Contrasting evolutionary hypotheses between two mediterranean‐climate floristic hotspots: the Cape of southern Africa and the Mediterranean Basin

Abstract

Aim The Cape of southern Africa and the Mediterranean Basin, two of the world's five mediterranean-climate biodiversity hotspots, are exceptionally species-rich and constitute a well-described example of ecological convergence. However, the area-adjusted plant species diversity of the Cape is on average more than double that of the Mediterranean Basin. Here, we investigate the causes of this diversity asymmetry by drawing on phylogenetic information from a variety of plant groups and focusing on three competing hypotheses: diversity disparities arising from differential clade ages, diversification rates or diversity limits.Location Cape of southern Africa and the Mediterranean Basin.Methods We reviewed a variety of studies in order to contrast the geography, geomorphic history, biogeographical connectivity and ecological context in the two hotspots. We also tested the relationship between clade age and species richness in both regions based on phylogenetic information from 39 clades.Results Clades are on average older in the Cape than in the Mediterranean Basin. Clade age is a strong predictor of species diversity in the Cape, suggesting that diversity-dependent regulatory mechanisms may be weak. In contrast, we failed to find a relationship between age and diversity in the Mediterranean Basin, indicating that a diversity limit may have been achieved in multiple clades. Main conclusionsThe Cape has a higher species density than the Mediterranean Basin owing to a combination of older clade ages, high rates of diversification in certain lineages and an exceptionally high upper limit to diversity. High richness in the Cape is linked to long-term lineage persistence in a heterogeneous but stable evolutionary context. In contrast, the climatically unstable Mediterranean Basin has offered fewer opportunities for diversity accumulation in the long term (owing to high extinction rates), but appears to be a hotspot of recent rapid speciation.

Variation in plant diversity in mediterranean‐climate ecosystems: the role of climatic and topographical stability

“…increased diversification rates towards the present). The limited available data are largely consistent with this hypothesis (Hopper et al ., ; Sauquet et al ., ; Valente et al ., ; Buerki et al ., ; Valente & Vargas, ).…”

“…Differences in Neogene plant diversification rates are unlikely to have produced these patterns: these rates, although variable, are highest in the Mediterranean Basin, intermediate in California and the Cape, and lowest in Australia (Linder, ; Madriñán et al ., ). Studies on lineages shared between MCEs indicate that contrasting diversification rates do not explain differences in extant species richness (Hopper, ; Sauquet et al ., ; Valente et al ., ; Buerki et al ., ; Valente & Vargas, ). Instead, we suggest that the persistence of older clades in the more stable MCEs of the Cape and Australia is a more likely explanation for contemporary diversity patterns.…”

“…Contemporary ecological factors do not explain all the variation in regional‐scale plant diversity of MCEs (Cowling et al ., ; Valente & Vargas, ). Energy regimes, as measured by potential evapotranspiration, are highest in the South West Australian Floristic Region and the California Floristic Province (Bradshaw & Cowling, ), which show contrasting diversity patterns.…”

“…Moreover, topographical heterogeneity – a surrogate for habitat diversity – is highest in the (relatively species‐poor) eastern Mediterranean Basin and central Chile and lowest in the (relatively species‐rich) South West Australian Floristic Region, whereas the Cape Floristic Region, which is the most species‐rich area by far, has moderate heterogeneity (Bradshaw & Cowling, ). It appears that variation in contemporary plant diversity patterns in MCEs is profoundly influenced by their respective environmental histories and their impacts on patterns of diversification (speciation minus extinction) (Cowling et al ., , ; Jansson & Dynesius, ; Linder, ; Hopper, ; Valente et al ., ; Valente & Vargas, ).…”

Using endemic-plant distribution, geology and geomorphology in biogeography: the case of Sardinia (Mediterranean Basin)

“…Furthermore, geology and geomorphology are important factors determining endemic distribution patterns (e.g. Tribsch & Sch€ onswetter, 2003;Valente & Vargas, 2013); indeed, bedrock chemistry has been shown to be significant for the delimitation of biogeographical units in several studies on mountain plants of south-eastern France (M édail & Verlaque, 1997), southern Spain (Mota et al, 2002) and the Swiss Alps (Wohlgemuth, 2002), among others.…”