Grazing regime alters plant community structure via patch-scale diversity in semiarid grasslands

Abstract

Selective grazing of livestock creates lightly and heavily grazed vegetation patches, which together contribute to the whole community in grazed grasslands. The intermediate disturbance hypothesis (IDH) predicts moderate grazing intensity can increase species diversity. However, grazing patchiness complicates predicted responses to grazing intensity from ecological theory and may influence how various management regimes affect biodiversity at the whole community scale. We examined effects of management regime and grazing intensity on plant species diversity, community composition, aboveground net primary production (ANPP), and soil compaction. Two management regimes (season-long continuous grazing and grazing and mowing alternated annually) and seven levels of grazing intensity were applied over the past 10 yr. We assessed how α diversity within patches and β diversity across patches contributed to the diversity of the whole plot and how these relationships responded to both grazing intensity and management regime. We further divided β diversity into nestedness and replacement components across lightly and heavily grazed patches within plots. The mixed grazing–mowing regime differed from the continuous grazing regime in that the former had a higher number of palatable species, higher species evenness, and higher Shannon-Wiener diversity, in both lightly and heavily grazed patches and the whole plots, and especially at moderate and high grazing intensities. The continuous grazing regime and the mixed grazing–mowing regime did not differ in total β diversity. However, the nestedness component of total β diversity was dominant in the continuous grazing regime. In contrast, species richness and ANPP did not differ significantly between the two management regimes, though soil hardness in heavily grazed patches was significantly higher under the continuous grazing regime than the mixed grazing–mowing regime. Loss of rare species under both management regimes, even at low to moderate grazing intensities, suggests that selective grazing and patch formation may not conform with the IDH. Our study indicates that the mixed grazing–mowing regime is more sustainable for long-term grassland management than the continuous grazing regime by controlling the creation of heavily grazed patches. These findings, integrating α-β-γ diversity and patch-scale approaches, provide a more thorough evaluation than the intermediate disturbance hypothesis of grazing management in terms of sustainability and biodiversity conservation in semiarid regions.