Nutritional mode and specialization alter protist consumer diversity effects on prey assemblages

Abstract

Mixotrophy is a widespread phenomenon and plays an important role in aquatic food webs by increasing the complexity of trophic interactions and enhancing trophic transfer up the food web. Nevertheless, the effects of mixotrophic consumers on lower trophic levels have not been investigated in the framework of recent biodiversity and ecosystem functioning research, although mixotrophic interactions can lead to enhanced variability of those relationships. The effects of ciliate consumer diversity and nutritional mode on algal prey biovolume and evenness as well as consumer biovolume were investigated in 2 freshwater microcosm experiments. Different species compositions in the consumer and algal assemblages of the 2 experiments resulted in partially different interactions. Increasing consumer richness decreased prey biovolume in both experiments. However, heterotrophic consumers decreased prey biovolume and evenness more than mixotrophs in one experiment, driven by a strong grazer that was not present in the other experiment. Consumer presence and richness affected prey evenness differently, preventing competitive dominance among algae in one experiment but leading to unbalanced grazing in an initially more even prey assemblage in the other experiment. Secondary production increased with consumer richness in both experiments but was differently affected by the consumers' nutritional mode due to altered competitive interactions and niche overlap in different species combinations. The present study demonstrated that consumer-specific characteristics, such as nutritional mode and feeding preferences, may alter consumer diversity effects on prey. © Inter-Research 2012.