|Overview||Natural Disturbances Regimes||Patterns of Speciation|
Although there has been widespread speculation for many years on the causal factors responsible for the high levels of vascular plant biodiversity in MTEs, few clear generalizations have emerged. What is known is that there are conditions promoting the coexistence of seemingly ecologically equivalent shrubs, geophytes, and graminoids (Restionaceae) in frequently burned shrublands on nutrient-poor soils of fynbos and kwongan communities.
Slow growth rates and diverse strategies of postfire regeneration and reestablishment appear to promote this coexistence. MTEs on less-infertile soils and with longer intervals between fires (chaparral, matorral, maquis) have lower diversity as short-lived species are excluded by rapidly growing shrub dominants. Open woodlands in the Mediterranean Basin and California, however, may have very high local diversity of annuals and short-lived perennials, where grazing maintains open habitat for species establishment. Regional topographic and climatic heterogeneity per se, which might be thought to be logical correlates of comparative diversity among the five MTEs, is a poor predictor of diversity.
Instead, natural selection has operated to allow for a fine-scale discrimination of habitats and niches under the selective pressures of stable climates, predictably frequent fires, and periodic drought that promote community turnover and diversification. Thus, fynbos in the southwestern Cape Region and kwongan communities in Southwestern Australia have evolved species-rich landscapes in topographically homogeneous areas through rapid speciation coupled with low extinction rates. High predictability of interannual rainfall and relatively limited summer drought periods may be an important factor in this low level of extinctions.