Coadaptation shapes ecological interactions in mixotroph-resource systems

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.