Introduction

The prophetic words of geographer Chauncy Harris (1956), who believed that global urbanisation was a ‘gigantic and pervasive revolution’, hold even more true now than when he uttered them in 1956. Whereas less than a third of the world's population lived in urban areas in 1950, over half currently lives in urban areas (World Bank,1984), nearly half of the Earth's terrestrial surface has been altered by human activities, and human-induced losses to biodiversity are occurring at unprecedented rates (Vitousek et al., 1997b). Urbanisation is thus among the most influential factors shaping biological communities today, but the scope and magnitude of its consequences have only recently been glimpsed.

Urbanisation is the process whereby humans convert indigenous ecosystems to a type of ecosystem in its own right, the urban ecosystem. An urban ecosystem is not merely an area under human domination; rather, it is characterised as an area of high-density human habitation (see McIntyre et al., 2000). The amount of energy consumed in an urban ecosystem is over 1000 times greater than that in other types of ecosystems (Odum, 1997). As such, cities and towns have an ecological ‘footprint’ (sensu Wackernagel and Yount, 1998) that extends far beyond recognisable urban boundaries (Luck et al., 2001). An urban ecosystem is at least as heterogeneous as native ecosystems in terms of three-dimensional structure, land-cover types, microclimate zones and resource availability. Within urban ecosystems, the presence of anthropogenic impervious surfaces and building materials may alter climatological and hydrological cycles (Arnold and Gibbons, 1996).