What does urbanization actually mean? A framework for urban metrics in wildlife research

Extensive research has demonstrated that urbanization strongly alters ecological processes, often perniciously. However, quantifying the magnitude of urban effects and determining how generalized they can be across systems depends on the ways in which urbanization is measured and modelled. We coupled a formal literature survey with a novel conceptual framework to document and synthesize the myriad of metrics used to quantify urbanization. The framework enables clear cataloguing of urban metrics by identifying (a) the urban component measured, (b) the method of measurement, (c) the metric's spatial scale and (d) the metric's temporal nature. Thus, the framework comprehensively captures the what, how, where and when of urban metrics. We documented striking variability in urban metrics with respect to which urban components were measured as well as how, where and when they were quantified. Overall, our survey revealed that they tended to be: (a) structurally focused, (b) methodologically simplistic, (c) spatially variable and (d) temporally static. Synthesis and applications. Many metrics are used to quantify urbanization or ‘urban-ness'. The variation in urban metrics complicates the development of theory, comparisons of findings across studies, and the implementation of management and conservation actions. To pave a clear path forward for more efficient and policy-relevant urban research, we systematically organized urban metrics using a simple, flexible and comprehensive framework. The framework clarifies what urbanization actually means in empirical practice and identifies several crucial areas for future research, including: (a) systematic assessments of urban metrics across multiple scales, (b) an increased and judicious use of more complex urban metrics aimed at evaluating both mechanistic and broad-scale correlative ecological hypotheses, and (c) an increased emphasis on the socio-economic aspects of urban effects.