Catastrophic vegetation dynamics and soil degradation in semi-arid grazing systems

摘要: When vegetation is drastically reduced as a result of drought or an increase in herbivore numbers, it does not simply recover if periods with normal rainfall follow herbivores are removed. These commonly recognized catastrophic phenomena semi-arid grazing systems general and the African Sahel particular. The main aims this thesis to provide effective explanation properties dynamics these predict under which conditions they might be expected. We start description Sahelian rangeland dynamics, reveal its properties. This exercise appeared very useful first step growth our ideas about because: 1) translated rather vague concepts into verifiable format by deducing hypotheses expected, 2) generated notion that soil degradation could somehow important factor attributing systems. contrast models emphasize feeding characteristics plant competition possible mechanisms underlying dynamics. tested hypothesis degradation, i.e. erosion run-off wind consequent loss water nutrients, sufficient explain mathematical modelling. Our model studies indeed show can effectively Soil cause positive feedback between resource (soil nutrients) availability biomass may lead collapse system. loop triggered grazing. argue on basis large body literature mechanism causing Furthermore, for site-specific loamy clayey soils case water-limited production, sandy nutrient-limited production. because have higher infiltration rates but more vulnerable nutrient through than soils. Based models, we hypothesized removal aboveground herbaceous would content production increased run-off. savanna Tanzania (East Africa). Indeed, consequence removal, reduction occurred. But evaporation litter ultimately outbalanced all other effects content. Several factors contributed importance evaporation, overriding research area was loam, lower percentage sand perentage clay, while primarily occurred light showers. Thus, conditions, when operate, another become prominent. further at certain range impact small initial differences cover amount resources magnify alternative states persist time due plant-soil feedbacks. system Burkina Faso (West Africa), where studied patchiness along gradient impact. occurrence likely persistence spatial pattern vegetated patches alternating bare explained infiltration. stress applicability comparing grasslands arctic salt marshes Hudson Bay Canada. both systems, herbivory shift, caused feedback, leading desertification. One assumptions density regulated biomass. In discussion, I investigated influence plant-herbivore system, depends As reproduction efficient, destabilize collapse. chapter also practical relevance approach finally objective ecological criteria pastoral managers base their decision how evade hazard rangelands. highlight three topics deserve priority reseach agenda concerning near future. Hereby, want put experimental empirical clear theoretical framework, whereby modelling should play role. are: heterogeneity formation, facilitation functional groups within layer feedbacks