species as a result of reduced predation pressure can destabilise pre-existing interactions. However, many of these interactions are either indirect or difficult to monitor and manage. Careful ecosystem management, including the construction of trophic web maps, is required to predict, mitigate and manage these unexpected consequences.