How does fear drive a marine food web? That’s right I said fear, as in blood-curdling-scream-I-think-I-just-wet-my-pants fear. Sharks just swim around and intimidate the hell out of other animals. What if a utopia state existed where sharks weren’t invited and a society developed of peace, love, and understanding? What would that society look like?
In actuality we are not too far from that state as shark populations continue to decline. Frid and colleagues publish a model this week in the journal Oikos addressing the impacts of shark declines and “fear-released systems”. The model is elegant containing foraging decisions by harbour seals on herring (near surface dweller) and walleye pollock (found deeper in areas preferred by sharks). Seals make decisions on how deep to dive and for how long, time breathing oxygen at surface, time spent traveling in water column, and how they prey upon the fish. All these affect both their risk of becoming a tasty shark treat and their own ability to fill up on tasty fish snacks. The important is that in the model seals don’t actually get eaten. It is simply the fear of being eaten that drives seal decisions.
The simulations suggest that fear, i.e. predation risk, is sufficient to drive seals toward underutilizing the riskier deep and thus ignore pollock. This scenario is an asymmetric trophic cascade, so name because the influence of top trophic level (sharks) on the next trophic level (seals) cascades to another trophic level (fish) and asymmetric because it differentially affects pollock and herring. With no shark risk seals switched to pollock.
Frid, A., G. Baker, G., M. Dill, L. (2008). Do shark declines create fear-released systems?. Oikos, 117(2), 191-201. DOI: 10.1111/j.2007.0030-1299.16134.x