A framework for transparent quantification of urban landscape gradients

Context: The urban gradient concept has been applied widely to test biological responses to urbanization. However, a general understanding of the effects of urbanization has remained elusive, largely due to inconsistencies in the definition of ‘urban’. Moreover, ambiguities about how to measure urban gradients exist, despite well-established best practices in landscape ecology of landscape gradient definition. 

Objectives: To describe current practices of urban gradient definition and characterize how existing approaches compare to established best practices. In particular, we seek to evaluate the extent to which a single decision framework exists across gradient definitions. 

Methods: We reviewed 250 urban ecology focused research articles that invoke the urban gradient concept. For each, we recorded metrics related to the methodological process of urban landscape quantification. 

Results: Our review revealed striking methodological consistency aligning with the best practices of gradient definition: (1) the selection of features representative of the urban landscape, (2) identifying associated data to characterize features, and (3) selection of an appropriate spatial scale. Apparent inconsistencies in urban gradient definition, thus, reflect ecological context of the study (i.e. species, location). This decision framework can be synthesized into a standardized landscape gradient framework that promotes candid reporting and justification for, decisions made regarding landscape features, data selection and processing, and spatial scale. 

Conclusions: As the global human population grows, it is imperative to understand ecological processes in urban regions to create socially and ecologically sustainable cities. Practicing the transparent decision framework described herein will help to standardize methods, and improve consistency in urban ecology research, and better understand the process underlying ecological patterns in human dominated landscapes.