Relative importance of plant-mediated bottom-up and top-down forces on herbivore abundance on Brassica oleracea

1. Arthropod communities are structured by complex interactions between bottom-up (resource-based) and top-down (natural enemy-based) forces. Their relative importance in shaping arthropod communities, however, continues to be under debate. Bottom-up and top-down forces can be affected by intraspecific plant variation, for example by differences in concentrations of secondary metabolites that affect herbivore abundance through plant quality (bottom-up) or attract natural enemies of these herbivores (top-down). 2. Our objective was to investigate whether herbivore abundance is more strongly affected by plant-mediated bottom-up or top-down forces. 3. We used a model system of four cultivars of Brassica oleracea that show a high degree of variation in several plant traits, resistance to herbivores and attraction of natural enemies. During two field seasons, we recorded the abundance of several herbivorous and carnivorous insect species. To assess the relative importance of bottom-up and top-down forces, we quantified chemical and morphological traits of the cultivars (bottom-up) and assessed parasitization of herbivores and predator oviposition on plants inoculated with a controlled number of herbivores (top-down). 4. We show that intraspecific variation in plant chemistry and morphology consistently affects the abundance of insect herbivores and their natural enemies, resulting in cascading effects on tritrophic interactions in the associated insect community. Foliar profiles of glucosinolates and leaf toughness appeared most important for these effects. Brassica oleracea cultivars that harboured the largest numbers of herbivores also harboured the largest numbers of natural enemies. Differences in the fraction of herbivores parasitized and in predator oviposition on plants inoculated with a controlled number of herbivores could not explain the differences in natural abundance of herbivores. 5. Although abundance of herbivores is most likely influenced by a combination of bottom-up and top-down forces, it appears that in the tritrophic system investigated, bottom-up forces (plant chemistry and morphology) were more important for herbivore abundance than plant-mediated top-down forces (attraction and arrestment of natural enemies).