Three factors affecting the invasion of Watersipora subtorquata in the marine fouling community of Humboldt Bay, California

There is concern and debate as to the effects and factors influencing species introduction and invasion. Experimental findings suggest that factors such as species richness and available free space may play crucial roles in determining the outcome of species introductions. Watersipora subtorquata was recently introduced to the fouling community of Humboldt Bay, CA, and is found in varying abundances at two marinas within the Bay. To investigate potential factors contributing to the varying success of Watersipora within Humboldt Bay, invertebrate fouling communities were established on ABS panels and groomed to control for species richness, species composition and free space. Measured response variables were: recruitment of new Watersipora colonies, total number of colonies, and colonies size. Panel communities were sampled at two-week intervals over an eight-week period during September and October of 2002. Free space, which is often limited in natural communities of higher richness, played a primary role in the success of Watersipora. This finding suggests that invasion will be less severe in communities where higher species richness limits available free space. However, Watersipora success was also facilitated by an interaction affect of free space and the presence of Balanus crenatus. The presence of Balanus also resulted in increased Watersipora success within higher richness communities, suggesting high species richness may also increase the probability that an interacting species will be present in the resident fouling community. This study found complex relationships among species richness, species composition, and free space in the invasion process. However, although these factors may affect the extent of the invasion, none precluded the possibility of invasion. It appears that the outcome of invasion will be affected by several factors of both the invaded community and the non-native species, and that factors such as species richness and composition may play several roles in the process of invasion.