The number and size of cities and towns around the globe are rapidly expanding. There is a tremendous call for information about urban ecosystems in order to address issues related to the conservation of natural resources and the development of sustainable planning and building practices, while also providing a safe and healthy environment for humans. Historically urban ecosystems have not been the object of study by ecologists and thus there is a crucial lack of knowledge regarding the structure and dynamics of these human dominated systems. The use of a comparative approach provides urban ecologists with the ability to quickly develop a greater understanding of the structure, function and dynamics of urban ecosystems.
Comparative studies involve the systematic assessment of the similarities and differences between entities or systems. They are a valued and well tested method of developing new understandings in a diversity of disciplines and have been successfully used in ecological and sociological research. To effectively elucidate the multiple dimensions of urban ecosystems and create sustainable cities in the future, urban ecologists need to develop comprehensive ecological and sociological knowledge bases for cities with a range of sizes, developmental histories and at local, regional and global scales. Such knowledge bases can provide important generalities or principles about the relationships between the structure, function and dynamics of urban ecosystems. By making these relationships clear we can progress the study of urban ecology from correlative relationships to the development of a more mechanistic understanding of cities and towns.
We examined the methods used by urban ecologists to study urban ecosystems over the past 20 years, and assessed their potential contribution to comparative analyses. Case studies that provide a contextual reference offered opportunities for productive meta-analyses. One of the major challenges to successful comparative studies is the identification of standardised, quantitative measures for classifying both the independent (predictor) and dependent (response) variables for meta-analyses and new empirical studies.
Comparative ecology allows us to expand our questions beyond the scope of an individual city and begin to ask whether specific ecological patterns and processes are common to most cities. When there are exceptions, a comparative approach also allows us to investigate what social, biophysical or climatic factors are moderating the response. To begin drawing generalisations from urban ecological research, it is imperative that we begin placing a greater research attention on the opportunities that are offered by a comparative ecology approach.