ABSTRACT
Models can be very simple, or highly complex, but they all contain certain basic elements. Simulation models are designed to trace out future changes. The largest and most complex simulation models currently in existence are the global circulation models, which attempt to predict future climate changes under different atmospheric and oceanic conditions. In the western region of Niger, on the border with Mali, a Sahelian nomadic livestock raising ecosystem has developed over centuries. Existence is precarious. Rain is often scarce and unevenly distributed. Nomads move herds in search of good grazing in a large area that is essentially a commons, with no private property rights. In periods of good rains, people and livestock build up. often overgrazing, which degrades the land, making the situation ripe for a collapse. In 1974 Anthony Picardi (1974) undertook a study to treat this ‘tragedy of the commons’ with an explicitly interacting ecological–social–economic model.
