A common view is that species in biodiversity hotspots diversify more quickly than species in less biodiverse areas. However, that seems not to be the case for a group of Australian wildflowers, the spikey-flowered Banksia. This genus is found in many parts of Australia, including the biodiverse Southwest Botanical Province, which is home to more than 7300 plant species in the midst of shrubland, semi-arid heath, and Mediterranean-climate forests. But although Banksia species richness in this hotspot is ten times higher than in the rest of the continent, it's not diversifying any more quickly than congeneric plants in other parts of Australia.
How did the biodiversity of the Southwest Botanical Province arose? If new species aren't arising more rapidly, perhaps they go extinct less frequently compared with similar plants in less diverse regions. A team from the Australian National University, Canberra constructed the a near-complete phylogeny of Banksia to test whether diversification rates have differed between lineages confined to the southwest Australian hotspot and those found throughout southern, eastern and northern Australia.
The geographic pattern of diversification in Banksia appears more complex than can be characterized by a simple hotspot vs. non-hotspot comparison, but in general, these findings contrast with the view that the high diversity of Mediterranean hotspots is underpinned by rapid radiations. Steady accumulation of species at unexceptional rates, but over long periods of time, may also have contributed substantially to the great botanical richness of these regions.
The team also looked at biodiversity within the confines of the Southwest Province where they found that Banksia plants in semi-arid heath and shrublands were diversifying more quickly than plants in the high-rainfall forests. Diversity is likely generated in these semi-arid regions, then migrates out to boost diversity in the adjacent forest.
Biodiversity hotspots are frequently found in Mediterranean-climate regions, where they rival tropical rainforests for flowering plant biodiversity. But these environments typically lack features such as high rainfall or productivity that are usually linked with high plant diversity. Indeed, some of the most species-rich Mediterranean communities are found in dry regions, on nutrient-poor soils. Understanding these apparent outliers on global biodiversity gradients may yield insights into the factors driving the diversification of flowering plants.