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

Abstract

AbstractEcological communities are composed of a multitude of interacting species, forming complex networks of interactions. Current global changes are altering community composition and, as such, we need to understand if the mechanisms structuring species interactions are consistent across different species compositions. However, it is a challenge to identify which aspects of species interactions are primarily driven by community structure and which by species identity. Here we compared the outcome of host-parasitoid interactions across four community modules that are common in host-parasitoid communities (i.e., host-parasitoid, exploitative competition, alternative host, and combined exploitative competition and alternative host modules) to measure the influence of structure and identity on the outcome of host-parasitoid interactions. In a laboratory experiment, we produced nine different species assemblage combinations per community module using a pool of three Drosophila host and three larval parasitoid species. We found that multiple parasitoid species enhanced host suppression due to sampling effect and weaker interspecific than intraspecific competition between parasitoids. Presence of an alternative host species had no general effects on host suppression nor on parasitoid performance, therefore showing no evidence of indirect interactions between host species nor any host switching behavior. However, effects of community 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 performance of the parasitoids depends on the identity of either the co-occurring parasitoid species, the alternative host species, both, or none. This implies that effects of co-occurring species should be taken into account for understanding ecosystem functioning and services. Understanding the effects of the biotic context on species interactions is essential when developing biological control strategies, especially in open agricultural systems.