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Description
The forest-forming giant kelp, Macrocystis pyrifera, and the communities it supports have been decreasing across their native ranges in many parts of the world. The sudden removal of giant kelp canopies by storms enhances space and light for the colonization ofopportunistic understory macroalgae, such as Desmarestia herbacea, which can inhibit M. pyrifera recovery and alter local community composition. Understanding the mechanisms by which D. herbacea interacts with M. pyrifera can therefore provide insight into patterns of kelp forest recovery following these disturbances, and can aid in predicting future community structure. This study experimentally tested the independent and combined effects of two likely competitive mechanisms, namely shade and scour, by which D. herbacea might inhibit recovery of M. pyrifera in the Point Loma kelp forest in San Diego, CA. We conducted field experiments on SCUBA to study the individual and combined effects of shade and scour by D. herbacea on the survival of M. pyrifera microscopic life stages, and the recruitment, survival and growth of its young sporophytes. Our results show that scour (i.e. physical abrasion by D. herbacea thalli) had the strongest effect on the survival of M. pyrifera microscopic life stages and recruitment of its young sporophytes, but shade and scour were both important for survival and growth of these sporophytes as they grew larger. Canopy-removing storms are increasing in frequency and intensity, and this change could facilitate the rise of opportunistic species, like D. herbacea, which might alter community succession and recovery of kelp forests. Understanding the mechanisms by which these opportunistic species suppress kelp recovery can therefore enhance conservation efforts.
