Concerns about the dramatic global species loss have triggered great interest in the question whether biodiversity loss affects ecosystem functioning. While experiments within this complex research field were initially conducted within one trophic level, mainly the producer level, this question has more recently also been addressed in a multi trophic context. Consumer diversity effects on food web dynamics have been shown to be highly context-dependent, determined by food web structure, and thus by the species-specific traits and associated trade-offs involved. To understand the consequences of consumer diversity loss on trophic dynamics and ecosystem functioning, we used experimental approaches (semi-continuous microcosm- and chemostat-experiments) and life observations to investigate the role of trait variation on two interacting trophic levels (freshwater ciliate consumers and microalgal prey). We focused particularly on the trade-offs between consumer-specific traits, such as consumer specialization (generalist, intermediate, specialist), grazing and growth rates (specialization versus grazing efficiency) and on the associated prey trait variation (edibility versus growth rate). Intraspecific trait variation was incorporated by including three different clones of the intermediate consumer and setting up ciliate combinations with either monoclonal or polyclonal populations of this species.
Results/Conclusions
Increasing inter- and intraspecific consumer diversity increased total ciliate biovolume. On the species level, total ciliate biovolume was high wherever the very productive generalist was included, indicating a positive selection effect for a competitively superior species. Polyclonal monocultures of the intermediate consumer 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, the generalist displayed specific grazing preferences within its dietary niche. Furthermore, it exhibited an induced offense, forming giant cells that fed on other ciliates. The specialist 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 variation determine the consequences of consumer diversity loss on ecosystem functioning.