A cross-genus comparison of grazing pressure by two native marine herbivores on native, non-native naturalized, and non-native invasive Sargassum macroalgae

Abstract

In marine systems, algal abundance and community composition is often heavily influenced by top-down control by herbivores. As a result, examining the extent to which native herbivores exert grazing pressure on non-native marine algae can provide valuable insight into mechanisms controlling invasion success. The purpose of this study was to examine the grazing preferences of two common intertidal and subtidal herbivores on three congeneric species of marine algae with unique colonization histories in San Diego, California, USA, to determine if grazing pressure, or lack thereof, may help explain invasion success. We provide evidence that neither native Sargassum agardhianum, nor non-native Sargassum horneri, are particularly palatable to purple urchins or black turban snails, but that non-native Sargassum muticum is consumed by both native herbivores. We also provide evidence that when given a choice of all three species neither herbivore exhibits a significant grazing preference for any algal species. We suggest that other mechanisms may determine the invasion success of the two non-native algal species and the overall distribution and abundance patterns of these species, and we discuss potential directions for future work.