Ecology, hydrology and alternate stable states in Sahelian grazing systems

Background/Question/Methods

Vegetation communities in much of the Sahelian region of West Africa were severely impacted by the drought of 1985-86. During the years of average rainfall that followed (1988-1992), there is evidence that the hydrological dynamics of seasonal pools in some areas have changed. In our study area of northern Mali, a previously seasonal pool used for transhumant grazing underwent an unexpected transition from ephemeral to perennial and has remained in that state ever since.  As a result of this change in lake state a small village has established beside the lake and large numbers of cattle now routinely remain in the watershed throughout the dry season. Here we use simplified models to explore hydrological transformation and alternative stable states in dryland grazing systems.  The African Carbon Exchange (ACE) model is a simplified model that simulates spatial and temporal variability in vegetation community dynamics in response to resource competition, climate variability and disturbance (including herbivory).  We use this model with prescribed herbivory and simple runoff calculations to examine the interactions that might contribute to this change in stable state.

Results/Conclusions

Initial simulations demonstrate that there are feedbacks between climate, vegetation, grazing and lake volume.  It is likely that biological and physical thresholds were exceeded during the drought to trigger a temporary state change in the lake from ephemeral to perennial, which then triggered a change in land-use and grazing patterns that now maintain the lake in its ‘perennial’ state.  Our simulations show how heavier grazing can contribute to increased runoff to the lake and lake volumes that can persist through the dry season with no long term increase in rainfall.  The existence of hysteresis in the relationship between rainfall, vegetation, grazing and lake-state indicates that the perennial lake-state is likely to be highly stable. We will discuss how the interactions and feedbacks operating at this lake may have broad implications for the ecology and management of the Sahel and other dryland regions.