Effects of grazing livestock on grassland functioning may depend more on grazing intensity than livestock diversity

Disentangling the effects of livestock diversity on ecosystem functioning and underlying mechanisms in managed ecosystems is important for distilling an integrated biodiversity−ecosystem functioning relationship theory (BEF) across multitrophic cascades in ecology, biodiversity conservation, and ecosystem management. In PNAS, Wang et al. (1) present fascinating results with a 5-y manipulated mixed grazing experiment with cattle and sheep on BEF in a managed Eurasian steppe in northern China. They claim that diversifying livestock promotes multidiversity and ecosystem multifunctionality in increasingly managed grasslands, and the link between multitrophic multidiversity and ecosystem multifunctionality is always stronger than that between single diversity components and functions. In principle, we agree with the notion that grazing may promote grassland multifunctionality via increasing multidiversity, and appreciate the positive multidiversity−multifunctionality relationships tested in statistics; however, we question the robustness that livestock diversity is that causally influential in shaping the grassland BEF relations.

According to the classical Intermediate Disturbance Hypothesis in ecology, species richness in communities is maximized at moderate levels of disturbance (2–4). In the research by Wang et al. (1), the designed grazing was undoubtedly a disturbance introduced to the steppe. Given that a moderate grazing intensity (i.e., 6.67 sheep units per ha⁻¹) was “equally” maintained for all of the plots including mixed vs. single grazing to ensure the observations were comparable (1), this would definitely result in significant biodiversity and biomass production promotion effects within the plots. However, due to spatially unequal vegetation patch fragment configurations throughout the plot installations to observations, higher soil heterogeneity, and uneven foraging of the grazing livestock (this is especially the case for the mixed grazing plots with a size double the single grazing ones and divergent livestock species) as well as a history gap among the selected plots in nature, maintaining the plots at an anticipated comparable moderate grazing intensity level was a tough challenge (5–7). In particular, in execution, it was difficult to assign the same group of experimental livestock individuals (16 sheep and 4 cattle with criteria) over the 18 grazing plots at 6 randomized block sites at the same moment, which added the possibility of confounding the effects of livestock diversity and grazing disturbance at a moderate intensity level on shaping the BEF patterns.

In this regard, we have no doubt that mixed grazing, particularly with those representative genetically divergent livestock species, may indeed play roles in regulating specific functions of an ecosystem (8), but what components of the mixed grazing scenarios, including grazer diversity, grazing intensity, or both or others, really account for the tested BEF patterns need to be clearly distilled from one another. We thus suggest a synergistic design considering a full spectrum of grazing disturbance levels (e.g., none, lower, moderate, and higher), richer livestock species or varieties (e.g., goats, horses, hares, geese), and diverse grasslands across geographic regions is necessary to draw a panorama of the relationships among livestock diversity, grazing intensity, grassland multidiversity, and multifunctionality. In this research paradigm, a partitioning analysis is needed to further verify what on earth governs the grassland BEF relationships.