Grazing effects on the species-area relationship: Variation along a climatic gradient in NE Spain
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Questions: Does grazing have the same effect on plant species richness at different spatial scales? Does the effect of spatial scale vary under different climatic conditions and vegetation types? Does the slope of the species-area curve change with grazing intensity similarly under different climatic conditions and vegetation types? Location: Pastures along a climatic gradient in northeastern Spain. Methods: In zones under different regimes of sheep grazing (high-, low-pressure, abandonment), plant species richness was measured in different plot sizes (from 0.01 to 100 m2) and the slope of the species-area curves was calculated. The study was replicated in five different locations along a climatic gradient from lowland semi-arid rangelands to upland moist grasslands. Results: Species richness tended to increase with grazing intensity at all spatial scales in the moist upland locations. On the contrary, in the most arid locations, richness tended to decrease, or remain unchanged, with grazing due to increased bare soil. Grazing differentially affected the slope (z) of the species-area curve (power function S = c Az ) in different climatic conditions: z tended to increase with grazing in arid areas and decrease in moist-upland ones. β-diversity followed similar pattern as z. Conclusions: Results confirm that the impact of grazing on plant species richness are spatial-scale dependent. However, the effects on the species-area relationship vary under different climatic conditions. This offers a novel insight on the patterns behind the different effects of grazing on diversity in moist vs. arid conditions reported in the literature. It is argued that the effect of spatial scale varies because of the different interaction between grazing and the intrinsic spatial structure of the vegetation. Variations in species-area curves with grazing along moisture gradients suggest also a different balance of spatial components of diversity (i.e. α- and β-diversity).