Optimal foraging by large herbivores maintains a dynamic balance of plant diversity in semiarid grasslands

Understanding the feedback between plant diversity and livestock grazing is fundamental to biodiversity conservation in grassland ecosystems. However, this plant–herbivore interaction has rarely been evaluated explicitly within the framework of optimal foraging theory. We hypothesized that herbivores adopt distinct foraging strategies in response to varying plant diversity, and that these context-dependent behaviors subsequently regulate plant diversity. In a semiarid grassland in Northeast China, we quantified livestock intake, foraging efficiency, and foraging costs, together with changes in plant diversity, aboveground net primary productivity (ANPP), plant functional group composition, and resource availability across three levels of plant diversity, thereby providing a pioneering and integrated test of how foraging decisions and community dynamics interact. In high-diversity grasslands, livestock maximized energy intake through a high-reward, high-cost foraging strategy. They showed strong foraging selectivity for preferred legumes and forbs, which facilitated the competitive release of unpreferred C 3 grasses, a dominant functional group that strongly structures community competition and productivity in semiarid grasslands, and consequently reduced plant diversity. In contrast, in low-diversity grasslands, livestock shifted to a low-reward, low-cost strategy that optimized foraging efficiency. Weakened foraging selectivity broadened their diet to include both preferred and unpreferred species, suppressing the dominance of C 3 grasses and enhancing plant diversity. Moreover, the effects of grazing on plant diversity were primarily mediated through the regulation of C 3 grass dominance, which was linked to changes in light availability and soil nitrogen content. Our findings indicate that optimal livestock foraging across contrasting diversity contexts maintains a dynamic balance in plant diversity by modulating the dominance of C 3 grasses. Thus, top-down control by large herbivores can promote species coexistence and enhance grassland community stability.