Explaining the uniqueness of the Cape flora: incorporating geomorphic evolution as a factor for explaining its diversification

Abstract

The plant diversity of the Cape Floristic Region is regarded as being exceptional in an ecological and evolutionary context. The region supports about double the number of species predicted by models based on water-energy variables for regional floras globally. However, contemporary diversity patterns are profoundly influenced by evolutionary processes contingent upon idiosyncrasies of history and geography. The relatively recent appearance of dated molecular phylogenies, and their optimization in relation to habitat and geography, has provided hitherto unsurpassed opportunities to generate knowledge about the evolution of the Cape flora. Almost all studies invoke climatic deterioration during the Mio-Pliocene as the major trigger of radiations and subsequent speciation of Cape clades. While some do show the importance of edaphic heterogeneity for clade radiation, the evolution of this heterogeneity is not considered. Here, we review the literature on the late Cenozoic geomorphic evolution of the Cape in order to assess the extent to which the changing nature of scenery and soils could act as a stimulus for plant diversification. Despite dating uncertainties associated with both the phylogenetic and geomorphic data, it appears that moderate uplift in the early and late Miocene, which significantly increased the topo-edaphic heterogeneity of the Cape was an important driver of plant diversification. In particular, the massive increase in heterogeneity after the late Miocene event probably acted in synergy with rapid climatic deterioration, to produce the extraordinarily rapid diversification recorded for some Cape clades at that time. A comparison of the plant diversity and palaeoenvironmetal patterns of mediterranean-climate regions provide insights regarding the "remarkable environmental conditions" of the Cape that have generated the high diversification and low extinction rates necessary to produce such a rich flora. These conditions are a gradual increase in topo-edaphic heterogeneity and relative climatic stability during the late Cenozoic.