Patterns, mechanisms and community consequences of variation in kelp forest canopies

Structurally complex vegetative habitat can have strong influences on the patterns, mechanisms and consequences of species interactions and the structure of a community. How spatial and temporal variation of biogenic habitat influences species interactions that determine spatial and temporal patterns of community structure and functions is not well understood. I investigated how environmental gradients and anthropogenic perturbations influence the physical structure of the canopy of giant kelp, Macrocystis pyrifera , forests and the consequences for community structure and functions. Examination of kelp forest canopies along an environmental gradient of oceanic swell revealed that kelp forests more exposed to swell support greater total abundance, species richness, and markedly different invertebrate assemblages than did more protected kelp forests. Additionally, abundance of an epibiont, Membranipora spp., was positively correlated with increased swell exposure, which contributed to the variation in community structure. I also experimentally assessed the effects of biogenic habitat and the epibiont on prey assemblages, predator-prey interactions and the foraging success of predators (juvenile rockfishes of the genus Sebastes). The cumulative effects of Macrocystis and Membranipora caused a shift in the species composition of the prey assemblage and increased growth rates of juvenile rockfishes by 100%, reflecting an increase in biomass and size of the substrate-associated prey. The results demonstrate how epibionts can modify predator-prey interactions and enhance the functional role of vegetation (i.e. nursery habitat for juvenile fishes). I also investigated how the reduction of canopy habitat by kelp harvesting influences the invertebrate assemblage. I found that when habitat reduction occurs later in the summer when the growth of Macrocystis is minimal, the canopy and associated assemblage did not recover. In contrast, reduction of habitat earlier in the summer, when Macrocystis is still growing, enables the canopy and invertebrate assemblage to recover. Additionally, I determined that partial reduction of the canopy habitat maintains partial functioning of the canopy. By understanding the patterns, mechanisms and consequences of variability in a biogenic habitat on its associated community, we can better understand, predict and manage how natural and anthropogenic impacts to kelp forest canopies alter ecosystem structure, functions and the services they provide humans