<b>Metabolites limiting predator growth wane with prey biodiversity</b>

Modeling and engineering microbiomes require a thorough understanding of predator-prey interactions. In soil, nematodes are the main consumer of bacteria, but predicting trophic interactions remains a challenge. Several prey traits potentially impact resistance to predation, but little is known about their effects in a multispecies context. Here we leverage synthetic bacterial communities of varying complexity to reveal the traits driving palatability. We set up predation assays between four nematode species and communities built from 122 bacterial isolates in monoculture or multispecies assemblages. Using nematode size as a measure of prey palatability, we show a strong variability among species, which can be predicted by prey metabolic and morphological properties. Crucially, the relevant traits depended on community complexity. Secondary metabolites drove palatability in monoculture, but this effect vanished under increasing prey biodiversity, where prey size became the dominant predictor of nematode size. This study has major implications for our understanding of microbial ecology in the context of predator-prey interactions. Idiosyncratic properties emphasized in the literature are of minor importance in biodiverse assemblages. In contrast, generalizable traits such as body size, compatible with state-of-the art community dynamics models, reliably predict predator-prey interactions. We conclude that research efforts to decrypt microbial ecology can be guided by deductions based on generalizable biological traits.