Climate played a primary role in driving intraspecific herbivory variability
Structural equation modeling revealed stronger effects of climate versus other environmental factors on both intraspecific herbivory variability and community-wide herbivory. Similarly, of all factors considered, temperature seasonality was the most often included factor in the best models; seasonality was negatively associated with community-wide herbivory, intraspecific herbivory variability and herbivory of the three focal plant species. These findings are consistent with previous studies of individual species which found that large-scale latitudinal variation in herbivore pressure was predominantly explained by climatic variables (e.g. , Moreira et al. , 2015; Moreira et al. , 2018; Loughnan & Williams, 2019). Furthermore, while climatic factors were strongly associated with herbivory in our three focal plant species, plant defense traits were only weakly linked to herbivory in these species. These findings suggest that (indirect) climatic effects were unlikely to be mediated by plant chemical defense traits. Instead, previous studies have found that climatic conditions strongly restrict the abundance and distribution of insect herbivores on the Qinghai-Tibetan Plateau (Xi et al. , 2013; Li et al. , 2019; Du et al. , 2020). Therefore, we speculate that climate may directly determine herbivore pressure on the Qinghai-Tibetan Plateau, primarily by altering the distribution and identity of insect herbivores affecting plant communities.
Apart from climatic factors, the effect of soil properties on herbivore pressure should not be ignored. In this study, although there was no overall association between the tested edaphic factors and intraspecific herbivory variability, soil nitrogen and water content were both positively associated with community-wide herbivory. Herbivore pressure has often been posited to be related to resource availability, both because high-resource environments sustain more herbivores (particularly generalists) (Weiblen et al. , 2006; Louthan et al. , 2015) and because plants growing in high-resource environments typically have higher tissue quality than plants growing in low-resource environments (Coley & Aide, 1991; Price, 1991). In addition, we found that soil pH was negatively correlated with community-wide herbivory and intraspecific herbivory variability. These negative associations may be mediated by the soil biota. For example, a recent study demonstrated that higher soil pH and nutrient availability (e.g. , of phosphorus and potassium) can increase microbial abundance and diversity; this can further suppress aboveground insect herbivory via enhancing plant tolerance (by increasing plant total biomass) and resistance (by decreasing concentrations of sugars and amino acids) (Jiang et al. , 2020). Therefore, future research on the soil biota may enhance our understanding of the factors driving latitudinal variation in herbivory.