Multi-scale biogeographic controls over plant communities on granite outcrops in the Southwest Australian Floristic Region

Granite outcrops are globally widespread habitat islands that harbour naturally fragmented areas of vegetation in 'pockets' of soil. These habitats are ideal for investigating the drivers of biogeographic patterns at different scales. However, beyond the tropics, understanding of the environmental drivers of community composition within and between these 'island on islands' is limited. In this study, we determined within- and between-outcrop patterns of plant biodiversity in the Southwest Australian Floristic Region - a global biodiversity hotspot. We measured plant community composition of soil-patches on nine granite outcrops. Outcrop floristic composition was mostly controlled by geographic location and outcrop topography (slope and aspect-northness), whereas plant community diversity (richness, abundance, evenness and Simpson diversity) was strongly controlled by soil-patch size, regardless of outcrop location or size. Outcrops with larger soil-patches hosted greater plant diversity on those soil-patches. Effective conservation of outcrops should focus on protecting the fragile soil communities on multiple rock outcrops at a regional scale to allow for the continuing persistence of extraordinary biodiversity in these regions.