Biogeography of the arid steppeland north of the Sahara

Abstract

The arid steppes north of the Sahara occupy an area of some 630,000 km2, between the isohyets of 100 and 400 mm of mean annual rainfall (MAR), from the Red Sea to the Atlantic Ocean. Physiognomically they may be subdivided into perennial grass, dwarf shrub, tall shrub, crassulescent, succulent and pulvinate (or tragacanthic) steppes. Floristically they include some 2630 species of vascular plants, with a 26% rate of endemism; endemism is distributed into an eastern center (130 species), a western center (365 species) and an overall steppic nucleus (165 species). The limits between the east, central and west centers are approximately the 19°E and 0° of Longitude, respectively; while the regional limits are the Atlantic Ocean to the West and the Red Sea and Suez Canal to the east.The main elements of phytochory, in species numbers, are: Mediterranean, 88%; Cosmopolitan, 5%; and Tropical, 4%. The Mediterranean element of phytochory includes: 26% Mediterranean taxa, sensu stricto; 40% Ibero-Maghribian; 11% Irano-Turanian; and 11% Mediterraneo-Saharo-Arabian.The archaı̈c element of Austro-Pan-African xerophytic flora, the so-calledRand-Flora , mostly composed of Tropical and Mediterraneo-Saharo-Arabian species, represents some 350 species, i.e. 13%.The fauna includes 129 species of mammals, 133 species of resident birds, and 87 species of reptiles.Large mammals are mainly of Africo-Tropical and small mammals of Mediterranean kinships. Birds are predominantly Mediterranean, while reptiles are evenly balanced between Mediterranean and Tropical taxa. There are eastern, central and western centers of distribution of these three main groups, limited by approximately the same meridian borders as for the Flora, the 19°E and 0° of Longitude. The Oriental province specifically includes 15 species of mammals, nine species of birds and 12 species of reptiles; the western province, in turn, is characterized by eight species of mammals, 15 species of birds and four species of reptiles. For all groups the proportion of the Tropical element tends to grow southwards, with aridity and temperature.