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Summary: Parasites can catalyze or inhibit interactions between their hosts and other species, but the ecosystem‐level effects of such interaction modifications are poorly understood.We conducted a large‐scale field experiment in temperate grasslands of China to understand how foliar fungal pathogens influenced top‐down effects of cattle on plant diversity and productivity.When foliar pathogens were suppressed, cattle grazing strongly reduced biomass of the dominant grass, Leymus chinensis, generating competitive release that significantly increased community‐level species richness and evenness. In the absence of grazing, pathogen attack on L. chinensis had no measurable effect on host biomass. However, pathogens disrupted top‐down effects of herbivory by inhibiting grazing effects on plant biomass and species richness. Mechanistically, fungal pathogens were linked to increased alkaloid and reduced nitrogen levels in leaf tissue, which appeared to deter cattle grazing on L. chinensis.In conclusion, foliar pathogens can suppress top‐down effects of large herbivores on grassland community composition and ecosystem function by modifying the strength of their host's interactions with dominant consumers. Parasites may act as modulators of ecosystem function when their direct effects on host abundance are overshadowed by powerful influences on host traits that modify their interactions with competitors, herbivores, or predators. [ABSTRACT FROM AUTHOR]

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 twofold 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.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 microenvironmental factors altered by cattle and ants on litter decomposition rates.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 microenvironments by increasing soil N availability, which in turn increased soil microbial biomass and accelerated decomposition process.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 frequently 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. A free Plain Language Summary can be found within the Supporting Information of this article. [ABSTRACT FROM AUTHOR]