The interactive effects of warming and eutrophication on food web structure and dynamics have been shown theoretically, but experimental evidence is lacking. We examined the effect of increasing temperature and nutrients on an experimental omnivory module where bacteria are consumed by the protist Tetrahymena pyriformis, and both are consumed by an omnivore protist species Euplotes sp.. We used a factorial design including two temperatures (22ºC and 25ºC) and two nutrient levels (half- and full- strength media), resulting in four treatments with six replicates each. We recorded the density of both protist species using fluid imaging and measured bacterial biomass through optical density (OD600) daily (excluding weekends) for 16 days. Fluid imaging also provided trait measurements—such as body size and shape—for the protists throughout the experiment. By simultaneously analyzing population dynamics and changes in protist traits over time, we examined how temperature and nutrients individually and interactively mediate food web dynamics and reshape community phenotypic composition.
Results/Conclusions
Our results showed that increasing temperature significantly increased initial population growth in both intermediate and omnivore predators regardless of nutrient level. An interactive effect between temperature and nutrients was only detected for the initial growth of the omnivore predator, which led to faster initial growth with lower nutrients at both temperatures. Higher nutrient level led to significantly higher maximum abundance in both intermediate and omnivore predators. In addition, we found significant interactive effects on maximum abundance of the intermediate predator, but only significant temperature additive effects on that of the omnivore predator. Both temperature and nutrients individually affected the extinction time of the intermediate predator, but no interactive effects were detected. For bacterial biomass, there were significant main effects from both temperature and nutrients, but no interactive effects. Lastly, body size and shape of both protists changed through time, but more analyses are needed to identify the major factors impacting trait changes. Overall, our work suggests important but overlooked interactive impacts of environmental factors on transient dynamics in a commonplace omnivore module, and interesting feedbacks on the phenotypes that drive the underlying consumer-resource interactions, representing the first experimental attempt to test existing theory on the topic.