INTRODUCTION

Madagascar, “La Grande Ile” off the coast of south-east Africa, is the fourth largest island on earth. Its 587000 km2 are topped only by the islands of Greenland, New Guinea and Borneo. The island broke off from Africa some 150 to 160 million and from India some 88 to 95 million years ago. Although Madagascar is separated from Africa only by the Mozambique Channel, no more than about 300 to 450 km wide, the prevailing winds and ocean currents were and still are unfavorable for repeated colonization of the island (Krause et al., 1997). Due to the long isolation and low rate of colonization events, the flora and fauna of Madagascar underwent impressive adaptive radiations, resulting in one of the world's most diverse arrays of endemic plants and animals (Myers, 1986; Mittermeier, 1988; Table 4.1).

Based on phytogeographic criteria, the evergreen forests of eastern Madagascar are distinguished from the deciduous formations of the west and south (Du Puy & Moat, 1996; Lowry et al., 1997; Fig. 4.1). The evergreen rainforests of the east receive between 1500 and more than 3000 mm of rain per year. The deciduous forests of the west and extreme north of Madagascar are subject to a distinct dry season of four to eight months without rain and annual precipitation of 500 to 2000 mm. In both vegetation types, annual rainfall decreases from the north to the south. Parts of the south and south-west of the island receive less than 500 mm of rain per year at irregular intervals with an extended dry season for more than eight months. Here, habitats are represented by dry deciduous, riverine and spiny forest characterized by Didieraceae and other succulent plants.