Abstract/Summary

Cities are novel and fast changing environments. We have little understanding of how urbanisation affects ecological patterns and processes. In Chapter One I review the literature (with an emphasis on arthropods) concerning the general effects of urbanisation on biodiversity, how urban greenspaces are structured, the main characteristics of urban ecological populations and communities, and how trophic dynamics and species interactions are affected by urban environments. The experimental chapters of this thesis focuses on addressing gaps in knowledge concerning how species interactions respond to increasing levels of urbanisation, by using aphids and their natural enemies and ant mutualists as a model system. In Chapter Two I explore the local and fine scale environmental drivers of naturally occurring assemblages of aphids, and their coccinelid and syrphid predators, in urban gardens. Ladybirds are the only group affected by increased urbanisation, while aphids and hoverflies vary as a function of host plant abundance and garden plant richness, which in themselves are indirect consequences of urbanisation. In Chapter Three, I investigate the main biotic and abiotic factors that affect the recruitment of naturally occurring predators, parasitoids and mutualists in experimental colonies placed on an urbanisation gradient. In Chapter Four, I build on this to consider how two aphid species, each differentially attended by mutualists, are influenced by urbanisation. In both chapters I found a higher sensitivity of predators to increased urbanisation, while ants appear to particularly benefit from the creation of these novel habitats. In Chapter Five I explore if there is a differential response of predator functional groups to urban green spaces, and I confirm my hypothesis that specialist predators respond more strongly to increased abundance and size of green space in urban areas. In Chapter Six I investigate which features of urban habitats have a greater influence in the outcome of interspecific competition, and my results suggest that an increased abundance of ants in urban habitats may act in concert with abiotic features of urbanisation to affect the dynamics of competition synergistically. In the seventh and final chapter, I discuss the main findings, contributions and future directions in the field of urban ecology.