Asexual endophytes and associated alkaloids alter arthropod community structure and increase herbivore abundances on a native grass.

Despite their minute biomass, microbial symbionts of plants potentially alter herbivory, diversity and community structure. Infection of grasses by asexual endophytic fungi often decreases herbivore loads and alters arthropod diversity. However, most studies to date have involved agronomic grasses and often consider only infection status (infected vs. uninfected), without explicitly measuring endophyte-produced alkaloids, which vary among endophyte isolates and may impact consumers. We combined field experiments and population surveys to investigate how endophyte infection and associated alkaloids influence abundances, species richness, evenness and guild structure of arthropod communities on a native grass, Achnatherum robustum (sleepygrass). Surprisingly, we found that endophyte-produced alkaloids were associated with increased herbivore abundances and species richness. Our results suggest that, unlike what has been found in agronomic grass systems, high alkaloid levels in native grasses may not protect host grasses from arthropod herbivores, and may instead more negatively affect natural enemies of herbivores.