Grasshoppers amensalistically suppress caterpillar performance and enhance plant biomass in an alpine meadow

Amensalism may be common between non-trophically linked animals in natural ecosystems, where variation among species in body sizes and foraging modes may give rise to one-sided interference. However, species and ecosystem-level consequences of animal-animal amensalism are largely unknown. In a Tibetan alpine meadow, dominant herbivorous grasshoppers trigger a death feigning anti-predator response of co-occurring grassland caterpillars despite posing no consumptive threat. We hypothesized that: 1 ) grasshoppers reduce the performance of caterpillars while incurring no cost to themselves; and 2) this amensalism reduces top-down control of plant composition and biomass. We tested these hypotheses by factorial manipulation of both herbivores within replicate field enclosures. Grasshoppers significantly suppressed caterpillar feeding, growth rate, survival, reproductive effort and delayed metamorphosis. In contrast, grasshopper performance was unaffected by the caterpillars. Suppression of caterpillar feeding decreased overall herbivore suppression of plant biomass by 58% and shifted the functional composition of the plant community (i.e. increased sedge: forb ratio). These results suggest that consideration of non-trophic interactions such as amensalism will help predict the consequences of species losses for the structure and functioning of ecosystems.