With over 300 chenopod species occupying its deserts and coasts, Australia is a recognised global centre of chenopod diversity but the biogeography and drivers of this diversity are poorly understood. The prominence of sandy and salty habitats within desert and coast, suggests that desert species initially derive from a coastal flora but the long-held, speculative notion of this ‘littoral connection’ has no systematic, biogeographic analysis. To understand chenopod expansion and diversification from coastal ancestors, species richness and distribution are examined in context of emerging arid landscapes, formulating hypotheses for phylogenetic testing.
Global molecular studies reveal at least 11 chenopod clades mostly arriving in Australia since the Late Miocene. Diversification in rich clades links to Late Miocene-Pliocene adaptive radiations, coinciding with the onset of continental aridity. From species richness and sharing patterns based on occurrence and assembly in 23 Physiographic Provinces, major areas of diversification can be identified, including variation that may suggest differing evolutionary history of the immigrant clades. Landscape routes of range expansion emerge from frequency analysis of species habitats categorized into 10 major land types of known landscape geochronology, based on their existing range and habitats as described in national Floras and herbarium collection records.
Results/Conclusions
Provincial distributions identify two large centres of chenopod diversity (Yilgarn and Eyre-Murray). Dissimilarity indices for provincial assemblages show a close relationship between the centres, sharing 128 species, though 165 species occur in only one. Principal inland habitats are saline land types of Riverine Desert (arid river/floodplain) and Desert Lake (saltlake), while xeric types of Sand Desert (dunefield) and Desert Upland (arid ranges) have relatively low chenopod diversity. Furthermore, 95 species are specialists of saline types and 6 species are specialists in xeric types. Of the 25 coastal species, those with inland populations outnumber coast specialists 3:1 and increasingly occupy inland marshes.
The relationship between the centres of diversity suggests hypotheses testable through phylogenetic analysis. Either the centres developed separately and later exchanged species through range expansion, or a continental flora divided in two and each centre diversified separately. The species rich Riverine Desert is the most likely route of range expansion from the coast, facilitating early diversification within the rich clades. It links past coastlines to the two centres of diversity through Neogene palaeodrainage, whereas Pleistocene formation of Desert Lake and Sand Desert post-date early adaptive radiations. Increasing salinization, more than adaptation to aridity, influenced chenopod expansion and diversification.