Weakness is a strength: An empirical test of the weak interaction hypothesis in a food web with omnivory

Background/Question/Methods

Trophic interactions rarely adhere to the paradigm of linear interactions depicted in simple food chains. Omnivory, defined as feeding on more than one trophic level, is prolific. However, theoretical analyses have demonstrated that the stability and ubiquity of food webs with omnivory is predicated on weak interactions.

In this paper, we experimentally dissect and modify predator-consumer interactions in a tri-trophic food web with omnivory to determine how the strength of predation and competition contributes to the persistence of the food web. We used a blue mussel (Mytilus edulis), zooplankton (Artemia franciscana), and algae (Isochrysis galbana) system to simulate strong and weak interactions. Here the inhalant siphon of the blue mussel facilitates predation on both zooplankton and algae, while zooplankton consume only algae.   

We constructed mussel-zooplankton-algae food webs in mesocosms and regulated the strength of mussel-zooplankton predation and competition to generate a combination of strong and weak interactions. Predation was manipulated with Nitex sleeves that either facilitated or weakened mussel predation on zooplankton. The mussel-algae interaction was weakened by supplementing each mesocosm with algae in accordance with the number of cells removed. No subsidies were provided in the strong competition treatment. A mesocosm with the predator removed served as the control.

Results/Conclusions

The predation results were consistent with the theoretical predictions – strong interactions drive the consumer towards extinction. Treatments with strong predation resulted in lower zooplankton biomass at the conclusion of the experiment relative to the weak predation experiment and strong competition reduced zooplankton biomass in the weak predation experiments. We expected to find weak competition would facilitate persistence at high predation, however the availability of algae induced increased filtration and consequently predation. The strong predation with weak competition treatment had significantly lower biomass (ANOVA; p<0.05). 

Our results suggest that persistence of the consumer is most likely in food webs with weak interactions and weak competition could facilitate the persistence of simple food webs where predation is weak. However in systems with a filter-feeding predator weak competition, through increased productivity for example, could generate higher predation on the consumer and have the opposite effect of theoretical predictions. Given the potential for the propagation of omnivory through species introductions and anthropogenic food web alterations (e.g. aquaculture), understanding how species traits affect the manifestation of predicted interactions is critical.