Grasslands are among the most widespread and diverse ecosystems on Earth, covering >40% of terrestrial surface and supporting a wide range of biodiversity and ecosystem services for humankind. Livestock grazing is the most widespread land use in grasslands, including in northern China, which is part of one of the largest remaining grasslands on Earth (i.e., the Eurasian steppe) where grassland is largely used to support livestock grazing for food production. In these ecosystems, grazing by livestock is a major driver of biodiversity and function. However, overgrazing is also one of the most pervasive and significant processes that degrades grassland, especially in northern China, where 90% of grasslands have been overgrazed and thus degraded.
The degradation of grasslands has attracted great attention in recent years and stimulated a large number of studies on the use of exclosures. In order to relieve the problem of grassland degradation, exclosure was extensively used to protect and restore degraded grassland ecosystems all over the world in recent decades. This strategy has been in consideration for more than a decade in degraded and overgrazed areas. However, less work has been done to study the relative effects of long-term grazing and long-term exclosure on grassland biodiversity and multifunctionality.
We used a network of field experiments with paired grazing conditions (including and excluding livestock) carried out across 10 sites experiencing decades of grazing to investigate the impacts of long-term grazing on biodiversity and ecosystem multifunctionality. These experimental sites situated along a 1100 km transect from east to west across an aridity gradient. The gradient in aridity includes the three major types of grasslands in northern China (meadow steppe, typical steppe, and desert steppe from wetter to drier) – the largest natural grasslands remaining on Earth. We evaluated the relatively long-term effects of domestic herbivore grazing by excluding livestock for an extended period of time to get comparable control. At each experimental site, a >10 years long-term grazing exclosure was established, and all these grasslands inside enclosure have greatly been different from that outside enclosure due to long-term prohibition of livestock grazing. We aimed to examine the effects of domestic herbivore grazing on biodiversity and ecosystem multifunctionality (EMF), and whether the effects depend on grassland aridity, and whether long-term grazing change the relative strength of plant and soil biodiversity in supporting EMF.
To our knowledge, this is the first large-scale multi-site experiment examining the effect of long-term domestic herbivore grazing on grassland biodiversity and ecosystem multifunctionality across an aridity gradient. Our cross-site work provides solid empirical evidence that relatively long-term impacts of managed grazing depend on site aridity, with the most arid sites experiencing more negative impacts on ecosystem multifunctionality. Our findings also provide field experimental evidence that aridity exacerbates the negative effects of long-term grazing on belowground biodiversity, and further strengthened their contributions to the decline in multifunctionality at arid grasslands. Thus, domestic herbivore grazing shifted the contribution for biodiversity drivers of multifunctionality from plant to soil biodiversity when moving from ungrazed to grazed ecosystems.
The messages for the land managers and policy makers are clear: long-term grazing can disrupt the associations between plant biodiversity and ecosystem multifunctionality, while long-term exclosures may disrupt the associations between soil biodiversity and ecosystem multifunctionality, thus either long-term overgrazing or long-term exclosure is not good grassland management strategy. Our study suggests that rotational exclosure and grazing would be a good management strategy to restore and improve ecosystem multifunctionality in natural grassland ecosystems. Moreover, livestock grazing should be given more caution in more arid grasslands wherein multifunctionality is vulnerable to livestock grazing and highly dependent on soil biodiversity. Our study also includes a series of implications for the management and conservation of grasslands under global change. Our results suggest that increased global warming and reduced precipitation amount may increase the risk of the negative effects of grazing on grassland biodiversity and function. To protect the long-term health of grasslands and avoid the degradation of grasslands in a changing world, we call for adjusting livestock amounts and the spatial geographical range suitable for grazing to adapt to a drier and warmer planet.