Sheep and Ships: Modelling Grazing and Erosion in a Warming World

Abstract

"Pastoralism is a major activity in the World's Drylands. It is often practised by marginalised communities in Common lands presumed fit for no other use by people in poverty. With global warming the resilient livelihood systems developed by pastoralists to cope with difficult seasonal and inter-annual climatic fluctuations may break down completely. Our premise is that vegetation cover is a key control on the rate of water erosion and that land degradation leads to a downward spiral in which more erosion leads to poorer pasture and consequently heavier grazing intensities as pastoralists are forced to herd on smaller and smaller available commons. Because of poverty they are forced to graze larger herds on smaller areas to avoid complete destitution. We have developed a digital model to investigate the probable impacts of lower rainfall on the balance between vegetative production and vegetal consumption. The model simulates a set of grazing 'styles' that comprise combinations of origins (sheds, farms, settlements), routes or transports (herded by shepherd, carried by wagons, or transhumant) and animal behaviours (walk, eat, rest,). We have developed an interactive P.C.-based programme in Java using an oops-UML approach to make accessible interactive software that is low in data and computing demands and designed to be user friendly for inexperienced users. This will be demonstrated. The results of simulations of the relationship between degradation and herd size for fixed grazing seasons (1) show that theoretical economic models of fisheries depletion by Schaefer, as modified by Clark for constrained catches, closely describe the behaviour of the grazing systems in common lands revealing yet another case of common-pool-resources (CPR) depletion, (2) provide a means for examining alternative management practices for the grazing in CPR and (3) offer a possibility of evaluating the impacts of climate change on grazing systems ranging from Nomadic to paddock, in the world's Drylands."