The Sahel is an area of chronic vulnerability to food insecurity; about 80% of their needs are met by local production in conditions which are characterized by poor soil fertility, low rainfall, underdeveloped marketing channels and marginal land. Rainfall is uncertain and the 1970s and 1980s were particularly dry years, although there seems to be some increase in the rainfall levels recently. Many areas lack the phosphorous, nitrogen, organic matter and water retention necessary for cultivation; the FAO estimates that only 4% of the area covered by the Sahelian countries is suitable for crops, although another 32% is marginally suitable for rainfed agriculture. Over the last twenty five years, human and animal populations have grown at an average annual rate of 2.6% and 1% respectively, and this has limited the mobility that was formerly enjoyed. Fallow periods have been reduced and marginal land cultivated. Institutional weaknesses and declining terms of trade have made for a sharp decline in the economic viability of Sahelian farmers. Sahelian farmers have responded to their environment by intensifying input where water is available, and by soil conservation and reclamation techniques. The adoption of yield-enhancing technologies for staple crops is more likely when there are important synergies with cash crops. Institutional changes such as the liberalization of the cereal market in Mali also have effects on the priority placed on agricultural production. Meat production also responds to market conditions, such as the 1994 CFA franc devaluation which boosted the competitiveness of Sahelian meat. Diversification, mobility and trade enable Sahelian farmers to purchase what they are not in a position to grow. This has contributed to urbanization, and urban growth in Sahelian countries averages at 5.9%. Foreign assistance provides some buffer against food insecurity, particularly in the form of emergency food aid; it is likely that this will remain a necessary part of Sahelian food security, at least in the short term.