Herbivores optimize the function of grassland ecosystem

Haiyan Ren, Valerie T. Eviner, Weiyang Gui, Gail W. T. Wilson, Adam B. Cobb, Gaowen Yang, Yingjun Zhang, Shuijin Hu, Yongfei Bai

Sheep flock and grassland from Creative Commons
Sheep flock and grassland from Creative Commons

Herbivores have been shown to stimulate plant production in grasslands and share a long co-evolutionary history with vegetation. Also, the changes in soil nutrients, biodiversity and soil microbial communities induced by grazing all have strong effect on grassland systems. Development of sustainable grazing systems that promote ecosystem resilience, enhance or maintain plant diversity, increase soil health, and maintain ecosystem multifunctionality and delivery of multiple ecosystem services is a global concern. Balancing these multiple objectives can be challenging because tradeoffs are common across multiple grassland management goals. Herbivore grazing intensity is one of the most crucial drivers for grassland management efforts. The number of livestock per unit area is a double-edged sword, which could help or hamper grassland development. We evaluated the effects of seven grazing intensities, based on an 11-year field experiment in a semi-arid Inner Mongolian steppe, and found that soil nutrients, plant nutrients, species diversity, biomass production and soil microbes had optimal values under different grazing intensity levels. Our results suggest that grassland ecosystem functions differ in their sensitivity to grazing intensity, and that a low grazing threshold is required to achieve multiple goals in the Eurasian steppe. Establishing thresholds of grazing necessary to maintain multiple functions is critical for sustainable rangeland management, and can increase the accuracy of predictions of grassland ecosystem responses to grazing pressure. We need more wide-spread assessment of grazing thresholds for multiple functions across diverse grasslands, because many mesic grasslands are predicted to become more arid under a changing climate, and thus the intensity of grazing that can be sustained is likely to decrease.

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