1. Theory predicts that a large fraction of phytochemical diversity – the richness of individual chemical compounds produced by plants – governs the complexity of interactions between plants and their herbivores. While the effect of specific classes of chemical compounds on plant resistance against herbivores has been largely documented, the effect of community-level variation in phytochemical diversity on plant-herbivore interactions has so far received minimal consideration. 2. We hypothesized that plant communities bearing on average higher levels of phytochemical diversity should sustain lower herbivory rates, overall. Yet, the magnitude of this effect could vary across different environmental conditions, potentially because of climate- mediated effects on phytochemical production and changes in herbivore community richness and composition. 3. To address these hypotheses, we used previous knowledge of species-level phytochemical make-up for more than 400 plant species of the Swiss Alps. Using common garden experiments, we estimated season-wide herbivore damage on low- (average 3500 unique molecules) and high- (average 4500 unique molecules) phytochemical diversity plant communities that were planted in the colline, mountain, and alpine vegetation sites along two elevation transects in the Alps. 4. We found that high-phytochemical diversity plant communities showed reduced levels of herbivore damage in the colline (low elevation) sites, but this pattern reversed in the alpine (high elevation) sites. Our results suggest that the outcome of phytochemical diversity on plant-herbivore interactions depends on the characteristics of the local herbivore communities, together with trade-offs between chemical defences and other plant traits (i.e. physical defences and plant palatability). 5. Synthesis – Phytochemical diversity is a key component of functional diversity, influencing community composition and dynamics. We show that the effect of phytochemical diversity on herbivory is environmental-dependent, generating ecological switches when moving from low to high elevation. Through upward movement of plants under climate change, phytochemical community structure will be likely modified, ultimately disrupting local community assembly processes.

Datasets included in the archive file are stored within the following subfolders: "Field_and_plant_data.xslx" : this subfolder includes field data and species trait information for each experimental plant. The first worksheet ("Plant_data") includes plant level information (species identity, plant community and phytochemical diversity group), location (region in Swiss alps and elevation of planting), herbivory estimations at four time points and plant biomass at the end of the season. The second worksheet ("Species_traits") includes the species-level information of phytochemical richness measured in our experimental plants by using HPLC-DDA methods, HPLC-DIA values of phytochemical richness from Defossez et al. (2021), and mean traits values retrieved from Descombes et al. (2020) and Pitteloud et al. (2020). "Metabolomic_raw_data.csv" corresponds to the molecular features matrix obtained from the experimentally-grown plants. Given are the spectra peaks heights for each molecular feature detected in each sample, as well as the sample information (species identity and location). Metabolomic data was adquired by ultra-high-performance liquid chromatography - high resolution tandem mass spectrometry using data-dependent analysis (DDA) and analyzed with MZMINE 2.53.