Transformation von der Halbwüste zu urbanen Systemen auf Fuerteventura : Auswirkungen auf die Phytodiversität

Fuertventura was selected as an example for rapid transformation from a natural resp. slightly ruderalized landscape into a network of urbanisations because this is of high interest from the ecological point of view. Fuerteventura is the second largest island of the Canarian Archipelago, its surface covers an area of 1700 km.. The climate of Fuerteventura is arid (less than 150 mm annual precipitation in the lowlands), the shortest distance to Africa is only about 100 km. Colonizing the oldest island of the Canaries by alien plants is a dynamic process which is in no way completed, it reflects cultural history and its impact on natural vegetation. Recovery of firewood and overgrazing by goat destroyed the succulent vegetation of the class Kleinio- Euphorbietea since the 15th and 16th centuries. A very specialized agriculture preferred in the last centuries some unusual crops like Mesembryanthemum crystallinum, Agave spp., Opuntia spp., Lycopersicon esculentum and Aloe vera. Many of them remain only as relics, some are running wild. Waterstress is the most important ecological factor for plant life in Fuerteventura. Therefore we studied the impacts on phytodiversity in episodic streams, examples of dry farming, water basins and reservoirs, as well as on irrigated gardens, crops and even urban forests. The recent flora of Fuerteventura contains some 760 species, but the differentiation between indigenous vs. adventitious is problematical. About 50 % of the total flora of Fuerteventura has been strongly favoured or even introduced by human activities. The overall estimated percentage of aliens reaches 35 %. During the last decades of the 20th century the whole plant collection of subtropical gardens has been imported. Together with these plants many weeds have been introduced unintentionally. Common alien weeds of irrigated gardens are Bidens pilosa, Conyza sumatrensis, Oxalis pes-caprae, Salpichroa origanifolia and others. Urbanisation causes the most serious impacts on endangered species and vegetation, anthropogenous disturbances are identified as drivers for transformations of the ecosystem. Especially the psammophilous vegetation (Androcymbium psammophilum, Asteriscus schulzii, Convolvulus caput-medusae, Pulicaria burchardii, Reseda famarae) is endangered by urbanisation as well as by outdoor activities.