How many principles of urban ecology are there?

Context The nature of urban ecology theory is controversial. Issues include whether urban theory is distinct, whether it has principles, and whether those principles differ from those for non-urban systems. Recently, Richard Forman enumerated urban ecology principles, while we have previously articulated different urban ecology principles. This raises the question of whether there are legitimately different sets of principles. Objectives Recognizing the legitimacy of Forman's principles, we wish to determine whether different methodological assumptions can lead to different sets of urban ecology principles. Methods We contrast Forman's 90 urban ecology principles with our much smaller set to determine why they differ in detail, empirical motivation, and generality. We identify the approaches that generate each set of principles, and seek an inclusive framework to integrate them. Results The alternative sets represent contrasting approaches to identifying principles: Forman's approach is more inductive, generating principles from a body of empirical cases, while our concise roster is more deductive, focusing on general ideas from which the details of specific cases can be derived. Both are legitimate. Conclusions Principles of urban ecology can arise from inductive empirical generalization within specified ecological, cultural, and historical contexts, as shown by Forman. However, urban ecology principles can also emerge from a more general, synthetic impulse as we have shown. Neither approach is necessarily better. Both can contribute to a comprehensive theoretical hierarchy that can advance urban ecology.