Urban areas are rapidly expanding, causing extensive habitat modifications that have significant consequences for the environment and wildlife. Understanding how wildlife communities respond to urbanisation has been a central goal for urban ecologists for the last few decades. Changes in species richness, abundance and community composition are commonly reported in wildlife communities within urban landscapes. These changes are influenced by local and landscape level environmental variables. Fragments of vegetation embedded within the urban landscape are important areas for conservation and wildlife within these fragments are affected by the characteristics of the fragment. These characteristics include: fragment area, isolation, urban intensity of the surrounding landscape and other ecological variables e.g. habitat features or biological interactions. Understanding these fragment features and characteristics that help retain biodiversity is important for informed efficient conservation of biodiversity. In addition to the more traditional style of urban studies which examine changes in wildlife communities as a result of environmental variables, urban landscapes provide scientists with an opportunity to investigate a broad range of questions. For example, questions on evolution, dispersal, behaviour, ecological processes and biological interactions are all possible within urban environments.
In this thesis, I investigate a number of questions related to the effects of urbanisation on wildlife. I firstly examine changes in bird diversity in urban habitats and a number of potential mechanisms that may help retain diversity within new housing developments and forest fragments. Urban dung beetle communities are also examined, first to determine how they are affected by environmental variables and secondly to investigate how changes in dung beetle communities has altered the efficiency of an important ecological process within urban forest fragments. Finally, I investigate the effect of competition for food resources on the morphology of a secondary sexual characteristic, utilising a common dung beetle species within urban forest fragments as a model system.
Field surveys were used to examine species richness, abundance and composition of birds within vegetated fragments in urban areas of Brisbane, Australia. I found that fragment area was positively correlated with species richness, a finding that has been commonly reported for birds and a number of other taxonomic groups. Fragment connectivity had no significant effect on species richness or the abundance of birds. In new urban developments, I found that the presence of water bodies within the forested fragments had a positive effect on bird diversity, significantly increasing species richness. This finding further highlights the importance of retaining waterways within urban landscapes and supports previous findings of other studies. Field surveys were also used to examine the importance of retaining isolated remnant trees within new urban housing developments for bird diversity. It was found that urban streets which retained at least 1 remnant tree with a diameter 3 greater than 50cm lead to increased bird abundance compared to urban streets with no remnant trees. Additionally, increasing the number of remnant trees increased both the abundance of birds and the number of species observed within a transect, possibly a result an increased availability of resources. Retaining trees within the urban landscape may be a useful mechanism to increase the bird diversity utilising urban areas and increase the functional connectivity between forest fragments.
To further examine that effect of resource availability on wildlife communities in urban areas we examined the effects of food resources and fragment characteristics on dung beetles communities. We surveyed dung beetle communities over two field seasons from 37 forest fragments. Path analysis models were used to examine the direct and indirect effect of fragment area, isolation, food availability (macropod dung), and vegetation characteristics on dung beetle species richness. We found that fragment area had a significant correlation with species richness, however, path models found that fragment area only had an indirect effect on species richness via an increase in food resources with increased area. Increased food availability resulted in increased species richness. Thus, the decline in macropods in small urban fragments has resulted in decreased food resources for dung beetles and a decrease in species richness. These findings provide a clear example of the trophic cascades likely to be common in urban forests.
To investigate the effect of changes in species composition on the functional capacity of a community, a subset of the forest fragments were chosen to investigate the burial rate of macropod dung. Dung beetles bury dung beneath the ground to lay eggs and reproduce on, doing so increases the nutrient cycling within the soil. We measured the amount of macropod dung that was removed from experimentally placed dung balls in forest fragments where macropods were present and fragments where no evidence of macropods was found. We found that dung beetle species composition differed significantly between these two fragment types, fragments where macropods were still present had increased dung removal. This provides us with evidence that changes in species composition can alter the functional capacity of the community and the change in dung beetle communities was likely driven by the decline in food resources.
A change in resource availability is likely to have significant ramifications on many aspects of an organism’s ecology and life history. To further examine this we utilised the altered resource availability observed in urban dung beetle communities to examine the effect on a secondary sexual trait, male horn size. Male dung beetles utilise horns in male-male combat, with increased horn size an important predictor of success. We measured the body size and horn size of males from a number of urban populations to examine the effect of population level, resource competition on 4 average horn size of a population. It was found that populations with increased competition for macropod dung (food) had decreased average horn length. Additionally, increased competition resulted in increased variance of horn size within a population, both findings support current theory on condition dependent sexually selected traits.
The findings of this thesis further our understanding about the effects of urbanisation on wildlife, investigating changes in wildlife communities and a number of cascading effects including ecological processes and important sexually selected traits. These findings help us to better understand how bird diversity responds to a number of environmental variables within forest fragments and how to better retain bird diversity within new housing developments. Dung beetles have not previously been studied in urban environments and these findings further our knowledge on some of the ecological cascades that can occur in urban forest fragments. Additionally, few studies have been able to show a relationship between food resources and population differences in reproductively important traits in wild populations. Changes in important sexually selected traits are likely to have consequences on the behaviour and evolution of organisms within urban environments.