Why is Table Mountain's flora unique?

In the latest issue of Veld & Flora (March 2011) Alice Notten's reply to Ronnie Glass's letter sums up why the fynbos is so interesting.

The letter asked the question: Fynbos is said to have the most plant species and cover the smallest area, yet they say the tropical rainforests have more species. What am I misinterpreting regarding our fynbos?

Alice Notten, the Chief nterpretive Officer at Kirstenbosch, replied:

I think the confusion arises from the way it is measured and compared. The figures that John Manning gives in his book Field Guide to Fynbos are species per 1000 km², calculated by dividing the total number of species by the surface area covered by that vegetation type. He says that the entire Cape Floristic Region (that includes fynbos, renosterveld and succulent Karoo) averages 94 unique species per 1000 km². The similar heathland communities of California and south-western Australia average 14 and 12 unique species respectively.

Southern Africa as a whole averages out at 8 unique species per 1000 km². Taking just the fynbos areas within the Cape flora, he gives an average of 150 to 170 unique species per 1000 km². Using this method fynbos figures are two or three times that measured for tropical rainforests (this figure for rainforests is not given in his book). So fynbos has a high number of species packed into a tiny area.

The other way of measuring is to measure out a quadrant and count the number of species found in that quadrant. On this scale the rainforests come out on top. Tropical rainforests range between 130 and 190 species, fynbos has 65, Renosterveld has 85 and California heathlands have 30.

Why then does fynbos show diversity but not remarkable diversity on the local richness scale yet enormous diversity for the vegetation as a whole?

Fynbos diversity does not lie in the number of species found at any particular site, it lies in the proportion of species shared between sites, i.e. fynbos has an exceptionally high number of highly localized species – species that are restricted to a single small area. If you look at the species lists for quadrants only kilometres apart, between half and two thirds of the species will be different for fynbos whereas in the rainforest the same species are scattered widely over a large area. Thus you will find more species per square kilometre in a rainforest than you will in fynbos, but if you move to another square kilometre, the fynbos species will have changed whereas those for the rainforest will remain much the same, so in total fynbos counts up more species over the whole area it covers than rainforests.

The photograph is by Alice Notten and shows proteas and ericas on Table Mountain.