A facilitation between large herbivores and ants accelerates litter decomposition by modifying soil micro-environmental conditions

1. Large herbivores and insects commonly coexist and play important functional roles in grassland ecosystems. The interactive effects of these two animal groups in shaping ecosystem processes and functioning are poorly understood. In a semi-arid grassland of northeastern China, we previously found a reciprocal facilitation between large herbivores (cattle; Bos tarurs) and ants: cattle grazing led to a two-fold increase in ant mound abundance compared with ungrazed sites, while the presence of ant mounds, in turn, increased the foraging of cattle during the peak of the growing season. 2. Here, by using a large-scale, 4-year (2010-2013) manipulative experiment, we further investigated how such a facilitation between large herbivores and ants can affect a key ecosystem process, litter decomposition. Using a set of small-scale reciprocal translocation litterbag experiments, we separated the effects of litter quality and soil micro-environmental factors altered by cattle and ants on litter decomposition rates. 3. A significant interaction between the experimental factors cattle grazing and ant presence showed that litter decomposition rate was at the highest levels when both cattle and ants were present, with only a small impact when each was present on its own. Mechanistically, cattle and ants exerted limited effects on litter quality (litter C:N ratio). However, these animals greatly altered soil micro-environments by increasing soil N availability, which in turn increased soil microbial biomass and accelerated decomposition process. 4. Synthesis. Our results demonstrate how positive interactions between two groups of animals, large herbivores and ants, can affect decomposition rates, with important consequences for ecosystem carbon and nutrient cycling. Large herbivores, either domestic or wild, often coexist and interact with a diverse of other fauna in terrestrial ecosystems. Assessing their interactive effects will help us to better understand their role in shaping ecosystem processes and functioning with important management implications.

1. 大型草食动物与昆虫常共存于草原生态系统中，并发挥关键的功能作用。目前学界对这两类动物类群调控生态系统过程与功能的交互效应仍知之甚少。在中国东北的半干旱草原中，我们此前曾发现大型草食动物（large herbivores）与蚂蚁之间存在双向促进作用：与未放牧样地相比，牛（Bos tarurs）放牧可使蚁丘（ant mound）的丰度提升一倍；而在生长季高峰期，蚁丘的存在又会提高牛的觅食活动。
2. 本研究依托2010-2013年开展的为期4年的大型控制性野外实验，进一步探究了大型草食动物与蚂蚁间的这种促进作用如何调控关键生态系统过程——枯落物分解（litter decomposition）。我们通过一系列小型双向移植分解袋（litterbag）实验，分离出了牛与蚂蚁所改变的枯落物质量（litter quality）与土壤微环境因子对枯落物分解速率的影响。
3. 实验因子（牛放牧与蚂蚁存在）间存在显著交互作用：当牛放牧与蚂蚁同时存在时，枯落物分解速率达到最高水平；而仅单一因子存在时，其影响则十分有限。从机制上看，牛与蚂蚁对枯落物质量（枯落物碳氮比（C:N ratio））的影响较为有限，但二者可通过提升土壤有效氮含量大幅改变土壤微环境，进而提高土壤微生物生物量并加速分解过程。
4. 综合分析。本研究结果阐明了大型草食动物与蚂蚁这两类动物类群间的正相互作用如何调控分解速率，这对生态系统碳循环与养分循环具有重要意义。无论是家养还是野生的大型草食动物，在陆地生态系统中常与多样的其他动物类群共存并发生相互作用。评估它们的交互效应，有助于我们更深入地理解其在调控生态系统过程与功能中的作用，同时具有重要的管理应用价值。