Bringing Back the Bettong: Reintroducing ecosystem engineers for restoration in Box-Gum grassy woodland

Abstract

The grassy woodlands of eastern Australia have declined in their range by more than 95% through clearing and fragmentation. This decline has coincided with the loss of many digging or soil-foraging species that are considered to be 'ecosystem engineers' because of their role in biopedturbation and effects on other species and processes. Ecosystem engineers are therefore a priority for reintroduction to restore biodiversity and ecosystem function. However, there are gaps in our knowledge of how digging animals affect their environment and potential impacts on recipient ecosystems. The aim of this thesis was to examine the impact of the eastern bettong (Bettongia gaimardi) on ecosystem processes, following their reintroduction to a fenced reserve. It consists of five chapters that explore different aspects of this topic and the implications for management and conservation. Chapter 2 examines biodiversity patterns as a backdrop to bettong reintroduction in a Box-Gum grassy woodland. We found that beetle assemblages differed in their composition among distinct ground-layer plant communities at log and tree microhabitats, while beetle communities in open microhabitats were more uniform. Sites with evidence of prior agricultural use also had altered beetle communities. These findings demonstrate the fine-scale structure of the grassy woodland ecosystem as a mosaic of plant and insect communities. Chapter 3 investigates the structural and abiotic effects of bettong and rabbit foraging pits. I found that bettong pits filled in faster than rabbit pits due to their deeper and narrower shape. I did not find any consistent effect on soil nutrients in foraging pits, unlike similar studies in arid areas. Bettong pits reduced daily temperature fluctuations compared to the soil surface. I therefore concluded that in mesic environments, the structural effects of digging may be more important than changes in soil nutrients. Chapter 4 tests the hypothesis that bettong foraging pits provide favourable conditions for seed germination compared to the soil surface. I found that seedling abundance was almost doubled in pits compared to the soil surface. Responses differed between species and years, with native species responding more strongly to the presence of pits than exotic species in the first year. The response was also stronger in denser grassland, suggesting that the driving mechanism for the increased germination is the creation of gaps and reduced competition from the grass canopy. Chapter 5 examines the impact of bettongs on a native geophyte, the early nancy (Wurmbea dioica). I found that bettongs consumed 13-24% of the plants that emerged each year, resulting in a decline in the Wurmbea population over 5 years. However, there was a shift in the population demographics toward younger plants, which may suggest that bettong digging increased recruitment and a possible feedback mechanism. Similar interactions between digging animals and geophytes have been described internationally, but this is the first study in Australia. Finally, Chapter 6 synthesises research on soil-disturbing ecosystem engineers in Australia. I argue that the goals of species reintroductions are often poorly defined, leading to missed opportunities for research and potential negative outcomes. I discuss the factors that should be considered when reintroducing ecosystem engineers and provide a framework for clarifying the goals of species reintroductions. The outcomes of my research suggest that reintroducing extirpated ecosystem engineers may contribute to restoration of grassy woodlands, but they may also have unexpected consequences. Translocations should therefore consider both trophic and engineering effects and be prepared to monitor and manage unexpected outcomes. These findings will inform management of sanctuaries and translocations, and contribute to restoration efforts in grassy woodlands in Australia and worldwide.