Establishment, spread, and impact of the introduced Japanese seaweed, Sargassum muticum, in the San Juan Islands, Washington

I used an experimental approach to investigate the establishment, spread, and impact of the introduced alga, Sargassum muticum, in native kelp communities of the San Juan Islands, WA. My results showed that disturbance alters the relationship between propagule input and recruitment of Sargassum because recruitment is a space-limited process for this species. In habitats with low levels of natural disturbance, space availability will limit Sargassum recruitment. In contrast, in habitats characterized by high levels of natural disturbance, dispersal limitation will be the dominant process regulating Sargassum recruitment. Additional results demonstrated that the identity of native algal functional groups that are important for resisting invasion by Sargassum change as the invasion progresses. This occurred because different native functional groups pre-empted key resources during the recruitment and post-settlement phases of the invasion. Because overall resistance to S. muticum will be due to the additive effects of resistance at different invasion phases, the sequential preemption of key resources by native functional groups is one mechanism that could produce a negative relationship between functional group diversity and invasibility. Once established, Sargassum has substantial effects on native kelp communities in the San Juan Islands, including direct and indirect effects at multiple trophic levels. The most important of these effects are: (1) the displacement of native kelp species due to competition for light, and (2) a negative, indirect effect on the native urchin, Strongylocentrotus droebachiensis, which arose due to competition by Sargassum with the urchin's preferred food resources (i.e. native kelp species). Finally, my results indicated that structurally complex habitats provide a spatial refuge for native kelps from competition with Sargassum because Sargassum has difficulty recruiting to vertical rock surfaces. In summary, my dissertation research has demonstrated that the Sargassum muticum invasion is mediated by a complex array of biological interactions, and other invasions are likely to be similar in this respect. These types of biological interactions will be important to examine in other systems where invasions are occurring and should be incorporated into theoretical models of the invasion process.