Multiple parasitoid species enhance top-down control, but parasitoid performance is context dependent

Ecological communities are composed of many species, forming complex networks of interactions. Current environmental changes are altering the structure and species composition of ecological networks, which could modify interactions, either directly or indirectly. To predict changes in the functioning of communities, we need to understand whether species interactions are primarily driven by network structure (i.e. topology) or the specific identities of species (i.e. nodes). Yet, this partitioning of effects is challenging and thus rarely explored. Here we disentangled the influence of network structure and the identities of species on the outcome of consumer-resource interactions using a host-parasitoid system. We used four common community modules in host-parasitoid communities to represent network structure (i.e. host-parasitoid, exploitative competition, alternative host and a combination of exploitative competition and alternative host). We assembled nine different species combinations per community module in a laboratory experiment using a pool of three Drosophila hosts and three larval parasitoid species (Leptopilina sp., Ganaspis sp. and Asobara sp.). We compared host suppression and parasitoid performance across community modules and species assemblages to identify general effects linked to network structure and specific effects due to species community composition. We found that multiple parasitoid species enhanced host suppression due to sampling effect, weaker interspecific than intraspecific competition between parasitoids, and synergism. However, the effects of network structure on parasitoid performance were species specific and dependent on the identity of co-occurring species. Consequently, multiple parasitoid species generally strengthen top-down control, but the performance of the parasitoids depends on the identity of either the co-occurring parasitoid species, the alternative host species or both. Our results highlight the importance of preserving parasitoid diversity for ecosystem functioning and show that other effects depend on species community composition, and may therefore be altered by ongoing environmental changes.