Elevation structures plant growth and defence but not resistance to slug herbivory

Abstract Predicting how plant resistance to herbivory varies along environmental gradients is a central goal in ecology, yet most empirical tests rely on insect herbivores and may not generalize across consumer guilds. Elevational gradients are expected to relax plant defence at high elevation because of reduced herbivore pressure, but whether this pattern holds for functionally distinct herbivores remains unclear. Here, we tested how plant resistance to herbivory varies with elevation when plants interact with terrestrial slugs, a group of generalist herbivores. We conducted a common garden experiment using six closely related Brassicaceae species spanning approximately 2,000 m of elevational distribution in the Swiss Alps. We quantified plant growth, chemical defence expression (glucosinolates), slug damage, and slug performance. Low-elevation species accumulated significantly more biomass and produced more than twice the total glucosinolate concentrations of high-elevation species. Despite elevational differences in growth and defence, neither slug damage nor slug performance differed systematically between high- and low-elevation plants. Instead, resistance varied primarily among species and was best predicted by plant growth rate: faster-growing species suffered higher levels of slug damage, consistent with growth-defence trade-offs. Glucosinolate composition differed with elevation and herbivory treatment, but variation in total glucosinolate concentration did not translate into reduced slug feeding or growth. Our findings demonstrate that elevational patterns of plant defence inferred from insect-based studies do not readily extend to molluscan herbivores. Rather than elevation per se, species-level trait variation, particularly growth strategies, emerged as a key determinant of herbivory outcomes, highlighting the importance of herbivore functional identity in trait-based frameworks.