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 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, they 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 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 Floristic 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.
Plant biodiversity is particularly notable in the MTE regions. These areas are home to ~45,000 species of vascular plants, roughly 16% of the world’s roughly 250,000 plant species. Nowhere outside of lowland tropical rainforests do ecosystems have higher regional diversities of species. For these reasons, MTEs have all been designated global ‘‘hotspots’’ of evolution. Vertebrate diversity within each region varies, and is less significant overall on a global basis. Even so, MTEs are rich in specific groups of reptiles and amphibians.
A characteristic ecological feature of MTEs is the dominance of evergreen shrublands. Many of these shrub species have sclerophyllous—small, hard, and leathery—leaves. The dense cover and biomass of these shrublands burn readily under dry, arid summer conditions. Each MTE region has a unique burn frequency. Many such species have evolved morphological, ecophysiological, and phenological adaptations that enable them to regenerate after fire. Most such characteristics involve resprouting and reseeding that is stimulated by fire. Also widely present in MTE sclerophyll shrub are adaptations to tolerate low soil nutrient availability and summer drought conditions.
Other vegetation forms are also present in MTE regions. Woodlands are widespread in most areas, particularly those with deeper or richer soils. Wetter sites host riparian woodlands or gallery forests. Oak woodlands dominated by both evergreen and deciduous species of Quercus are widespread in California and the Mediterranean Basin. 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.
Along drier coastal and arid interior margins, the evergreen, sclerophyll shrublands of California and Chile commonly grade to a plant community dominated by drought-deciduous shrubs. Known as sage scrub in California and coastal matorral in Chile, this community may also dominate early successional disturbance sites or arid microsites in evergreen shrublands. 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.