Differences in herbivore feeding preferences across a vertical rocky intertidal gradient

Primary producers such as plants and macroalgae can vary in palatability to their herbivorous grazers; this leads to variation in the intensity of herbivory, which can play an important role in setting the composition and diversity of producer assemblages. However, despite strong gradients in the composition and abundance of herbivores across intertidal gradients, little is known about how macroalgal palatability and associated herbivore defenses vary across strong vertical gradients in marine rocky intertidal zones. Plant defense theory predicts that decreasing intensity of herbivory with increasing tidal elevation should result in higher intertidal species being more palatable and less defended than their lower intertidal counterparts. In this study, we examined the relative palatability of 9 pairs of closely related macroalgal species that occupy different elevations across this vertical gradient, to 3 of the most common local herbivores. We conducted controlled paired-choice feeding assays for every herbivore-algal species combination. Although we found no evidence for a vertical gradient in palatability consistent across all 3 herbivores, there were many significant, species-specific herbivore preferences driven by morphological and/or chemical properties of the macroalgae. In general, herbivores consumed more of the algae that they did not co-occur with: the lower intertidal herbivores Strongylocentrotus purpuratus and Tegula brunnea preferred higher intertidal algae overall, and the higher intertidal herbivore Tegula funebralis preferred lower intertidal algae. Our results suggest that the intensity of herbivory may change with tidal elevation in more complex ways than previously suspected, and that studies of macroalgal palatability and antiherbivore defenses in these systems will need to examine the relative impacts of a range of herbivores on algal community structure.