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Invasions of fouling organisms that colonize artificial structures are modifying coastal environments. Our goal was to assess harbor conditions including dissolved copper pollution, position in relation to open coast, temperature, and artificial structure density (substrate for fouling) as determinants of macroinvertebrate recruitment. Settlement panels were deployed over a gradient of human coastal modification (28 sites in California and 2 in southern Australia). Non-indigenous macroinvertebrates dominated panel cover (overall by six-fold compared to indigenous taxa). Marinas and sites of heavy shipping showed high macroinvertebrate diversity, contrasting with open-coast sites of lower human impact [Santa Catalina Island (SCI) and one mainland coastal site] where there was low macroinvertebrate fouling. At two SCI sites with low vessel traffic, invertebrates that were rare on exposed panels were more common in a protected space between plastic strips indicating larvae amount does not completely explain low invertebrate diversity, suggesting a contributing role of predators or larval recruitment pattern. While dissolved copper levels correlated strongly with artificial structure measured at the water surface, the pollution gradient was not supported as a driver of macroinvertebrate diversity in semi-partial analysis. Density of artificial structure was supported as a better predictor of macroinvertebrate diversity, including separately analyzed non-indigenous and indigenous groups, than dissolved copper, distance from open coast or temperature variation in semi-partial correlation analysis. An artificial structure density measure may therefore increase power in predicting abundance of fouling organisms and could be useful in moderating the influence of non-indigenous species.