Independent Group Leader University of Zurich, Switzerland
Abstract: Species interactions are typically studied in a pairwise manner, however, in natural systems two species rarely occur in isolation. Higher-order interactions occur when a third species alters the interaction between a focal pair via an indirect pathway. If the focal interaction being altered is between a predator and its prey, this is considered a trophic interaction modification (TIM). There is growing evidence that TIMs are widespread and can significantly impact species interactions. Currently, empirical estimates of TIMs use short-term experiments, however, such approaches may not be able to fully capture the systems’ dynamics. The goal of this study was to estimate the direct and indirect species interactions in a simple community using time-series data to quantify the impact of a modifier species on the focal trophic interaction.
To investigate our research question, we assembled two ciliate communities each of which contained a predator (Spathidium sp) and a prey species (either Colpidium striatum or Dexiostoma campylum) which competed for the basal bacterial resource with a non-consumable modifier (Paramecium caudatum). We used a ‘ministat array’ composed of small-scale flow-through cultures (i.e., 20 mL chemostats) to culture each community in replicate (n = 10) for 60 days. We measured population densities thrice weekly which, were used to estimate species interaction strengths and quantify the effect of the trophic interaction modification.
In the absence of the predator, the modifier (Paramecium) was competitively superior to both prey species. Intriguingly, the introduction of the predator had a negative effect on the modifier species via a non-consumptive effect. The negative effect of the predator on the prey was reduced at high densities of the modifier for the strong (Colpidium) but not the weak (Dexiostoma) competitor. The observed long-term dynamics of our populations were the product of the interacting competitive and trophic interactions. Our results suggest that the presence of a modifier species can decrease the strength of the predator-prey trophic interaction. However, the mitigation of the trophic interaction appears to be dependent on the competitive relationship between the prey and the modifier. To our knowledge, ours is the first study to utilize a continuous flow-through system to study the modification of trophic interactions. The ministat array allowed us to maintain constant environmental conditions (e.g., resource availability) over an extended period. We believe this system is a valuable experimental approach to add to the community ecologist’s toolkit.
