Trophic interactions are typically studied in a pairwise manner. However, accumulating evidence suggests that such an approach is often insufficient for understanding and predicting community dynamics. 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). Past studies have focused on documenting the existence and magnitude of TIMs, however, the underlying processes and long-term consequences remain elusive. To address this gap, we experimentally quantified the density-dependent effect of a third species on a predator's functional response.

We conducted short-term experiments with two ciliate communities each of which contained a predator (Spathidium sp), prey (either Colpidium striatum or Dexiostoma campylum), and non-consumable ‘modifier' species (Paramecium caudatum). We generated response surfaces for consumption by varying the density of the prey (9 densities) and modifier (6 densities) in a fully factorial manner. To quantify the short-term impact of the modifier on the predator's functional response we used a hypothesis-driven model selection approach. Additionally, we investigated how community dynamics differ when models account for TIM compared to when they are strictly pairwise with numerical simulations.

In both communities, increasing modifier density weakened the trophic interaction strength, due to a negative effect on the predator's search clearance rate. However, the magnitude of the impact differed with food web context. Simulated long-term dynamics indicate quantitative differences, such as time to extinction and population cycle period, between models that account for TIMs or include only pairwise interactions. Thus only considering pairwise interactions may limit the predictability of community dynamics.

To our knowledge, ours is the first study to utilise a functional response surface to (i) identify the density dependence of a TIM, (ii) pinpoint the processes impacted by a modifier species and (iii) model the long-term impact of the TIM on community dynamics. Our findings demonstrate that TIMs are important to understand and predict community dynamics and highlight the need to move beyond focal species pairs to understand the consequences of species interactions in communities.