Loss of foundation species: disturbance frequency outweighs severity in structuring kelp forest communities

Disturbances often cause the disproportionate loss of foundation species but understanding how the frequency and severity of disturbance to such organisms influence biological communities remains unresolved. This gap in knowledge exists in part because of the rarity of ecologically meaningful studies capable of disentangling different elements of disturbance. Hence, we carried out a long-term (9 yr), large-scale (2,000 m² plots), spatially replicated (4 sites) field experiment in which we manipulated disturbance to a globally distributed marine foundation species, the giant kelp Macrocystis pyrifera, and tracked community responses over time. To distinguish the effects of disturbance frequency and severity on the biodiversity and composition of temperate rocky reef communities, we simulated the repeated loss of giant kelp from destructive winter waves across a background of natural variation in disturbance. By following the response of over 200 taxa from the surrounding community, we discovered that the frequency of disturbance to giant kelp changed the biomass, diversity, and composition of community guilds in a manner commensurate with their dependence on the physical (i.e., benthic light and space), trophic (i.e., living and detrital biomass), and habitat (i.e., biogenic structure) resources mediated by this foundation species. Annual winter disturbance to giant kelp reduced living and detrital giant kelp biomass by 57% and 40%, respectively, enhanced bottom light by 22%, and halved the seafloor area covered by giant kelp holdfasts. Concomitantly, the biomass of understory algae and epilithic sessile invertebrates more than doubled, while the biomass of rock-boring clams, mobile invertebrates, and fishes decreased 30-61%. Frequent loss of giant kelp boosted understory algal richness by 82% and lowered sessile invertebrate richness by 13% but did not affect the biodiversity of mobile fauna. In contrast to changes driven by disturbance frequency, interannual variation in the severity of disturbance to giant kelp had weaker, less consistent effects, causing only modest changes in assemblages of sessile invertebrates, mobile invertebrate herbivores, and fishes. Our results broaden the foundation species concept by demonstrating that repeated disturbance to a dominant habitat-forming species can outweigh the influence of less frequent but severe disturbances for the surrounding community.