Plant defense trait measurements
We investigated whether abiotic and biotic factors influenced intraspecific variation in herbivory by altering the susceptibility of individual plants to herbivore damage. We also quantified plant chemical defenses in three common species (Polygonum macrophyllum , Saussurea pulchra and Taraxacum mongolicum ). Among all species identified in the study survey, these three species had the broadest latitudinal distributions (Figure S1).
We measured the total flavonoid, saponin and tannin contents; these compounds are widely used by plants as toxins (i.e. , flavonoids and saponins, Treutter, 2005; Faizal & Geelen, 2013) and digestibility reducers (i.e. , tannins, Feeny, 1968). They are also protective against abiotic stresses; for example, flavonoids play a role in drought resistance and UV radiation stress tolerance (Treutter, 2005). The quantification of these defense chemicals is described inSupplementary Methods .