Climate‐induced upslope shift of orthopteran herbivores imposes greater herbivory through trait complementarity

Abstract The asynchronous upward shift of insect herbivores and plants towards higher elevations following climate warming is anticipated to generate novel plant–insect interactions. As generalist herbivores shifting to higher elevations might incorporate novel plant species into their diet, high‐elevation plant communities could experience increased herbivory pressure. Accordingly, we investigated the impact of lower‐elevation orthopterans on high‐elevation plant communities and some of the mechanisms enabling orthopterans to establish at higher elevations. We conducted a common garden experiment at two elevations in the Swiss Alps, exposing high‐elevation plants to different orthopteran communities. This field experiment was complemented by a cafeteria choice experiment to assess the diet preferences of the orthopteran species included in the first experiment, along with functional trait measurements. The field experiment highlighted that high‐elevation plants belonging to the Liliopsida [= monocotyledons] clade experienced higher herbivory rates when fed upon by a mixed group of lower‐ and higher‐elevation orthopterans than when plants were attacked by single‐elevation communities. The cafeteria experiment revealed diet conservatism, with orthopterans favouring plants sharing functional traits with their home‐elevation diets. Additionally, our trait analysis indicated that orthopterans can be categorized into either Liliopsida or Magnoliopsida [= dicotyledons] feeders based on their dietary traits and functional characteristics, explaining the observed feeding niche complementarity. In summary, upslope‐shifting orthopterans may increase herbivory pressure on high‐elevation plant communities. High‐elevation Liliopsida, in particular, are highly vulnerable due to the functional traits of lower‐elevation orthopterans that drive their feeding preferences towards this plant group. Read the free Plain Language Summary for this article on the Journal blog.