Climate change and forage-intake are important components of livestock population systems, but our knowledge about the effects of changes in these properties on livestock is limited, particularly on the Northern Tibetan Plateau. Based on corresponding independent models (CASA and TEM), a human-induced NPP (NPP_H) value and forage-intake threshold were obtained to determine their influences on livestock population fluctuation and regrowth on the plateau. The intake threshold value provided compatible results with livestock population performance. If the forage-intake was greater than the critical value of 1.9 (kg DM d^-1 sheep^-1), the livestock population increased; otherwise, the livestock population decreased. It takes four years to transfer a disturbance in primary productivity to the next trophic level. The relationships between livestock population and NPPH value determined population dynamics via the forage-intake value threshold. Improved knowledge on lag effects will advance our understanding of drivers of climatic changes on livestock population dynamics.