Data from: Effects of grazing intensity on plant richness and diversity: a meta-analysis

Most of our knowledge of the effect of grazing on grassland structure is based on grazed-ungrazed contrasts. The effects of grazing in the most common scenario, where grazing intensity varies from low to high grazing intensity, are less known. The objectives of this paper were to (1) quantify the effect of stocking rates on species richness and diversity of grasslands world-wide, and (2) evaluate the response under different environmental and experimental conditions. We conducted a meta-analysis of experiments with at least two levels of controlled stocking rates and evaluated their effect on species richness and diversity. The results showed that the response of richness and diversity to either reducing or increasing stocking rate from a moderate level mostly fell within the range ± 25% or ± 5 species. Mean response of species richness and diversity to increasing stocking rate from moderate to high levels was negative. Mean response to lowering stocking rate from moderate levels was not different from 0. However, overall, species richness significantly decreased as stocking rate increased. The response of richness and diversity to stocking rate was not related to mean precipitation, productivity or aridity. However, the most negative responses of richness to stocking rate were larger in arid, low productivity systems than in subhumid and humid systems. The effects of grazing on richness and diversity found in this review were smaller than the effects on species composition shown by the literature. Thus, grazing drastically changes species composition, but the net change of species and diversity is much smaller.当前学界对放牧影响草地结构的认知，大多基于放牧样地与未放牧样地的对照研究。但针对放牧强度从低到高梯度变化这一最常见的场景，其放牧效应的相关认知仍较为匮乏。本研究旨在达成两个目标：其一，量化全球尺度下载畜率（stocking rate）对草地物种丰富度与多样性的影响；其二，评估不同环境与实验条件下的物种响应特征。我们针对至少设置两个及以上可控载畜率水平的实验开展了元分析（meta-analysis），并解析其对物种丰富度与多样性的影响效应。研究结果显示，当载畜率从中等水平上调或下调时，物种丰富度与多样性的响应幅度大多处于±25%或±5个物种的区间内。当中等载畜率向高载畜率提升时，物种丰富度与多样性的平均响应呈负值；而从中等载畜率下调的平均响应则与0无显著差异。但整体而言，物种丰富度会随载畜率升高而显著降低。载畜率对物种丰富度与多样性的影响，与研究样地的年均降水量、草地生产力或干旱度并无显著关联。不过，在干旱、低生产力的草地生态系统中，物种丰富度对载畜率的负向响应幅度，显著高于半湿润与湿润生态系统。本综述发现，放牧对物种丰富度与多样性的影响，小于已有文献报道的对物种组成的影响。由此可见，放牧虽会大幅改变草地物种组成，但物种数量与多样性的净变化幅度却相对较小。