Shallow phylogeographic histories of key species in a biodiversity hotspot

Biodiversity hotspots may result from the retention of lineages through past climatic changes, local diversification of lineages or the accumulation of lineages derived from elsewhere. Different phylogeographic structuring is anticipated for taxa derived under these scenarios. Here we examine phylogeographic variation in four macroalgae that are dominant in a marine biodiversity hotspot and provide habitat for a range of other taxa, potentially influencing their diversity as well. Samples of Ecklonia radiata (Phaeophyceae), Macrocystis pyrifera (Phaeophyceae), Phyllospora comosa (Phaeophyceae) and Lessonia corrugata (Phaeophyceae) collected from 34 sites in south-east Australia – a recognized temperate marine biodiversity hotspot – were sequenced for the chloroplast Rubisco spacer region (rbcL) and mitochondrial cytochrome c oxidase subunit I (COI). Phylogeographic variation was limited to single shallow breaks within E. radiata and L. corrugata, corresponding to a recognized transition between biogeographic provinces, while P. comosa and M. pyrifera lacked spatial variation. The limited phylogeographic variation observed, in conjunction with phylogenetic relationships to other populations or congeneric species, suggests that each of these dominant habitat-forming species are recent arrivals (< 3 million years ago [Mya]) into the biodiversity hotspot. This contrasts starkly with expectations that dominant taxa in hotspots should reflect lineages that have adapted and persisted in these environments and raises concerns for the future of these ecosystems under climate change scenarios.