Department of Plant Sciences, University of California, Davis, California, 95616, USA.
Mpala Research Centre, PO Box 555, Nanyuki, 10400, Kenya.
Ecol Appl. 2017 Jan;27(1):143-155. doi: 10.1002/eap.1422.
Wild herbivores and livestock share the majority of rangelands worldwide, yet few controlled experiments have addressed their individual, additive, and interactive impacts on ecosystem function. While ungulate herbivores generally reduce standing biomass, their effects on aboveground net primary production (ANPP) can vary by spatial and temporal context, intensity of herbivory, and herbivore identity and species richness. Some evidence indicates that moderate levels of herbivory can stimulate aboveground productivity, but few studies have explicitly tested the relationships among herbivore identity, grazing intensity, and ANPP. We used a long-term exclosure experiment to examine the effects of three groups of wild and domestic ungulate herbivores (megaherbivores, mesoherbivore wildlife, and cattle) on herbaceous productivity in an African savanna. Using both field measurements (productivity cages) and satellite imagery, we measured the effects of different herbivore guilds, separately and in different combinations, on herbaceous productivity across both space and time. Results from both productivity cage measurements and satellite normalized difference vegetation index (NDVI) demonstrated a positive relationship between mean productivity and total ungulate herbivore pressure, driven in particular by the presence of cattle. In contrast, we found that variation in herbaceous productivity across space and time was driven by the presence of wild herbivores (primarily mesoherbivore wildlife), which significantly reduced heterogeneity in ANPP and NDVI across both space and time. Our results indicate that replacing wildlife with cattle (at moderate densities) could lead to similarly productive but more heterogeneous herbaceous plant communities in rangelands.
野生食草动物和牲畜在全球范围内共享大部分牧场，但很少有控制实验涉及它们对生态系统功能的单独、附加和交互影响。虽然有蹄类食草动物通常会减少地上生物量，但它们对地上净初级生产力 (ANPP) 的影响因空间和时间背景、放牧强度以及食草动物的身份和物种丰富度而异。一些证据表明，适度的放牧水平可以刺激地上生产力，但很少有研究明确测试食草动物身份、放牧强度和 ANPP 之间的关系。我们使用长期围栏实验来研究三组野生和家养有蹄类食草动物（大型食草动物、中型食草野生动物和牛）对非洲热带稀树草原草本生产力的影响。我们使用实地测量（生产力笼）和卫星图像，测量了不同食草动物群体（分别和不同组合）在空间和时间上对草本生产力的影响。生产力笼测量和卫星归一化差异植被指数 (NDVI) 的结果都表明，平均生产力与总有蹄类食草动物压力之间存在正相关关系，这主要是由于牛的存在。相比之下，我们发现，草本生产力在空间和时间上的变化是由野生食草动物（主要是中型食草野生动物）的存在驱动的，这显著降低了 ANPP 和 NDVI 在空间和时间上的异质性。我们的研究结果表明，用牛（以中等密度）替代野生动物可能会导致牧场中类似的但更具异质性的草本植物群落。
