The facilitative role of an introduced bryozoan (Watersipora spp.): structuring fouling community assemblages within Humboldt Bay

Studying the impacts on a community following the introduction of an exotic species is of increasing importance in the field of marine science. Recent investigations of marine biological invasion have primarily focused on studying the negative interactions resulting from species introductions. However, there is growing interest and need to include the influence of positive interactions when investigating the organization of recently invaded communities. Facilitation often occurs when a sessile species modifies the physical structure of a community by creating habitat that benefits others. Following recent introductions along the California coastline, the cheilostome bryozoan, Watersipora spp., has been shown to impact marine communities both through positive and negative interactions. Initially Watersipora may dominate a community by out-competing other sessile organisms for occupancy of primary substrate. However, as colonies collide and begin growing upward, they develop into a three dimensional structure that appears to serve as a new habitat utilized by other species for settlement and refuge. In order to investigate the role of Watersipora spp. within the fouling community of Humboldt Bay, CA, experimental panels were deployed at Eureka Public Marina. Of the 20 panels, ten were initially seeded with Watersipora recruits, while the other ten were left blank (as controls). By monitoring experimental fouling panels for recruit survival, changes in percent cover, and final species diversity, it was determined that the presence of Watersipora spp. greatly alters the Humboldt Bay fouling community following its introduction. Initially, during the first two months of deployment, Watersipora's presence decreased the survivorship of new recruits that settled nearby. During the following months, rapid colony growth and competition by Watersipora reduced the percent cover of dominant sessile invertebrates (especially tunicates), by decreasing the establishment and successful growth of neighboring individuals and colonies. However, while Watersipora has the ability to outcompete sessile invertebrates for primary substrate, it also provides a complex three dimensional structure that can be utilized as habitat. After eight months of community development, experimental panels were destructively sampled to account for differences in final species diversity between treatments. Sampling revealed that the presence of Watersipora ultimately lead to the establishment of a more species diverse community. This increase in diversity was primarily due to the complex habitat modification provided by Watersipora that was not present on control panels.