The relevance of consumer competition and feeding traits, as well as their trade-offs, in determining multispecies trophic interactions

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The impact of biodiversity loss on food web dynamics and ecosystem functioning strongly depends on food web complexity and configuration, the trophic position and specific traits of the organisms, such as consumer specialization, growth and grazing rates. The functional traits of organisms and associated trade-offs play a key role for ecosystem functioning. The proposed project aims at evaluating the importance of inter- and intraspecific consumer trait variation for biomass and trait dynamics in a freshwater ciliate consumer - microalgal prey system. We will study the consumer trade-off between starvation resistance and maximum grazing rate by investigating the relevance of associated consumer traits in response to altered regimes of resource supply. We will focus on two types of starvation resistance that are linked to different traits: 1) the ability to reduce basal metabolism at the expense of reproduction, and 2) the ability of photosynthetic carbon fixation in addition to phagotrophy (mixotrophy). Furthermore, we will investigate how inducible offense (formation of giant morphotypes capable of intraguild predation) and inducible defense (decreasing grazing susceptibility) interact with this trade-off by altering species interactions. Laboratory experiments will be conducted manipulating inter- and intraspecific consumer trait variation in different food webs under different regimes of resource (prey and light) supply. Providing resources either continuously or in pulses, entailing periods of resource depletion, will strongly affect standing trait variation and biomass-trait feedbacks. Furthermore, we will investigate the combined influence of inter- and intraspecific consumer trait variation on the mutual interplay between biomass and trait dynamics at different hierarchical levels (clones, species, communities) in more complex food webs over a longer time. All laboratory experiments will be complemented by mathematical models that will help to optimize the experimental design and to identify underlying mechanisms regarding population and community dynamics. This joint approach will broaden the existing experimental and theoretical knowledge on the interplay between biomass and trait dynamics in complex food webs and will substantially enhance our understanding of the consequences of consumer trait variation, i.e. the adaptive potential of consumers that are subjected to resource fluctuations, for ecosystem processes and functioning.