Résumé
Desertification currently affects the livelihood of more than 200 million people. A primary aim of the international community is to stop desertification to enhance agricultural productivity and combat hunger of many millions of people. Desertification, especially the shift of productive semi-deserts into non-productive full-deserts, can have a "sudden" or "catastrophic" character, indicating the existence of critical transitions over "thresholds" or "tipping points". These catastrophic events result from the interplay between two different ecological interactions among the plants making up the vegetation in semi-desert ecosystems. The first is facilitation, where plants support each other in terms of collectively attracting water and nutrients and provide the soil with organic matter compounds. Facilitation acts at relatively small spatial scales. Second, there is competition between plants for the same resources (water, nutrients), but then on a relatively large spatial scale. The process of facilitation helps plants to survive, together, under harsh conditions. Such survival under harsh conditions requires though a critical vegetation biomass, below which the plants cannot adequately acquire the necessary resources. Similarly, when conditions becomes gradually harsher (e.g. less resources, higher grazing intensity), then at a critical point, the vegetation cannot attract enough resources anymore to survive and the ecosystem will collapse into a non-vegetation bare soil desert ecosystem. We will present the outcome of a study in which spatial explicit models that simulate desertification is combined with field observations on semi-deserts under various land use and climate regimes. The results showed that particular non-random patterns in the spatial distribution of the vegetation are indicative the proximity of a critical threshold. The model results closely resembled the patterns that were observed in the field trials.