Vegetation and Ecosystem Dynamics

Within the Mediterranean region a number of distinctive vegetation communities can be recognized, comprising some 25,000 species, of which about 50 per cent are endemic. Broadly defined, these originated with the establishment of a mediterranean-type climate about 3.2 million years ago, since when they have been subject to the vicissitudes of glacial–interglacial climate changes, plus the intensification of human impact during the last 10,000 years (Chapters 4 and 9). These communities are dynamic, responding to environmental changes at a variety of scales, both spatial and temporal. This chapter explores the characteristics of these communities and examines the relationships between ecosystem dynamics and biodiversity, and ecosystem response to disturbance. For example, each year fires burn out of control and are the subject of regular news stories during summer months. While fires may be economically devastating and lead to loss of life (Chapter 19), ecologically their incidence is an important dynamic component of Mediterranean ecosystems and may, indeed, be crucial to the successful propagation and spread of plants and communities regarded as typically Mediterranean. Associated animal populations generally recover quickly despite inevitable loss of life in some populations. Thus understanding the role of fire and other disturbance factors such as grazing is key to understanding Mediterranean vegetation communities and ecosystem dynamics. The chapter concludes with an evaluation of the likely response of vegetation communities to potential atmospheric and land use changes. While a number of distinct vegetation communities have been identified, a common characteristic is an ability to survive hot, dry summers and cool, wet winters, together with frequent disturbances. Many of the communities are dominated by shrubs, and Mediterranean evergreen sclerophyllous shrublands are recognized as one of the defined ecosystems of the world (di Castri 1981). Such shrublands are at the centre of a continuum of communities which vary along gradients of moisture availability, temperature, and nutrient availability, usually determined by substrate, and human activity. At the extreme ends of these gradients, but still Mediterranean, are sclerophyllous woodlands, coniferous and deciduous forests, savannas and grasslands grading into steppe and semi-desert shrublands, and heathlands.