Wallace's Line and plant distributions: two or three phytogeographical areas and where to group Java?

Abstract

Wallace's Line or its variants divide the Malay Archipelago or Malesia into a western and eastern area, but is this suitable for plant distributions? Indeed, all boundaries satisfactorily divide Malesia into two parts, stopping far more species east or west of a line than disperse over the boundary. However, phenetic analyses (principal components analysis, nonmetric multidimensional scaling analysis and the unweighted pair group method with arithmetic mean) of 7340 species distributions revealed a stronger partitioning of Malesia into three instead of two regions: the western Sunda Shelf minus Java (Malay Peninsula, Sumatra, Borneo), central Wallacea (Philippines, Sulawesi, Lesser Sunda Islands, Moluccas, with Java), and the eastern Sahul Shelf (New Guinea). Java always appears to be part of Wallacea, probably because of its mainly dry monsoon climate. The three phytogeographic areas equal the present climatic division of Malesia. An everwet climate exists on the Sunda and Sahul Shelves, whereas most of Wallacea has a yearly dry monsoon. During glacial maxima, the Sunda and Sahul Shelves became land areas connected with Asia and Australia, respectively, whereas sea barriers remained within Wallacea. Consequently, the flora of the two shelves is more homogeneous than the Wallacean flora. Wallacea is a distinct area because it comprises many endemic, drought tolerant floristic elements.