Trophic complexity alters the diversity-multifunctionality relationship in experimental grassland mesocosms

Higher levels of diversity within trophic levels are necessary to sustain multiple ecosystem functions, but this diversity-multifunctionality relationship peaks at intermediate percent-function thresholds. The presence of multiple trophic levels, or trophic complexity, affects ecosystem multifunctionality but its effect on the diversity-multifunctionality relationship has not been experimentally tested. To test the sensitivity of the diversity-multifunctionality relationship to trophic complexity, we simultaneously manipulated plant diversity and trophic complexity in a multifactorial tall-grass prairie mesocosm experiment at Cedar Creek, Minnesota, USA. Trophic complexity altered the diversity-multifunctionality relationship, by lowering the height of the peak as well as by shifting to a negative effect at lower thresholds. These outcomes are consistent with decreases in both the "jack-of-all-trades" effect and complementarity among plant species. Our findings suggest that trophic complexity, which is experiencing widespread declines on a global scale, is important for sustaining ecosystem multifunctionality in the face of equally widespread declines in biodiversity.