Biogeographic Regions Within Indomalaya: An Integrated Approach Using Bird Distributions

ABSTRACT Aim Biogeographic regions define areas of shared species distributions and provide a framework for testing evolutionary hypotheses. Recent quantitative methods have improved the delineation of these regions, but each relies on assumptions about how species distributions are summarised and varies in sensitivity to spatial scale. Investigating how these assumptions affect biogeographic inferences is an important but often overlooked part of biogeographic analyses. Here, we compare three modern delineation methods and evaluate their ability to infer biologically meaningful biogeographic regions for birds across Indomalaya. Location The Indomalayan Biogeographic Realm. Methods We apply three area delineation methods ( NDM , Ecostructure, and Hierarchical Clustering) to the distributions of 1544 native resident bird species of Indomalaya at four spatial scales (i.e., grid resolutions of 0.5°, 1°, 2°, 5°). We compare the results and discuss the strengths and weaknesses of each method with respect to issues related to analytical resolution, input data, and regional geomorphology. Results Each method produced different sets of areas and was differently impacted by analytical resolution. Each method also displayed unique problems when identifying small areas and continental areas with more diffuse boundaries. Nonetheless, all methods consistently found four large biogeographic units congruent with our understanding of the geological history of Indomalaya. Main Conclusions While the three methods in our study showed broad agreement in results as well as similar sensitivity to analytical resolutions, each method displayed different strengths and weaknesses in relation to the challenges of delineating biogeographic areas. NDM and hierarchical clustering are more effective at identifying regional‐scale areas of high endemicity, assuming there is strong spatial signal for the former and sufficient taxon dissimilarity between areas for the latter. Ecostructure and hierarchical clustering are effective at partitioning a large study area into broad, continuous biogeographic areas, but the former is more effective at visualising uncertainty of continental boundaries. By integrating results across multiple methods, we gain a more enriched view of the complex biogeographic history in the Indomalayan Realm.