Plant diversity of Holocene dune landscapes in the Cape Floristic Region: The legacy of Pleistocene sea-level dynamics

Coastal dune landscapes are subject to harsh environmental conditions and, owing to Pleistocene sea-level fluctuations, comprise small, fragmented features. These environments present several challenges for plant colonization and persistence, and dune floras are therefore likely to be species-poor but endemic-rich. Here, we explore an explicitly historical hypothesis — the Pleistocene disturbance hypothesis — to explain patterns of plant diversity on Holocene dunes of the Cape Floristic Region (CFR). This hypothesis predicts that: (1) dunes would not show the west–east decline in diversity evident in inland habitats as the unique selective regime of the dune environment would override the evolutionary diversity patterns evident elsewhere in the CFR; (2) dune diversity would be lower than in adjacent, inland habitats that were not exposed to extreme disturbance regimes during the Pleistocene. We tested this by compiling regional-scale plant species diversity data from dune and interior lowland habitats of the southwestern and southeastern CFR and subjecting the data to species–area analysis. Our results were consistent with the Pleistocene disturbance hypothesis: diversity patterns of dunes did not differ between the southwestern and southeastern subregions, and interior sites were richer in species than dune sites. However, the difference in diversity between interior and dune sites (1.42×) was remarkably marginal. We propose that this is a consequence of the CFR's glacial physiography, where a large expanse of dune habitat was exposed at length along the Palaeo-Agulhas Plain during Pleistocene sea-level lowstands. This vast habitat area would have facilitated the development of a species-rich dune flora.