Site-specific properties and irreversible vegetation changes in semi-arid grazing systems

There is an urgent need to develop a mechanistic understanding of how site-specific properties can lead to irreversible vegetation changes. We show, by means of a bifurcation analysis of two mathematical models, how site-specific properties determine the resilience of vegetation changes in semi-arid grazing systems. The models predict that if available soil water limits plant growth, the vegetation supported by sandy soils is generally resilient to herbivore impact and rainfall fluctuations, unlike the vegetation on clayey soils. This depends on the capacity of vegetation communities to improve the structural and water-holding capacities of the soil. In contrast, if plant growth is nutrient limited, vegetation on sandy soils is generally not resilient to herbivore impact and fluctuations in external nutrient input, unlike the vegetation supported by clayey soils. This is affected by the nutrient retention capacity of vegetation communities. We stress the applicability of the general theory provided by this model to the Sahel environment. The model predictions are consistent with field observations documented in the literature.