The Nature Conservancy, 258 Main Street, Suite 200, Lander, Wyoming, 82520, USA.
Department of Zoology and Physiology, University of Wyoming, Laramie, Wyoming, 82071, USA.
Ecol Appl. 2018 Mar;28(2):323-335. doi: 10.1002/eap.1649. Epub 2018 Jan 18.
Rainfall and herbivory are fundamental drivers of grassland plant dynamics, yet few studies have examined long-term interactions between these factors in an experimental setting. Understanding such interactions is important, as rainfall is becoming increasingly erratic and native wild herbivores are being replaced by livestock. Livestock grazing and episodic low rainfall are thought to interact, leading to greater community change than either factor alone. We examined patterns of change and stability in herbaceous community composition through four dry periods, or droughts, over 15 years of the Kenya Long-term Exclosure Experiment (KLEE), which consists of six different combinations of cattle, native wild herbivores (e.g., zebras, gazelles), and mega-herbivores (giraffes, elephants). We used principal response curves to analyze the trajectory of change in each herbivore treatment relative to a common initial community and asked how droughts contributed to community change in these treatments. We examined three measures of stability (resistance, variability, and turnover) that correspond to different temporal scales and found that each had a different response to grazing. Treatments that included both cattle and wild herbivores had higher resistance (less net change over 15 years) but were more variable on shorter time scales; in contrast, the more lightly grazed treatments (no herbivores or wild herbivores only) showed lower resistance due to the accumulation of consistent, linear, short-term change. Community change was greatest during and immediately after droughts in all herbivore treatments. But, while drought contributed to directional change in the less grazed treatments, it contributed to both higher variability and resistance in the more heavily grazed treatments. Much of the community change in lightly grazed treatments (especially after droughts) was due to substantial increases in cover of the palatable grass Brachiaria lachnantha. These results illustrate how herbivory and drought can act together to cause change in grassland communities at the moderate to low end of a grazing intensity continuum. Livestock grazing at a moderate intensity in a system with a long evolutionary history of grazing contributed to long-term stability. This runs counter to often-held assumptions that livestock grazing leads to directional, destabilizing shifts in grassland systems.
降雨和食草作用是草原植物动态的基本驱动因素，但很少有研究在实验环境中检验这些因素之间的长期相互作用。理解这些相互作用很重要，因为降雨越来越不稳定，本地野生食草动物正被牲畜所取代。放牧和偶发性低降雨被认为会相互作用，导致群落变化大于任何单一因素。我们通过肯尼亚长期禁牧区实验（KLEE）的 15 年中的四个干旱期或干旱期，检查了草本群落组成的变化和稳定性模式，该实验由六种不同的牛、本地野生食草动物（如斑马、羚羊）和大型食草动物（长颈鹿、大象）组合而成。我们使用主响应曲线来分析每个食草动物处理相对于共同初始群落的变化轨迹，并询问干旱如何导致这些处理中的群落变化。我们检查了与不同时间尺度相对应的三种稳定性度量（抵抗力、变异性和周转率），发现每种稳定性度量对放牧都有不同的响应。包括牛和野生食草动物的处理具有更高的抵抗力（15 年内净变化较小），但在较短的时间尺度上变化更大；相比之下，放牧较轻的处理（没有食草动物或只有野生食草动物）由于持续、线性、短期变化的积累，抵抗力较低。在所有食草动物处理中，干旱期间和干旱后群落变化最大。但是，虽然干旱导致放牧较轻的处理发生定向变化，但它也导致放牧较重的处理的变异性和抵抗力增加。放牧较轻的处理中（尤其是干旱后）的大部分群落变化是由于适口性草 Brachiaria lachnantha 的盖度大幅增加所致。这些结果说明了食草作用和干旱如何共同作用，导致放牧强度连续体中处于中等至低等水平的草原群落发生变化。在具有长期放牧进化历史的系统中，以中等强度放牧的牲畜促进了长期稳定性。这与通常认为的牲畜放牧导致草原系统发生定向、不稳定变化的假设背道而驰。
