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Community‐level plant palatability increases with elevation as insect herbivore abundance declines

Descombes, Patrice, Marchon, Jérémy, Pradervand, Jean‐Nicolas, Bilat, Julia, Guisan, Antoine, Rasmann, Sergio, Pellissier, Loïc
The journal of ecology 2017 v.105 no.1 pp. 142-151
Spodoptera littoralis, altitude, biotic factors, carbon nitrogen ratio, dry matter content, grasshoppers, herbivores, insect larvae, leaves, palatability, plant communities, secondary metabolites, toxicity
Plants protect themselves against herbivore attacks through a myriad of physical structures and toxic secondary metabolites. Together with abiotic factors, herbivores are expected to modulate plant defence strategies within plant assemblages. Because the abundance of insect herbivore decreases in colder environments, the palatability of plants in communities at higher elevation should shift in response to both abiotic and biotic factors. We inventoried grasshopper communities to document changes in herbivore abundance along elevation gradients and quantified associated shifts in plant palatability. We measured plant palatability by measuring the growth of Spodoptera littoralis generalist caterpillars fed with the leaves of 172 plant species. We related plant palatability to leaf traits and elevation at the species and community levels. In congruence with the decrease in grasshopper abundance with elevation, we found that the mean palatability level of plant communities increases with elevation. In addition, plant palatability was negatively associated with the community‐weighted mean of leaf dry matter content. At the species level, plants with high carbon‐to‐nitrogen ratio were less palatable, while we found no effect of species mean elevation on plant palatability. Synthesis. Our results suggest that plant communities at higher elevation are composed of species that are generally more palatable for insect herbivores. Shift in plant palatability with elevation may thus be the outcome of a relaxation of the in situ herbivore pressure and changes in abiotic conditions.