Introduction

Comparative studies of cities are important for understanding the structure and dynamics of urban ecosystems (McDonnell and Hahs,Chapter 5). Comparative studies also contribute to the development of urban ecology. However, as McDonnell and Hahs point out, information on non-human components of urban environments is very limited. It is more problematic in cities where environment data are not available because of financial difficulties, lack of necessary investigation techniques and human resources. Even in cities where information is available, the usefulness of the data for comparative study can still be doubtful because the data are often not collected in a uniform way. The lack of information poses a challenge when conducting a comparative study of cities (McDonnell and Hahs, Chapter 5).

Non-human environmental data can be gathered through ground surveys. Methods such as biotope mapping (Sukopp and Weiler, 1988; Breuste, Chapter 21; Wittig, Chapter 30), urban–rural gradient analysis (McDonnell et al., 1997; Zipperer and Guntenspergen, Chapter 17; Carriero et al., Chapter 19; Pouyat et al., Chapter 20) and others (Rogers and Rowntree, 1988; Nowak et al., 1996) have been used successfully in many cities. However, these methods require extensive funding and labour support that are not always easy to get. There are also other restrictions that make ground surveys infeasible: for example, the cities under study may be inaccessible for political and safety reasons. A comparative study of cities therefore needs a tool to provide reliable, uniform and cost-effective data.