Results and Discussion
Our synthesis provides global-scale evidence that both multi-diversity and ecosystem multifunctionality (EMF) are significantly altered by intensified grazing disturbance in grasslands. However, multi-diversity and EMF responded differently to grazing intensity. Specifically, light and moderate grazing increased multi-diversity, but heavy grazing reduced multi-diversity (Fig. 1). The similar results were found for the grazing effects on individual biodiversity, where light and moderate grazing intensity had slightly positive effects on diversity of plant species and soil microbes, and heavy grazing decreased the diversity of plant species and functional groups as well as functional diversity (Fig.S3). This result is different from the previous meta-analysis that included a much smaller sample size than ours and showed little effect of grazing intensity on plant species diversity(Herrero-Jáuregui & Oesterheld 2018).
The EMF decreased consistently with increasing grazing intensity, and the reduction of EMF response under heavy grazing (-19.3%) was significantly stronger than that under light (-5.8%) and moderate grazing (-11.1%) (Figs. 1, 2). Moreover, increasing grazing intensity had significant negative effects on individual ecosystem functions (Fig. S3). Compared to no grazing, all grazing intensities significantly reduced above- and belowground biomass, net ecosystem productivity (NEP), ecosystem respiration (ER) and gross ecosystem productivity (GEP). In addition, heavy grazing further decreased soil moisture, organic carbon and total phosphorus content, but had a non-significant effect on soil nitrogen content (Fig. S3). This is because overgrazing decreases aboveground plant biomass, leading to less plant litters inputs into the soil and thereby less carbon and phosphorus storage, but the loss of nitrogen can be mitigated by increases in nitrogen inputs through N-fixation of legumes and herbivore dung and urine(He et al. 2011; Martinsen et al. 2011; Sitters et al. 2020).
The negative effect of grazing intensity on multi-diversity becomes stronger over time (grazing duration) (Fig. 2). On the one hand, long-term grazing disturbance may facilitate a shift in plant community composition from weedy and annual plants to grazing-tolerant species, thereby leading to a limited decline in plant species richness and functional diversity (Marriott et al. 2009; Lyseng et al. 2018; Fischer et al. 2019). On the other hand, the accumulation of herbivores trampling and removal of aboveground biomass by grazing may reduce carbon and nitrogen inputs through litters into the belowground processes, and promote the evaporation of soil moisture by increasing bare land area, leading to lower soil microbial activity and plant regrowth rates(Liu et al. 2016; Sitters et al. 2020). Furthermore, even low-intensity grazing may also aggravate biodiversity loss and grassland degradation with the accumulation of grazing years. This result suggests that the magnitude of grazing effects estimated on multi-biodiversity and EMF may be underestimated when only short experimental durations are considered.
The shift in livestock type from small livestock (e.g., sheep and goat) to large livestock (e.g., cattle and yak) had a significantly positive effect on multi-diversity (Fig. 2). Moreover, a similar tendency towards a positive effect was also found in the interaction between grazing intensity and larger livestock type (Fig. 2). This is likely because of the distinctive diet selectivity and grazing behavior of large and small livestock, which leads to differences in vegetation structure and soil properties (Tóth et al. 2016; Wang et al. 2019; Gao & Carmel 2020). Previous research has found that cattle are likely to select the dominant perennial grasses that are higher and have greater aboveground biomass(P et al. 2014) and thereby alleviate competitive exclusion by alleviating ground-level light limitation(Hautier et al. 2009; Borer et al. 2014), leading to increased abundance of subordinate species and prevent rare species extinctions(Olff & Ritchie 1998). Compared with cattle grazing, sheep grazing is more likely to threaten plant species and functional diversity by increasing extinction of rare palatable species due to their greater selective preference for forbs with high abundance in the short-grass community (Tóth et al. 2016; Zhang et al. 2018).
Additionally, increasing aridity index had a significantly negative effect on the grazing-induced responses of multi-diversity and EMF (Fig. 2). This result suggests that increasing grazing intensity reduces the biodiversity of arid grasslands more strongly than that of humid grasslands, as reported previously for plant species diversity (Herrero-Jáuregui & Oesterheld 2018; Gao & Carmel 2020). Importantly, we found a significant negative interaction effect between grazing intensity and aridity on multi-diversity index (Fig. 2), indicating a negative synergistic effect of grazing intensity and aridity. This may be attributed to the idea that herbivores may further aggravate water stress and limitation of nutrient resources of dry grasslands, causing decreases of plant and soil microbial diversity (Ren et al. 2018; Gao & Carmel 2020; Zhang et al. 2020). Therefore, our results suggest that the predicted increasing drought events might aggravate the negative effect of grazing disturbance in thefuture.
Compared with the single biodiversity-ecosystem function relationship, there was a stronger positive relationship between multi-diversity and EMF responses when integrated a great number of biodiversity groups and functions (Fig. S10). Further, we found that the slope of the multi-diversity and EMF relationship increased significantly by 66.6% from light grazing to heavy grazing (P =0.005) (Fig. 3). The underlying mechanism may be attributable to increased abiotic facilitation under a certain extent of increased stress of disturbances, as predicted by the “stress gradient hypothesis”(Baert et al. 2018). On the one hand, the environmental stress caused by increasing grazing intensity may strengthen the positive BEF relationship by promoting the dominance effects that high functional adaptive species increasingly replace low functional vulnerable species(Baert et al. 2018; Guo et al. 2019). On the other hand, although extremely high environmental stress was predicted to weaken the BEF relationship by inhibiting the functioning of all species(Baert et al. 2018), we did not detect this under any grazing intensities, which may be due to that the levels of heavy grazing in most studies are not sufficient to cause extreme environmental stress. Overall, our results suggest that increasing grazing intensity strengthens the dependence of EMF on multi-diversity globally.
The structural equation modeling reveals that an increase in grazing intensity decreased EMF not only directly, but also indirectly via reducing multi-diversity (Fig. 4). Grazing duration had a direct negative effect on the response ratio of EMF, and the changing livestock type from small to large size mainly had an indirect positive effect on EMF by increasing multi-diversity. In contrast, rising aridity index had an indirect negative effect on the response ratio of EMF via decreasing multi-diversity (Fig. 4). These findings established a unique and critical role of multi-diversity in mediating EMF under the integrative impacts of grazing intensity, duration, livestock type and climatic factors at the global scale, which expands our understanding based on the regulation of single-trophic biodiversity on ecosystem functions under land use and environmental changes at a local scale, as shown by previous studies(Allan et al. 2015; Hautier et al. 2015).
In conclusion, this study provides new insights into the effects of grazing intensity interacting with livestock type, grazing duration and aridity on the multi-diversity, EMF, and their relationship. We found that intensifying grazing disturbance strengthens the dependence of EMF on multi-diversity, and reduces ecosystem functioning through extensively decreasing biodiversity. Moreover, the negative grazing effects are stronger in drier grasslands with smaller livestock and longer grazing duration. The findings extend our current understanding on the grazing management practices in promoting biodiversity conservation and sustainability of ecosystem services (Schönbach et al. 2011; Kemp et al. 2013; DeLonge & Basche 2018). We strongly suggest to carefully modify the grazing duration and select proper livestock types while optimizing the grazing intensity. Establishing optimal adaptive management strategy based on biodiversity conservation is critical for preventing grassland degradation and promoting sustainable development of global pastoral areas, especially when extreme drought events are predicted to occur more intensively and frequently under global climate change.