The role of large, isolated trees in supporting urban biodiversity

Abstract

Large, isolated ‘specimen’ trees are a significant part of urban landscapes, yet their role in supporting wider biodiversity remains largely unexamined. The aims of this thesis were to examine how urban, isolated trees contribute to the maintenance of bird and arthropod communities. These aims were met through the compilation and analysis of a dataset on the behaviour of native and exotic birds in forty study trees, representing four genera. A survey was undertaken on the arthropod assemblages on a subset of these trees. Finally, GIS information was incorporated into the bird observation dataset to place the trees within a landscape context.

The data presented in chapter two highlighted both bird and tree interspecific differences in how the trees were used by birds. For some bird species, most notably silvereyes (Zosterops lateralis), some of the study trees were used extensively for foraging. For other bird species, native nectivores in particular, the observed behaviour was strongly suggestive of the trees fulfilling a stepping-stone role. In many cases, the movements of birds were correlated with tall vegetation and other large trees, providing further evidence that the isolated study trees were playing a role in the maintenance of landscape connectivity. Significant differences were found between exotic cosmopolitan birds commonly associated with urban areas, such as blackbirds (Turdus merula), and native forest birds. These differences suggest that the stepping-stone role provided by isolated trees may be more important for the native forest birds than the exotic bird species. In chapter three a survey of the arthropod fauna of urban trees was examined. Existing sampling methods had to be adapted to suit the particular challenges associated with sampling urban trees. Significant differences in the arthropod abundance, richness, and diversity were found. These differences were related to both the sampling method used, and the species of tree sampled. This arthropod data was linked to silvereye foraging in chapter four. In contrast to many native bird species, silvereyes thrive in urban areas. This makes them a useful study organism to examine what factors make some species robust to urban development, while others suffer localised extinction. The silvereyes showed an ability to incorporate exotic trees with high arthropod abundances into their foraging patterns, in spite of the general trend for native fauna to be associated with native vegetation. As lack of arthropod prey is a limiting factor for many urban bird species, this may be an important factor enabling silvereyes to thrive in urban areas. In chapter five the bird species richness associated with urban trees was examined, and analysed with regards to spatial data on three scales. At every spatial scale analysed, significant differences were found. The findings suggest that native birds have more narrow requirements regarding which trees they will utilise, and that they are more sensitive to the contexts in which urban trees are situated. These results show that by tailoring urban tree plantings and the contexts in which they are planted, the contribution they make to supporting native birds can be enhanced.