Unravelling trophic interaction modifications and community dynamics using multispecies functional responses

Background/Question/Methods

Studies of predator-prey dynamics have primarily focused on interactions between pairs of species. However, two species rarely exist in isolation, and in multispecies systems a modifier species may alter the focal interaction through indirect pathways. Such trophic interaction modifications (TIM) often go unnoticed despite increasing evidence that TIM are (i) as important as direct effects, and (ii) their proper quantification is necessary for the accurate prediction of species coexistence and community dynamics. The goal of this study was to quantify the effect of a modifier species on focal predator-prey pairs using a multispecies functional responses (MSFR). We used protists to measure the consumption rate of a predator (Spathidium sp.) in the presence of a modifier (Paramecium caudatum) for two prey species (either Colpidium striatum or Dexiostoma campylum). We varied the density of the prey (9 densities) and modifier (6 densities) in a fully factorial manner (54 combinations) to generate a response surface. By fitting a MSFR model to the number of prey consumed, we were able to estimate parameters that define the strength and stability of trophic interactions with and without the modifier species.

Results/Conclusions

In the absence of the modifier species, prey consumption was greater (i.e higher space clearance rate and lower handling time) for the Spathidium-Dexiostoma species pair than for Spathidium-Colpidium. The predator displayed density-dependent search rates for both prey species (i.e. Hill exponent > 1) suggestive of stable consumer-resource interactions. In the presence of the modifier species, the strength of the trophic interactions changed for both species pairs, and search rates remained density-dependent. MSFRs are rarely empirically estimated due to difficulties quantifying starting densities and consumption in many systems. To overcome this logistical hurdle, we have taken advantage of methodological advances in classification from video analyses that allow for precise estimation of a single species abundance in a multispecies context. Our study illustrates the value of functional response surfaces to understand trophic interaction modifications and the resulting community dynamics in multispecies systems. Our results suggest mechanisms that allow for the stable community dynamics frequently observed in nature, in contrast to the unstable dynamics often predicted from pair-wise species interactions.