Desertification, the degradation or even destruction of arid ecosystems through man's use, is not new. Deleterious changes, measured on the time scale of human archaeological history, have accompanied many civilizations located in the arid and semi-arid areas of the world; for example in China (Hou, 1985) and in the Middle East (Evenari et al., 1985). However, this chapter will concentrate on changes that have occurred over the past few decades.

   Desertification is a severe and dramatic disturbance to arid ecosystems. It is named for its transformation of the landscape but its effects can be seen at all trophic levels. For example, during a period of severe landscape degradation in Australia at the end of the last century, the numbers of sheep in the rangelands of New South Wales declined from 13 million in 1890 to 4--5 million in 1900, and since that time has changed very little (Perry, 1968).

   Humans are the secondary consumers or top predators in a pastoral system. Sinclair and Fryxell (1985) estimate that earlier this century approximately 50 million nomadic pastoralists used, and were supported by, the Sahelian rangelands. During the epochs of desertification that occurred in the Sahel in the periods 1969--75 and 1984- ?, at least one million humans starved, along with 40-50% of the population of domestic stock, and consequently some 7--10 million people became dependent on external food aid (e.g. Wade, 1974; Le Houerou and Gillet, 1986). Similarly the permanent devastation of Australian arid hunter--gatherer systems by the introduction of exotic domestic herbivores and the European rabbit was complete in less than 50 years. Productive systems that had supported man for approximately 2000 generations or more were irreversibly altered by two generations of European pastoralists.

   Desertification approaches in extent the global scale of impacts that the 'greenhouse' climatic change may bring. Therefore a disturbance with these attributes should provide some understanding of the structure and functioning of arid ecosystems.