The Sahul-Sunda floristic exchange: Dated molecular phylogenies document Cenozoic intercontinental dispersal dynamics

Abstract

Aim: The aim was to characterize the temporal dynamics of the Sahul-Sunda floristic exchange using published dated molecular phylogenies. Location: The Sahul and Sunda shelves in Australasia and Southeast Asia. Methods: Dated molecular phylogenies were compiled from the literature for plant clades that contained at least one node representing a biogeographical disjunction between the Sahul and Sunda shelves. For these nodes the age, ancestral geographical area and propagule type were determined. Results: We analysed 49 clades from 21 published phylogenies representing a diverse set of angiosperm lineages. The inferred age of the disjunctions ranged from c. 33 Ma to c. 1 Ma; the earliest age marked the onset of the Sahul-Sunda floristic exchange. Disjunctions (resulting from dispersal/migration events) occurred at the rate of 0.41 per 2 Myr between 34 and 12 Ma. Thereafter the rate sharply increased, coincident with the shelves effectively merging. For nearly two-thirds (63%) of the nodes Sunda was the ancestral area, and for 90% the ancestral species possessed zoochorous propagules. Main conclusions: There is strong support for a dynamic model of floristic exchange between Sahul and Sunda. Fewer (18%) disjunctions occurred prior to Sahul and Sunda merging around 12 Ma, which we attribute to a combination of the effect of overwater dispersal barriers and relatively stable, saturated species assemblages resistant to the establishment of newly arrived lineages. The exchange, once underway, was strongly asymmetrical; eastwards migration into Sahul predominated over the reverse by a factor of c. 2.4. As zoochorous lineages were overrepresented among the successful dispersers, we infer a strong role for localized animal dispersal across narrow water barriers.