A spatially explicit delineation of transition zones for biogeographic regionalization

O_LIA central goal of biogeography is to partition the worlds biota into meaningful biogeographical regions. While naturalists have long delineated biogeographic regions using hierarchical hard clustering approaches, they have often acknowledged the existence of transition zones. However, such transition zones have not been empirically identified or distinguished from hard clusters. Transition zones are outstanding areas for understanding the processes that shape biotic assembly; however, a coherent framework for delineating them is lacking. C_LIO_LIHere, we propose a framework to empirically identify and analyze transition zones through a new metric, the "Transition Zone Index (TZI)". The TZI decomposes the proportional contribution of multiple biotas to a spatial unit (called a map cell) derived from a Grade of Membership model into a single quantitative measure per cell using Shannon entropy principles. The transition zones can then be quantified and presented as a gradient, scaled from TZI = 0 (indicating distinctive bioregions) to TZI = 1 (corresponding to the highly transitional areas with maximally admixed biotas). Finally, we implement spatial regression models to detect the predictors of these transition zones. C_LIO_LIWe demonstrate the application of our framework using vascular plants of southern Africa. Through the spatial variation of TZI, our approach represents transition zones as a continuous gradient of biotic turnover between bioregions. These transition zones were found to be either previously delineated as discrete clusters or incorporated into neighboring clusters. Our approach effectively captures both the intensity and width of transitional dynamics, showing that these patterns are primarily shaped by precipitation. C_LIO_LIBiogeographic transition zones are a measurable gradient, and our framework provides a robust quantitative foundation for detecting and analyzing the drivers of these areas. This improves our ability to understand the patterns and processes underlying biogeographic transition and provides important implications for conservation planning and biodiversity management. C_LI