A mechanistic exploration of the effects of human land-use on the balance between native and invasive mammal species

Invasive species and habitat degradation are among the main threats to global biodiversity. Many studies describing invaded ecosystems have found that human activities are often linked to an increase in the occurrence and abundance of invasive species, usually at the cost of native biodiversity. With its evolutionary isolation and unique ensemble of fauna, invasive species are one of the greatest environmental problems facing Australia. Australia’s southern island, Tasmania, represents a microcosm for studying the combined impacts of invasive species and land use, with all major land uses within a discrete island and at a smaller geographical scale.
In this thesis, I set out to answer the following question: “At what level of human activity do we start having more invasive than native mammal species in Tasmania?” To do so, I measured the diversity and abundance of native and invasive species in the mammal communities in the three major anthropogenic land-use types (undisturbed, plantations, and agricultural land), replicating them in each bioregion across Tasmania. I then focused on the mechanisms that could be influencing the population growth rates of nine native (Tasmanian devil Sarcophilus harrisii, spotted-tailed quoll Dasyurus maculatus, eastern quoll D. viverrinus, brushtail possums Trichosurus vulpecula, eastern bettong Bettongia gaimardi, long-nosed potoroo Potorous tridactylus, eastern barred bandicoot Perameles gunnii, southern brown bandicoot Isoodon obesulus, swamp rat Rattus lutreolus) and two highly invasive (the feral cat Felis catus, and the black rat R. rattus) mammal species and analysed these mechanisms at the community level. Specifically, I measured: 1) stress levels through faecal glucocorticoid metabolites; 2) diet composition and resource competition through a combination of environmental DNA and stable isotope analyses; and 3) viral load and risk of disease spill-over and spillback.
In Chapter 1 of my thesis, while conducting a systematic literature review, I found that research in invasion biology, which often informs management actions, is: 1) skewed towards measuring changes in species richness and abundance instead of studying the multiple relationships of alien species in the invaded ecosystem, knowledge of which is important to avoid unexpected outcomes during management; and 2) relegating human activities to the discussion of the study instead of assessing them as a factor influencing the formation, maintenance, strength, and direction of these relationships.
Chapter 2 of my thesis “sets the scene” for the chapters to follow. I analyse camera trap data from my study sites across land-use type and bioregion using Hierarchical Modelling of Species Communities (HMSC, a Bayesian multivariate species distribution modelling approach that accounts for species interactions based on the presence-absence, and abundance of sympatric species in the analysis). I find that land-use indeed influences the structure and composition of the mammalian community. The next three chapters delve into the mechanisms that might underly these patterns.
In Chapter 3, I use scat samples collected from trapped animals across my study sites to compare the concentration of faecal glucocorticoid metabolites (FGM) as an indicator of adrenocortical activity (often associated physiological responses) of Tasmanian devils, spotted-tailed quolls, eastern quolls, brushtail possums, Tasmanian bettongs, long-nosed potoroos, and southern brown bandicoots in different land-uses. I discovered two major patterns: first, land use is an important variable explaining the variation in FGM concentrations in all species; and second, animals inhabiting plantations generally present with lower FGM concentrations. I discuss the limitations and caveats of FGM interpretation and how studying patterns of FGM concentrations in multiple species in community assemblages can offer a short-term way around these limitations.
In Chapter 4, I analyse the diet of native and invasive mammal species inhabiting different land-uses at different time scales using scat and blood samples. I found regionalisation in the diet of smaller species (rodents, brushtail possums and eastern quolls) but not in the two larger predators which consume widespread prey species, and that land-use further defines what each species feeds on. My results also show dramatic changes in the diets of native swamp rats possibly resulting from land-use and/or densities of sympatric black rats, raising questions about the potential long-term impacts of dietary shifts on life?history traits and evolutionary changes in native species.
In Chapter 5, I extracted RNA from faeces to compare the risk of disease (i.e. virus) spill-over, spill-back and exposure in native and invasive carnivores, omnivorous brushtail possums, and native and invasive rats in different land uses. I identified 58 vertebrate-associated viruses, 50 of which are novel. I found that the virome composition of Tasmanian mammal host species is surprisingly homogenous among land-uses, bioregions, status (native/invasive), and trophic levels. Supported by the findings in Chapter 4, I discuss the homogenisation of viromes in the context of the distribution of invasive species (i.e. black rats and feral cats) in Tasmania.
I restricted my study to the austral winter, the most challenging time of year in this temperate environment when stressors are likely to be most evident, to avoid additional effects of seasonality and mating seasons in my target species. In doing so, my thesis provides a first glimpse into how animals perceive their surroundings and some of the mechanisms by which they respond to the challenges the environment bestows upon them at one specific point in time. Instead of finding the answer to one question, my thesis, with all its restrictions, has uncovered patterns and questions that will each require exhaustive research to be fully understood. This highlights just how little we know about the mechanisms that drive the interactions in invaded ecosystems that result in ecological and evolutionary changes, and that will ultimately determine the success or failure of native biodiversity.