Inter-specific interactions around resources, such as nesting sites, are an important factor by which invasive species impact native communities. As resource availability varies across different environments, competition for resources and invasive species impacts around those resources change. In urban environments, changes in habitat structure and the addition of introduced species has led to significant changes in species composition and abundance, but the extent to which such changes have altered competition over resources is not well understood. Australia’s cities are relatively recent, many of them located in coastal and biodiversity-rich areas, where conservation efforts have the opportunity to benefit many species. Australia hosts a very large diversity of cavity-nesting species, across multiple families of birds and mammals. Of particular interest are cavity-breeding species that have been significantly impacted by the loss of available nesting resources in large, old, hollow-bearing trees. Cavity-breeding species have also been impacted by the addition of cavity-breeding invasive species, increasing the competition for the remaining nesting sites. The results of this additional competition have not been quantified in most cavity breeding communities in Australia. Our understanding of the importance of inter-specific interactions in shaping the outcomes of urbanization and invasion remains very limited across Australian communities. This has led to significant gaps in the understanding of the drivers of interspecific interactions and how such interactions shape resource use in highly modified environments. This knowledge deficit limits the effectiveness of conservation and management efforts to mitigate the loss of nest sites for native species and the effectiveness of ongoing management actions, such as the addition of artificial nesting boxes.
To address these gaps, I examined the changes in native and invasive alien species use of resources across urban gradients and examined the factors shaping inter-specific interactions to provide a mechanistic framework for understanding competition at the community scale and invasive species impact on native cavity-nesting species. The research presented here provides the first community-level description of competitive interactions, describes a mechanism driving interaction intensity, and provides a method to predict where and when invasive species impacts on nesting are likely to occur. Additionally, I show that both habitat structure and predation are important processes for urban cavity-nesting birds. My work highlights that many birds make some use of urban habitats. While there remains much to learn about how urban environments can be improved to support long term
persistence of individual species, conservation and management efforts can enhance the opportunity to help a large number of birds in Australian cities.
This study established a large-scale field project that included eight field sites across southeast QLD and NSW. The study was designed along both regional-scale invasion gradients, with focus on the Common (Indian) myna (Acridotheres tristis), and local-scale urbanization gradients. I discovered that interaction webs around tree hollows in urban environments are dominated by a small number of highly aggressive species, including the invasive common myna and that interaction frequency between species was positively correlated with the overlap in preferred breeding sites (breeding niche; Chapter 2). To investigate how breeding niche overlap can be applied to a community with many unstudied invasive species, I examined the extent of niche overlap between species in the cavity-breeding birds of Tasmania detailing the potential interaction network for the island (Chapter 3). I discovered that niche overlap predicted important inters-specific interactions and that most native cavity-nesting species potentially face increased competition for nesting sites as a result of the addition of multiple invasive species.
In Chapter 4, I examined how birds use supplementary nesting sites (i.e, nest boxes) varied across urban environments in different landscape contexts across New South Wales and Queensland), and across cities which have been invaded for different lengths of time. This was aimed at understanding the importance of artificial nest boxes as supplementary resources in areas with limited breeding cavities due to human impacts. There was high variation in the rates of nest box use both by invasive and native species such that the importance of nest boxes as supplemental nesting sites will differ across cities. I found that many nesting attempts in the nest boxes failed, highlighting the need to account for both competition and predation when designing and deploying nest boxes.
Finally, in Chapter 5, I explored patterns in the community composition of urban bird communities. This allowed me to test the impacts of regional invasion dynamics and local variation in habitat on avian community richness and abundance. I discovered that increasing habitat structure and decreasing abundance of despotic species (i.e., species which influence community composition through aggressive interactions such as the noisy miner) predicted greater native species richness. More broadly the work presented in this thesis highlights that urban conservation efforts will be improved by a better including into management actions and future work the factors influencing breeding dynamics and resource use.