Introduction

The human population is growing rapidly (Hopfenberg and Pimentel,2001), in particular in Africa, the Middle East and Asia since 1950, and is becoming more urbanised (McDonnell et al., 1997; Cohen, 2003). This influx of people from the rural landscape into cities creates major environmental concerns. Many cities are overpopulated and becoming more so, putting enormous pressures on urban green spaces. Furthermore, many cities are expanding in size, infringing on more natural suburban and rural environments.

What makes cities different from other environments? Obviously, and most importantly, cities and their suburbs are places where humans live. Ecologists agree that the main difference between urban and rural areas is the intensity of the impact of humans on nature (McDonnell et al., 1997; Niemelä, 1999b). Humans have modified and constructed their environment to such an extent that most of the city surface is covered by artificial substrates (see also Breuste, Chapter 21). Not only buildings and roads, but also the soil beneath lawns is likely to be of foreign origin. As a result of human activities, a considerable amount of waste, exhausts, and sometimes toxic substances, are released into the environment (see also Carreiro, Chapter 19). Furthermore, urban environments are characterised by the presence of exotic species, and species that have been modified by selective breeding, i.e. domestic animals (see also Catterall, Chapter 8; Nilon, Chapter 10; Natuhara and Hashimoto, Chapter 12).