When ‘Higher’ means ‘Hungrier’: Climate and population trait differences drive increased insect herbivory with elevation in a perennial subalpine wildflower

Because short growing seasons severely constrain plant growth and biomass accumulation in high elevation habitats, herbivory can profoundly impact both individual fitness and community dynamics in these settings. All else being equal, climate change is expected to increase the activity of insect herbivores as their metabolic rates rise with temperature. However, montane species may have more complex responses than those in agricultural or lowland ecosystems, since many factors that shape plant-insect interactions, including temperature, shift with elevation. From 2016 to 2018 we conducted field observations of grasshopper herbivory on subalpine lupines in Mt. Rainier National Park and combined these with multiple leaf trait analyses and a set of manipulative feeding trials to explore how insect herbivory varies along a climatic gradient, and whether differences in plant or insect herbivore phenotypes that are influenced by a population's climatic history can explain these patterns. We found a significant increase in herbivory with elevation that was related to both abiotic drivers, particularly snowmelt timing, and population traits, particularly leaf nutrition and grasshopper feeding rates. Our results suggest that some high-elevation plants may already be experiencing ecologically meaningful levels of insect herbivory that could intensify with climate warming. They also highlight the complexity of predicting how species interactions will change with warming in alpine and subalpine ecosystems, where environmental plasticity or local adaptation driven by elevational differences in climate may lend tremendous complexity to ecological dynamics.