Re-Thinking the Urban Biotic Homogenization Hypothesis

The Urban Biotic Homogenization (UBH) hypothesis predicts that urban areas are ecologically homogenous at both intra- and inter-city scales. This hypothesis has played a central role in the field of urban ecology over the past two decades. However, the body of literature testing this hypothesis shows mixed results, with only half of studies finding support for UBH. Here we argue that the UBH hypothesis has been overstated in magnitude and scope. While UBH does occur in certain scenarios, metacommunity theory suggests that in many scenarios, biotic heterogeneity should arise. Furthermore, unique social-ecological processes can create or maintain biotic heterogeneity at the local patch to landscape scales. Urban biotic homogenization versus diversification may be scale-dependent, though this is largely overlooked in research. Thus, we argue that biotic heterogeneity as a phenomenon may explain a majority of urban biodiversity's species diversity and distribution. We present three core tenets of urban ecosystems, which we argue facilitate biotic heterogeneity rather than homogeneity: metacommunity dynamics, human dimensions of management and variation in urban typologies within and across cities. We present examples from the literature and a series of novel research questions that can catalyse investigations into the relationships between urbanisation and biotic heterogeneity, and what applications can come from such research.