Introduction

As of 2003 more of the world's population lives in urban than in rural settings, and this provides us with immense challenges in the planning, management and conservation of urban areas. Cities in sub-Saharan Africa are growing faster than in any other region as a result of migration, reflecting people's hopes of escaping rural privation more than actual opportunity in the cities (United Nations Population Fund,1996). This sociological phenomenon is also backed by statistical reason: in South Africa a new resident moving into a city has a 60% chance of securing a work opportunity, while the general chances for someone remaining in a rural area are about 40% (Policy Co-ordination and Advisory Services, 2003). In our view, this increase in urbanisation provides enough reason to study the ecology of cities, towns and settlements.

According to Niemelä (1999a) an important reason for the study of urban ecosystems is that the creation of healthy and pleasing urban settings should be based on ecological knowledge of human impacts on urban ecosystems. Detailed ecological information is needed for better urban spatial planning (Niemelä, 1999a). Certain ecological processes in cities are the same as in rural areas, but aspects such as invasions by alien species and external control of succession are more prevalent in urban areas (Trepl, 1995). In South Africa, another justification for studies of urban environments is to conserve the high biodiversity of the area, because the process of urbanisation has resulted in the fragmentation of previously intact natural environments.