Food web context modifies predator foraging and weakens trophic interaction strength

Trophic interaction modifications (TIM) are widespread in natural systems and occur when a third species indirectly alters the strength of a trophic interaction. Past studies have focused on documenting the existence and relative magnitude of TIMs, however the underlying processes and long-term consequences often remain elusive. To address this gap, we experimentally quantified the density-dependent effect of a third species on a predator’s functional response to identify the processes impacted by, and consequences of TIMs. To do so we conducted short-term experiments with two ciliate communities each composed of a predator, prey, and non-consumable ‘modifier’ species. 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 effect differed with prey species identity. Using parameters estimated from our experimental observation we simulated long-term dynamics and observed quantitative differences between models that account for TIMs or include only pairwise interactions. Our study is a clear demonstration that TIMs are important to understand and predict community dynamics and highlights the need to extend past pairs of focal species to understand the consequences of species interactions in communities.