The positive impact of grazing herbivores on plant diversity has been frequently reported in fertile grasslands. Grazing-induced heterogeneity is often evoked as an explanation for this influence. However, the relative importance of plant community heterogeneity induced by herbivores compared to other mechanisms linked to grazing remains unclear. We investigated this question by looking at 3 plant communities found in wet grasslands situated along the French Atlantic coast and traditionally grazed by horses and cattle. An experimental design set up in 1995 allowed us to compare the consequences of cattle-grazing, horse-grazing, and grazing abandonment on plant community diversity and heterogeneity. Floristic measurements made in 2007 showed that cattle and horses both had a positive impact on species richness and Shannon diversity index and that patchiness only occurred in grazed situations. The relative importance of grazing-induced patchiness within the overall positive effect of grazing on plant diversity was assessed by partitioning the diversity gain due to grazing into its additive within- and among-patch components. Grazing-induced patchiness entirely explained the increase in plant richness, whereas it accounted for only a small part of Shannon diversity. Grazing-related processes operating at the patch scale explained the main part of the increase in Shannon diversity. These processes make only a limited contribution to species recruitment, but they result in a more even species abundance distribution.
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