Species richness and diversity indices (Shannon-Wiener index, Simpson dominance index and Pielou evenness index) in alpine grassland ecosystems (alpine meadow, alpine steppe and desert steppe) under grazing—excluded and freely grazed sites were investigated along the Northern Tibetan Plateau Alpine Grassland Transect during summer 2009 and 2010. We found that species richness and diversity have not been significantly altered by short—term grazing exclusion since 2006 at vegetation and regional scales. Species richness and diversity were mainly driven by growing season precipitation (GSP), which accounted for over 87 % of the total variation observed. Species richness and diversity at grazingexcluded and freely grazed sites appear to respond to growing season precipitation in parallel. Species richness exponentially increased with GSP while diversity indices showed positively linear relationships with GSP. This indicates that GSP on the Northern Tibetan Plateau is crucial in regulating species richness and diversity and should be taken into account in future studies on alpine grassland conservation.