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

Mediterranean heathlands with extremely low soil-nutrient concentrations (t he 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 d iversities: First, poor soils favour shrubs that are killed by fire and ree stablish from seed ('seeders'). Otherwise, the frequent fires in most medit erranean heathlands favour shrubs that can re-sprout ('sprouters'). Second, the numeric dominance of seeders on poor soil lowers their extinction rate s. Third, seeders have relatively short generation times and thus increased speciation rates. Elevated speciation rates coupled with depressed rates o f extinction lead to enhanced diversities. We elucidate this scenario and d iscuss evidence that favours the first factor. The evidence comes from 23 p reviously unanalysed sample plots surveyed by R.H. Whittaker and from two s upplemental data sets. In mature fynbos and kwongan, 90 and 93% respectivel y of the shrub cover belongs to shrubs that re-seed after fire. In maquis ( Israel), chaparral (California) and matorral (Chile), the proportion is con siderably smaller. Mature strandveld, a South African shrubland superficial ly like fynbos but with richer soil, has only 29% seeders, although it is p hysically adjacent to fynbos. We suggest that nutrient-poor soil may favour seeders because the extra investment in underground organs is not worth th e cost: pulses of nutrients released by fire lie mostly on top of the soil, inaccessible to new growth sprouting from subterranean lignotubers or epic ormic buds.