Effect of grazing rate on biomass and element composition of Xanthoparmelia camtschadalis Grassland unilization and feed budget of Tan-sheep under grazing conditions in Ningxia arid grassland Influence of Grazing Rate on Population Structure of Stipa grandis Feeding Ecology of Sinocalanus tenellus (Copepoda:Calanoida) Grazing on Phytoplankton in Saline-alkaline Ponds