Introduction

The Sahel (dry thornbush savana) of Africa is one of the simplest tropical ecosystems. Its biotic community comprises a small number of plant and animal species and its climate is characterized by a single and short rainy season followed by a long period of drought. This ecosystem has not been drastically modified by man, except for the replacement in some areas of most large wild ungulates by domestic species. The « Ferlo septentrional » in Northern Senegal (fig. 1) has been selected as a suitable area for long-term ecological studies, as its vegetation and fauna were reasonably well known and the proximity of the Richard-Toll field station (tig. 2) afforded logistic facilities. Furthermore the region is only partially inhabited by nomadic cattle herders (Fulani), whose herds have an average standing crop biomass of about 30 Kg/ha (fresh weight) . This facilitates comparison of « wild » and « managed » components of the same ecosystem type. A field base has been established at Fété Olé (fig. 3), close to the one square-kilometer reference quadrat, located outside the area regularly visited by fulani cattle herds. All the data presented in this first set of papers have been gathered within this quadrat, or in the 100 Km2 area surrounding it. The ORSTOM/SAHEL Project forms part of the French contribution to the International Riological Programme (PT Sec tion) and the support given by the ORSTOM (French Overseas Research Organization) is gratefully aknowledged. Investigations will be continued for a further two years, and progress reports will continue to be published in this journal