Why biogeographical transition zones matter

Biogeographical transition zones are areas of overlap, replacement or partial segregation between different biotas, which are sets of taxa sharing similar geographical distributions as a result of a common evolutionary history and ecological affinities. Transition zones occur in a few particular areas of the world where historically contingent processes have allowed different biotic elements to coexist and interact. Evolutionary or historical biogeographers have traditionally characterized transition zones by the overlap of different biotic elements. More recently, authors applying track analyses have detected transition zones by the presence of nodes or by conflicting results in cladistic biogeographic analyses. Ecological biogeographers have taken a different approach, usually applying quantitative methods, for example, species turnover indices to analyse the strength and breadth of biogeographic transition zones. Some authors consider transition zones to represent outstanding areas for analysing the principles shaping biotic assembly, whereas others have dismissed them, sometimes as artificial. Biogeographical transition zones are particularly interesting for a truly integrative biogeography, being natural laboratories where the interaction of historical and ecological processes can be analysed fruitfully, and are especially appropriate for an integrative approach, epistemologically grounded in integrative pluralism. A coherent conceptual framework to analyse transition zones, however, is still lacking.