Inselberg floristics exemplify the coast to inland OCBIL transition in a global biodiversity hotspot

Abstract

We examined the floristics of granitoid inselbergs in the hitherto poorly documented south-eastern region of the Southwest Australian Floristic Region (SWAFR) and adjacent Great Western Woodlands, addressing several hypotheses of OCBIL (old, climatically buffered, infertile landscapes) theory. We found exceptional taxon richness (1550 taxa on 89 inselbergs, with 58 well-sampled inselbergs and 1493 taxa chosen for detailed analyses). Granite inselberg endemism declined towards the arid inland, although taxon richness did not. OCBILs are likely to be found up to 500 km inland, not ~300 km as previously hypothesized. Hybrids were extremely rare on the 58 inselbergs analysed, whereas rare species, including singletons, were abundant. Conversely, exotic weeds were less common than in the whole SWAFR flora (8.2% vs 12.8%). Granite plant communities were distributed in bands parallel to the south coast, approximating the general transition from the Esperance and Boylya Floristic Districts across the SWAFR boundary north into the Arid Zone’s Great Western Woodlands. Positive correlations were found between several plant life forms and inselberg area. There was a decrease from the coast inland for most life forms, except for annual and graminoid herbs that increased in taxa inland. Thus, inselberg floristics exemplify the coast-to-inland OCBIL transition in this global biodiversity hotspot.