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.