Inter- and intraspecific consumer trait variations determine consumer diversity effects in multispecies predator−prey systems

This study investigated how inter- and intraspecific trait variations determine consumer diversity loss effects in a short-term microcosm experiment, using consumer and prey biovolume production and composition as the main response variables. Three levels of ciliate diversity were created, all feeding on a 3-species microalgal prey mixture. Ciliates differed in consumer specialisation, feeding on 1 (specialist, S), 2 (intermediate, I) or all 3 (generalist, G) microalgal species. Intraspecific trait variation was incorporated by including 3 different clones of I and setting up ciliate combinations with either monoclonal or polyclonal populations of I. Both increasing inter- and intraspecific consumer diversity increased total ciliate biovolume. On the species level, total ciliate biovolume was high wherever G was included, indicating a positive selection effect for a competitively superior species. Polyclonal I monocultures exceeded the biovolume of all monoclonal ones (transgressive overyielding) based on complementary differences of clone-specific feeding niches. This effect was also observed in multispecies combinations. Both inter- and intraspecific consumer diversity decreased prey evenness. Despite being able to feed on all prey species, G displayed specific grazing preferences within its dietary niche. Furthermore, G exhibited an induced offence, forming giant cells that fed on other ciliates. S responded with an inducible defence, escaping predation by the intraguild predator. Overall, our study demonstrated highly complex trophic interactions driven by consumer selectivity, grazing rates, selective feeding and phenotypic plasticity, and indicated that both inter- and intraspecific consumer trait variations determine the consequences of consumer diversity loss on ecosystem functioning.