| Overview | Natural Disturbances Regimes | Patterns of Speciation |
Although the five mediterranean-climate regions share many aspects of natural climatic and environmental disturbance regimes, there are significant differences as well. While all five regions experience characteristic summer drought, for example, the magnitude of this drought is particularly severe in California and Chile, where 6–8 months or more may pass without measurable rainfall. Extreme drought such as this is rare in South Africa and southwestern Australia where summer months frequently have light showers.
The dense canopies of evergreen sclerophyllous shrubs make fire an important component of natural disturbance regimes. Natural fires, however, are significant ecological events in consuming aboveground vegetation in only four of the five MTEs. Central Chile, where natural fires are a rare event, is the exception among MTEs. This mediterranean-climate region is protected from summer storms and lightning moving westward across Argentina by the high Andean Cordillera. The native flora of Chile shows little evidence that fire has been an important ecological disturbance regime in the evolution of life history characteristics.
Natural fire frequencies are quite different among the other four mediterranean-climate regions. In South Africa, for example, fynbos vegetation in the Cape Region commonly burns at intervals of 10–15 years, while in California natural frequencies are thought to be 30–50 years to as much as a century or more.
There are other strong environmental differences in addition to reduced drought stress, which make the Cape Region of South Africa and southwestern Australia distinct from the other three MTEs. These two areas lie in geologically ancient and stable landscapes, resulting in highly leached and nutrient-poor soils. In contrast, earthquakes, volcanic activity, orogenic uplift, and other dynamic processes create natural disturbance regimes in California, Chile, and the Mediterranean Basin. Unlike South Africa and southwestern Australia, the younger landscapes of these three regions have experienced tremendous changes in climate regime and landscape structure in Quaternary and Holocene times and these changes have had profound impacts on community structure and speciation. The remarkable patterns of speciation in fire-sensitive shrub lineages in the Cape Region of South Africa and southwestern Australia likely have resulted from a combination of relatively mild and stable Quaternary climatic conditions coupled with high fire frequencies in these nutrient-poor habitats.