Evaluation des indicateurs de la désertification dans les écosystèmes steppiques,
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This study examines the effects of livestock grazing on plant communities of arid steppe ecosystems, in central steppe rangelands of Algeria, Hodna basin. In addition, how land use changes over the past 30 years.Desertification is a worldwide concern; in Algeria, land degradation threats more than 20 million hectares of steppe rangelands. Overgrazing can generate a regressive chain reaction leading to degradation of vegetation cover and loss of biodiversity. The use of indicators to assess desertification is suitable to detect the state of the health of rangeland ecosystems. We conducted a comparative study of the plant community structure and diversity in grazed and ungrazed areas. We compared alpha and beta diversities using the Hill Index between the two treatments, and used the additive partitioning of beta diversity to test whether the difference in plant species composition is due to species spatial turnover or nestedness. Besides, by using the multiplicative diversity partitioning, we examined the effects of grazing on beta diversity at two spatial scales (i.e. among-transects, and among-sites). In addition, we assessed soil surface conditions; vegetation, litter, bare ground, biological soil crusts and Stipa tenacissima cover. For change detection, we used diachronic study by comparing land cover of three years 1984, 2000 and 2014. For alpha diversity, grazing reduced significantly the diversity of the annual species; however, perennials were not affected significantly. The results revealed a significant compositional difference between grazed and ungrazed areas. Essentially, the additive partitioning of beta diversity indicated that ~74% of the overall beta diversity was due to species turnover and ~26% was due to nestedness. The analysis of beta diversity at different spatial scales showed that grazing increased beta diversity at small scales and decreased beta diversity at coarse scales (among-sites). In addition, grazing activities have largely reduced values of the vegetation cover and diversity in grazed areas. The cover of the most dominant species (Stipa tenacissima), was not affected by grazing. The high value of bare ground cover indicated regressive trend, this can be interpreted as a process of desertification. Nevertheless, the protection from grazing increased significantly the species diversity of annual species and vegetation cover. Land cover changed regressively from 1984 to 2014. These results suggest that grazing changes the composition of plant community and affects mainly the annual plants. Furthermore, grazing increases both compositional variation at small spatial scales and vegetation homogeneity at coarse spatial scales. This study highlights the importance of protection and conservation as an effective management tool for maintaining the plant community structure and diversity in threatened ecosystems. It is recommended to use controlled grazing and increase the number of protected areas.