|Overview||Natural Disturbances Regimes||Patterns of Speciation|
Mediterranean-climate ecosystems (MTEs) occur in only five regions of the world: California, Central Chile, the Mediterranean Basin, South Africa, and Southwestern Australia. These regions lie on the western edges of continents between 30°and 40° latitude. Their unique climate pattern is characterized by dry summers with little or no rain and mild wet winters. Although the Mediterranean-climate regions comprise only about 2% of the earth’s land area, they account for 16% of the world’s plant species.
Although frosts may occur throughout much of the MTE regions, these are infrequent and relatively mild in lowland areas. Mean annual precipitation is as low as 250 mm in coastal areas of the MTEs and reaches to about 900 mm at the upper margins of the classic evergreen shrub zone. Mean annual rainfall is generally as low as about 120 mm at the floristic transition between mediterranean and desert biome regions.
Higher montane areas with winter rainfall regimes where the majority of precipitation falls as winter snow are also present within the traditional boundaries of MTEs. Wet temperate forests with a winter rainfall regime are generally not included within the mediterranean-climate region of Chile, but California includes the moist coast redwood forests within the California floristoc province. Although typically excluded from the boundaries of MTEs, adjacent arid regions that exhibit winter rainfall regimes (e.g., the Mojave Desert in California, Atacama Desert in Chile, Succulent Karoo in South Africa, and northern Sahara Desert) have a mediterranean-type climate in the broad sense.
Biodiversity is particularly notable in the MTE regions for vascular plant species. Although the combined area of these five regions is little more than 2% of the land area of the Earth, MTEs are home to ~45,000 species of vascular plants 18% of the world’s total of about 250,000 species. Nowhere outside of lowland tropical rainforests are ecosystems with higher regional diversities of species, providing a strong justification for each of these regions being designated as a global ‘‘hotspot’’ of evolution. Vertebrate diversity is variable, and less significant overall on a global basis, but highly diverse for many specific groups of reptiles and amphibians.
A characteristic ecological feature of MTEs is the widespread dominance of evergreen shrublands dominated by species with sclerophyllous leaves. The dense cover and biomass of these shrublands burn readily under dry summer conditions with low humidity, although with differing frequencies characterizing individual MTE regions as described below. Thus, there are evolved morphological, ecophysiological, and phenological adaptations to postfire regeneration of these stands through resprouting and firestimulated reseeding. Also widely present in MTE sclerophyll shrub are adaptations to tolerate low soil nutrient availability and restricted summer water availability.
Despite the general characterization of MTE regions as having dominance by evergreen shrublands, other vegetation forms are also present. Woodlands are widespread in most MTEs, particularly in areas with deeper or richer soils, or as riparian woodlands or gallery forests in wetter sites. Oak woodlands dominated by species of Quercus are widespread in California and the Mediterranean Basin, with both evergreen and deciduous species as dominants. These communities can take the form of closed canopy evergreen woodlands, grading into shrublands as in live oak woodlands of Southern California and the maquis of Europe, or open savannas of deciduous oaks that are widespread in both regions.
Central Chile once had widespread dry and wet sclerophyll woodlands, but the dominance of these has been dramatically reduced by human activities. Evergreen woodlands dominated by Eucalyptus are widespread in western Australia. Only the Cape Region of South Africa of all of the MTEs is largely lacking in woodlands, with such communities restricted to scattered stands of relict Afromontane forest along the southern coast in areas lacking in strong seasonal drought.
The evergreen, sclerophyll shrublands of California and Chile commonly grade off along drier coastal margins and at their arid interior margin to a plant community dominated by drought deciduous shrubs. This community, which may also dominate early successional disturbance sites or arid microsites in evergreen shrublands, is termed sage scrub in California and coastal matorral in Chile. Structurally similar communities with mixed dominance of low evergreen (and more rarely deciduous shrubs) are called phrygana in Greece and batha in the eastern Mediterranean Basin.