Mixotrophs combining autotrophy and heterotrophy are ubiquitous in aquatic environments and significantly influence ecosystem functioning. Mixotrophs may adapt their nutritional mode in response to selection, becoming more heterotrophic or more autotrophic over time. This may dynamically interact with adaptations in the defense level of their prey organisms (resources) as population dynamics are shaped by both competitive and predatory interactions. Here, we developed a trait-based mixotroph-resource model comprising a mixotrophic consumer (e.g. ciliate or algae) and a resource (e.g. autotrophic algae or bacteria) competing for inorganic nutrients. The model involves trade-offs between autotrophic and heterotrophic growth for the mixotroph, and between defense capacity against predation and maximum growth rate for the resource. We investigated the population and trait dynamics for different scenarios, in which none, one or both species were able to adapt their traits in response to selection. Under specific combinations of fixed traits, either species could dominate whereas the mixotroph often gained dominance when it could adapt its trait to exert both strong predation and competition pressure on the resource. Trait adaptation in the resource promoted its dominance only when the mixotrophy trait was fixed, whereas it played a minor role under coadaptation. Moreover, antiphase cycles often emerged when the resource and the mixotroph adapted their traits independently or interactively, with the species dominating that was able to adapt to the current selection pressure. Overall, our findings demonstrate that mixotrophy trait adaptation substantially affects species composition and the shape and stability of population dynamics in food webs.

We developed a trait-based model depicting a mixotroph (M) and a resource (R) in competition for nutrients, involving adaptations in mixotrophy (o) and defense (u) traits, respectively. The mixotrophy trait refers to the ability of M to obtain nutrition from both autotrophy and heterotrophy. The defense trait refers to the ability of R to resist predation from M. Regarding the ecological dynamics, our model tracks the flows in the different pools of particulate organic nutrients, i.e. M and R, and dissolved inorganic nutrients, i.e. N. The population sizes of the species are thus expressed in units of nutrients, e.g. nitrogen or phosphorus. To describe the temporal dynamics of the continuous defense (u) and mixotrophy (o) traits of the two different species, we employed the fitness gradient approach from quantitative genetics. Hence, u and o are assumed to change at a rate proportional to the genetic variance of the traits and the individual local fitness gradients.