Although many studies have investigated how community characteristics such as diversity and disturbance relate to invasibility, the mechanisms underlying biotic resistance to introduced species are not well understood. I manipulated the functional group composition of native algal communities and invaded them with the introduced, Japanese seaweed Sargassum muticum to understand how individual functional groups contributed to overall invasion resistance. The results suggested that space preemption by crustose and turfy algae inhibited S. muticum recruitment and that light preemption by canopy and understory algae reduced S. muticum survivorship. However, other mechanisms I did not investigate could have contributed to these two results. In this marine community the sequential preemption of key resources by different functional groups in different stages of the invasion generated resistance to invasion by S. muticum. Rather than acting collectively on a single resource the functional groups in this system were important for preempting either space or light, but not both resources. My experiment has important implications for diversity–invasibility studies, which typically look for an effect of diversity on individual resources. Overall invasion resistance will be due to the additive effects of individual functional groups (or species) summed over an invader’s life cycle. Therefore, the cumulative effect of multiple functional groups (or species) acting on multiple resources is an alternative mechanism that could generate negative relationships between diversity and invasibility in a variety of biological systems.
Introduced algae have become a prominent component of the marine flora in many regions worldwide. In the NE Pacific, the introduced Japanese alga Sargassum muticum (Yendo) Fensholt is common and abundant in shallow, subtidal, rocky habitats, but its effects on subtidal, benthic communities in this region have not previously been studied. I measured the response of native species to experimental manipulation of S. muticum in field experiments in the San Juan Islands of Washington State. Native canopy (brown) and understory (red) algae were more abundant in plots from which S. muticum had been removed, and the native kelp Laminaria bongardiana (the most abundant species of brown alga in the absence of S. muticum) grew more than twice as fast in plots where S. muticum was absent. The negative effects of S. muticum on native algae appear to be a result of shading, rather than changes in water flow, sedimentation, or nutrient availability. S. muticum also had a strongly negative indirect effect on the native sea urchin Stronglyocentrotus droebachiensis by reducing abundances of the native kelp species on which it prefers to feed. My results indicate that S. muticum has a substantial impact on native communities in this region, including effects at multiple trophic levels. Because of their worldwide distribution and capacity to alter native communities, non-indigenous algae are potentially important agents of global ecological change.