Bottom-up effects of plant diversity on multitrophic interactions in a biodiversity experiment

An eight-year study of the effects of biodiversity on ecosystem functioning across the food web suggests that plant diversity has strong bottom-up effects on biotic interactions. The experiment by Scherber et al. manipulated plant species in specially seeded grassland plots and monitored species richness of herbivores, carnivores and all other relevant groups, and their interactions. The effects are consistent across groups, but are stronger at adjacent trophic levels and in above-ground groups rather than below-ground groups. Responses to increasing plant diversity were generally positive, but were negative for biological invasion, pathogen infestation and hyperparasitism. The effects of biodiversity on ecosystem function are usually studied within trophic levels. These authors conduct a large experiment across trophic levels to show how manipulations of plant diversity affect function in different groups. The effects are consistent across groups, but are stronger at adjacent trophic levels and in above-ground rather than below-ground groups. Biodiversity is rapidly declining1, and this may negatively affect ecosystem processes2, including economically important ecosystem services3. Previous studies have shown that biodiversity has positive effects on organisms and processes4 across trophic levels5. However, only a few studies have so far incorporated an explicit food-web perspective6. In an eight-year biodiversity experiment, we studied an unprecedented range of above- and below-ground organisms and multitrophic interactions. A multitrophic data set originating from a single long-term experiment allows mechanistic insights that would not be gained from meta-analysis of different experiments. Here we show that plant diversity effects dampen with increasing trophic level and degree of omnivory. This was true both for abundance and species richness of organisms. Furthermore, we present comprehensive above-ground/below-ground biodiversity food webs. Both above ground and below ground, herbivores responded more strongly to changes in plant diversity than did carnivores or omnivores. Density and richness of carnivorous taxa was independent of vegetation structure. Below-ground responses to plant diversity were consistently weaker than above-ground responses. Responses to increasing plant diversity were generally positive, but were negative for biological invasion, pathogen infestation and hyperparasitism. Our results suggest that plant diversity has strong bottom-up effects on multitrophic interaction networks, with particularly strong effects on lower trophic levels. Effects on higher trophic levels are indirectly mediated through bottom-up trophic cascades.