Fish predators reduce kelp frond loss via a trait‐mediated trophic cascade

Abstract Although trophic cascades were originally believed to be driven only by predators eating prey, there is mounting evidence that such cascades can be generated in large part via non‐consumptive effects. This is especially important in cascades affecting habitat‐forming foundation species that in turn, influence associated communities. Here, we use laboratory and field experiments to identify a trait‐mediated indirect interaction between predators and an abundant kelp in a marine temperate reef system. Predation risk from a microcarnivorous fish, the señorita, suppressed grazing by the host‐specific seaweed limpet, which in turn, influenced frond loss of the habitat‐forming feather boa kelp. This trophic cascade was pronounced because minor amounts of limpet grazing decreased the strength required to break kelp fronds. Cues from fish predators mitigated kelp loss by decreasing limpet grazing; we found 86% of this indirect interaction between predator and kelp was attributed to the non‐consumptive effect in the laboratory and 56% when applying the same effect size calculations to the field. In field manipulations, the non‐consumptive effect of señorita was as strong as the total predator effect and most importantly, as strong as the uncaged, “open” treatment with natural levels of predators. Our findings demonstrate that the mere presence of this fish reduces frond loss of the feather boa kelp through a trait‐mediated trophic cascade. Moreover, despite large volumes of water, current flow, and wave energy, we clearly demonstrate a strong non‐consumptive effect via an apparent chemical cue from señorita, suggesting that chemically mediated trait‐driven cascades may be more prevalent in subtidal marine systems than we are currently aware.