Biologists still strive to identify the ecological and evolutionary
drivers of phytochemical variation that mediates biotic interactions. We
hypothesized that plant species growing at sites characterized by high
herbivore pressure would converge to produce highly toxic blends of
secondary metabolites, independently of phylogenetic constraints. To
address the role of shared evolutionary history and ecological niches in
driving variation in plant phytochemistry, we combined targeted
metabolomics, with insect herbivore bioassays on Cardamine species
growing along the entire elevational gradient of the Alps. We observed
that plant species cluster according to similar habitat-mediated
plant-growth forms and chemical profiles, independently of phylogenetic
relationship. We also showed that novel indices summarizing functional
phytochemical diversity better explain plant resistance against chewing
and sap-feeding herbivores than classic diversity indices. We conclude
that the functional axis of phytochemical diversity should be integrated
with the functional axis of plant growth forms to address convergence
along large-scale ecological gradients.