Disentangling the effects of defoliation, excreta return and trampling from livestock on grassland ecosystem compositional and functional temporal stability

Grazing animals can shape plant community composition and functioning through three mechanisms: defoliation, excreta return (dung and urine deposition), and trampling. However, little field research has been done to explore the independent effects of these grazing mechanisms on grassland compositional and functional (ANPP) temporal stability, though it is essential for preserving a sustainable grazing system. To fill this gap, we conducted field experiments in two grasslands (typical and meadow steppe) in the eastern Eurasian temperate steppe to explore the individual and combined effects of defoliation, excreta return and trampling on plant diversity, species asynchrony, dominant species stability, and consequently grassland temporal stability. We showed that defoliation played a predominant role in regulating grassland temporal stability in both steppes. Defoliation significantly increased community compositional and functional temporal stability in typical steppe, but decreased community compositional temporal stability and showed no effect on functional temporal stability in the meadow steppe. This was mainly due to different responses of the dominant species on the two steppes. Grassland temporal stability was also dependent on trampling from animal hooves, which caused a decrease in community functional temporal stability in the meadow steppe, yet had no effect on the typical steppe. Although excreta return alone did not affect grassland temporal stability, it significantly promoted community compositional temporal stability when combined with defoliation in the typical steppe. Species asynchrony and dominant species stability largely contributed to grassland community compositional and functional temporal stability. Our simulated grazing experiment demonstrated that maintaining ecosystem temporal stability in grazed grasslands is a complex process driven by various mechanisms. Defoliation and trampling by herbivores jointly influenced both compositional and functional temporal stability, and external nutrient addition may be necessary for maintaining grassland temporal stability during livestock grazing management, given the dominant impact of herbivore defoliation. Our findings help improve our understanding of grazing practices regulating grassland temporal stability in temperate steppes. Moreover, this study suggests that grassland management should consider the response strategies of dominant species to grazing disturbance, offering valuable guidance for sustainable grazing practice and future policy decision-making.