What makes nutrient-poor mediterranean heathlands so rich in plant diversity?

Mediterranean heathlands with extremely low soil-nutrient concentrations (the fynbos of South Africa and the kwongan of Australia) have plant species diversities several times greater than one would expect from their areas. A combination of three factors provides a sufficient explanation for these diversities: First, poor soils favour shrubs that are killed by fire and reestablish from seed ('seeders'). Otherwise, the frequent fires in most mediterranean heathlands favour shrubs that can re-sprout ('sprouters'). Second, the numeric dominance of seeders on poor soil lowers their extinction rates. Third, seeders have relatively short generation times and thus increased speciation rates. Elevated speciation rates coupled with depressed rates of extinction lead to enhanced diversities. We elucidate this scenario and discuss evidence that favours the first factor. The evidence comes from 23 previously unanalysed sample plots surveyed by R.H. Whittaker and from two supplemental data sets. In mature fynbos and kwongan, 90 and 93% respectively of the shrub cover belongs to shrubs that re-seed after fire. In maquis (Israel), chaparral (California) and matorral (Chile), the proportion is considerably smaller. Mature strandveld, a South African shrubland superficially like fynbos but with richer soil, has only 29% seeders, although it is physically adjacent to fynbos. We suggest that nutrient-poor soil may favour seeders because the extra investment in underground organs is not worth the cost: pulses of nutrients released by fire lie mostly on top of the soil, inaccessible to new growth sprouting from subterranean lignotubers or epicormic buds.