Effects of grazing on C:N:P stoichiometry attenuate from soils to plants and insect herbivores in a semi-arid grassland

Understanding the processing of limiting nutrients among organisms is an important goal of community ecology. Less known is how human disturbances may alter the stoichiometric patterns among organisms from different trophic levels within communities. Here, we investigated how livestock grazing affects the C:N:P ecological stoichiometry of soils, plants (Leymus chinensis), and insect herbivores (Euchorthippus spp.) in a semi-arid grassland in northeastern China. We found that 3 years of grazing significantly enhanced soil available N and leaf N content of the dominant L. chinensis grass by 15% and 20%, respectively. Grazing also reduced (soluble) C:N of L. chinensis leaves by 22%. However, grazing did not affect total C, N, or P contents nor their ratios in Euchorthippus grasshoppers. Our results reveal that the effects of grazing disturbances on elemental composition attenuated from lower to higher trophic levels. These findings support the theory that organisms from higher trophic levels have relatively stronger stoichiometric homeostasis compared to those from the lower trophic levels. Moreover, grasshopper abundance dropped by 66% in the grazed areas, and they reduced the feeding time on their host L. chinensis grass by 43%, presumably to limit the intake of excess nitrogen from host plants. The energetic costs associated with the maintenance of elemental homeostasis likely reduced individual performance and potentially explains the decrease in grasshopper abundance in the grazed areas. Our study suggests that a comprehensive investigation of stoichiometric properties of organisms across trophic levels may enable a better understanding of the nature of species interactions, and facilitate predictions of the consequences of environmental changes for community organization in the future.