Dominant species determines Drosophila-parasitoid food web dynamics and stability more strongly than complexity, warming or invasion

Ecological communities face major challenges of biodiversity loss, invasive species, and warming due to human action. Factors that determine the dynamics, stability and functioning of network of interacting species have been debated in ecology since its beginning as discipline. Community complexity and dominant (keystone) species have been proposed as the most relevant factors, however there is lack of knowledge on how these factors will interact with warming and invasion. To determine the effect of complexity, warming or invasion and their combinations on stability and functioning of a food web we performed a microcosm laboratory experiment using Drosophila-parasitoid food webs as a model. During five months, we recorded the dynamics of twelve different food webs (different combinations of flies and parasitoid species with two complexity levels) subjected to two temperature regimes (ambient and warming) and invasion or not by Drosophila suzukii in a factorial design. We did not observe any major effect of complexity or invasion. However, species composition had major effect, as webs with Drosophila simulans present were rapidly dominated by it, reducing diversity, increasing parasitism rate, reducing stability and bringing the community on the verge of collapse. Warming in turn increased diversity, overall fly population and temporal stability, while decreasing parasitism rate. Invasion of D. suzukii was not successful in any of the networks, possibly due to its low competitive ability. Our results imply that dominant species can have very strong effects on dynamics and stability, compared to food web complexity.