Trophic effects of an invasive alga in a temperate reef system

Invasive species are one of the leading causes of habitat change and are increasing in occurrence due to human-mediated factors like travel and trade. Invasives are capable of changing community composition, reducing biodiversity, and affecting food webs. Sargassum horneri is an invasive brown alga on rocky reefs in the Southern California Bight, which is native to the western Pacific. In 2003 it was discovered in California and by 2006 it had colonized the leeward side of Santa Catalina Island. Since then, studies have been conducted on its distribution around Catalina Island and the Southern California Bight, life history, effects on nearshore fish communities, and the resilience of some MPAs to invasion by it. However, the various effects S. horneri has on the trophic interactions in its invaded range have yet to be explored. This study aimed to test (1) whether three common invertivorous fishes foraged less on invertebrates in S. horneri than in native algae, (2) whether any differences in fish foraging rate among algae might be related to prey availability, which (3) might be related to differences in the chemical defenses; and finally (4) test whether the rate of growth of one common fish species was related to the abundance of the invasive alga. I studied foraging behavior of the fishes rock wrasse (Halichoeres semicinctus), garibaldi (Hypsypops rubicundus), and California sheephead (Semicossyphus pulcher) at six sites along the leeward, west end of Catalina Island, to determine if their feeding rate differed among the invasive and native algae. I recorded the algal composition where the foraging observations took place to quantify the resources available. I collected three of the most abundant native brown algae, Sargassum palmeri, Dictyopteris undulata, Zonaria farlowii, and the invasive S. horneri, to compare the epifaunal invertebrate assemblages associated with them to determine if the invasive has a lower abundance and diversity (Shannon-Weiner index). I measured the concentration of phenolic compounds, which brown algae produce to deter herbivores and epiphytes, from the algal samples to determine if the invasive alga is more chemically defended than the native algae. Finally, I collected rock wrasse to assess whether their growth was related to the abundance of the invasive alga and epifauna at the six sites where they were collected. All fishes foraged less among the invasive compared to native algae despite S. horneri being as, or more abundant than the native species. This result was consistent with the finding that the invasive alga harbored a lower abundance of epifaunal invertebrates than the three native algae. In addition to harboring lower densities of invertebrates, S. horneri also tended to harbor lower diversity of invertebrate species, though this was not true at all six study sites. This lower density and diversity of invertebrate assemblage associated with the invasive S. horneri, however, was not related to its chemical defenses: the total phenolic content (TPC) measured from S. horneri was not higher than the three native algae, suggesting this invasive species uses an alternative defense against grazing and epiphytism. Algal composition varied spatially among the six study sites and temporally, but these spatiotemporal differences were not driven by differences in S. horneri cover in the two years sampled. Rock wrasse otolith growth differed among sites, years, and ages, but the spatiotemporal variation in growth was not related to spatiotemporal variation in the density of S. horneri. Nor was growth rate related to epifaunal abundance at each site. This study provides a better understanding of the trophic and community effects of the invasive alga S. horneri on a nearshore temperate reef system. It revealed that despite the low chemical defenses and relatively similar densities of Sargassum horneri at six reefs at Santa Catalina Island, epifaunal invertebrates were much less dense and diverse on it than on surrounding native algae, thereby limiting prey availability to invertivorous fishes, and apparently altering their foraging behavior. Understanding the effects of invasive species across multiple trophic levels may help identify shifts in ecosystem functions caused by invaders.