Trophic cascade in a seaweed-epifauna-fish food chain

Trophic cascades have been described in many ecosystems where predation on a herbivore affects biomass or productivity of primary producers. While trophic cascades can be particularly strong on rocky reefs, the majority of studies are restricted to sea urchin herbivores despite the presence of numerous other grazers. The present study examines the potential for predation by small fishes on New Zealand rocky reefs to suppress small invertebrate grazers (epifauna) living on brown seaweeds, with cascading effects on host seaweeds and their epiphytic algae. A trophic cascade was identified in experiments run in outdoor mesocosms. When epifaunal densities were reduced either artificially or by fish predation, seaweeds were more fouled, but less damaged, than where epifaunal populations flourished. These experimental findings were not fully consistent with patterns observed at field sites with varying fish densities. Variation in epifaunal taxonomic structure and seaweed palatability potentially dampened the cascading effect of fish predation on epifauna. Trophic cascades with non-urchin herbivores warrant further study, with a combination of laboratory and field work necessary to understand systems in which herbivores are difficult to manipulate.