Kelp Morphology and Herbivory Are Maintained Across Latitude Despite Geographic Shift in Kelp-Wounding Herbivores

AbstractHerbivores can drastically alter the morphology of macroalgae by directly consuming tissue and by inflicting structural wounds. Wounds can result in large amounts of tissue breaking away from macroalgae, amplifying the damage initially caused by herbivores. Herbivores that commonly wound macroalgae often occur over only a portion of a macroalga's lifespan or geographic range. However, we know little about the influence of these periodic or regional occurrences of herbivores on the large-scale seasonal and geographical patterns of macroalgal morphology. We used the intertidal kelp Egregia menziesii to investigate how the kelp's morphology and the prevalence of two prominent kelp-wounding herbivores (limpets and amphipods) changed over two seasons (spring and summer) and over the northern extent of the kelp's geographic range (six sites from central California to northern Washington). Wounds from limpets and amphipods often result in the kelp's fronds being pruned (intercalary meristem broken away), so we quantified kelp size (combined length of all fronds) and pruning (proportion of broken fronds). We found similar results in each season: herbivores were most likely to occur on large, pruned kelp regardless of site; and limpets were the dominant herbivore at southern sites, while amphipods were dominant at northern sites. Despite the geographic shift in the dominant herbivore, kelp had similar levels of total herbivore prevalence (limpets and/or amphipods) and similar morphologies across sites. Our results suggest that large-scale geographic similarities in macroalgal wounding, despite regional variation in the herbivore community, can maintain similar macroalgal morphologies over large geographic areas.