Spread and impact of the introduced Japanese seaweed, Sargassum muticum, in native kelp forests of the San Juan Archipelago (yr. 3)

J. Timothy Wootton, The University of Chicago
$34,326

The introduction of species into habitats outside their normal geographic range is accellerating as human mobility increases. Introduced species have the potential to radically alter the structure and function of native ecosystems and are a leading threat to biodiversity. The Japanese alga Sargassum muticum has been invading kelp forests in the San Juan Islands of Washington since its introduction nearly fifty years ago. Subtidal kelp communities in the San Juan Islands are important habitat for a wide variety of animals including invertebrates, fish, mammals, and birds. Furthermore, the kelp species that dominate these plant assemblages add three-dimensional structure to the habitat and are important sources of carbon in coastal food webs. Therefore, any widespread effects of S. muticum on these algal communities are likely to have broader consequences for the nearshore ecosystem in this region. We are using an experimental approach to explore the dynamics and consequences of the S. muticum invasion in the San Juan Islands.

In 2003, our research will focus on three projects: 1) continued monitoring of removal experiments in order to compare long and short-term responses of the native community to S. muticum removal, 2) testing the hypothesis that spatially complex habitats provide a refuge for native kelp species from competition with S. muticum, 3) investigating the relationship between the number of invading propagules and recruitment success using S. muticum as a model system. Our emphasis is on field experiments in each of these projects, but we also employ field surveys, lab experiments, and other approaches where appropriate.

Our results will provide additional insight into the impacts of this ongoing invasion on the native plant and animal biota of the San Juan Islands. In particular, our work in 2003 may contribute valuable information about long versus short-term consequences of invasions. Additionally, our research into the role of habitat complexity in mediating competition between S. muticum and native kelp species should provide important information for resource managers about how to manage habitats and focus eradication efforts in ways that minimize the effects of invasive species. Finally, experimentally testing the relationship between the number of invading propagules and recruitment success will allow resource managers to make more informed decisions about how to manage potentially invasive species in order to minimize the risk of them becoming established.

Britton-Simmons, K. H. 2004. Direct and indirect effects of the introduced alga, Sargassum muticum (Yendo), in subtidal kelp communities of Washington state, USA. Marine Ecology Progress Series. (pdf)

Britton-Simmons, K. H. In Preparation. Understanding biotic resistance to non-indigenous species: Experimental evidence for the importance of functional group diversity. To be submitted to Ecology Letters.

Klinger, T., D. K. Padilla, and K. Britton-Simmons. 2006. Two invaders achieve higher densities in reserves. Aquatic Conservation: Marine and Freshwater Ecosystems: 16: 301-311. (pdf)

Britton-Simmons, K. H. July 2002. Understanding biotic resistance to invasive species: An experimental approach. American Society of Naturalists Annual Meeting, Banff, Canada.

Britton-Simmons, K. H. August 2002. Spread and Impact of the introduced Japanese seaweed, Sargassum muticum, in native kelp communities of the San Juan Islands. Marine Ecosystem Health Program Science Symposium, Orcas Island, WA.

Britton-Simmons, K. H. August 2002. How do native communities resist invasion by non-indigenous species? Friday Harbor Laboratories Summer Seminar Series, Friday Harbor, WA.

Britton-Simmons, K. H. August 2002. How do native communities resist invasion by non-indigenous species? Presented by Kevin Britton-Simmons. Guest Lecture to Marine Botany Class, Friday Harbor Laboratories, Friday Harbor, WA, August 2002

Britton-Simmons, K. H. April 2003. Establishment, spread, and impact of the introduced Japanese alga, Sargassum muticum, in native kelp communities of the San Juan Islands. Natural History Seminar Series, The University of Chicago, Chicago, IL

Britton-Simmons, K. H. May 2003. Establishment, spread, and impact of the introduced Japanese alga, Sargassum muticum, in native kelp communities of the San Juan Islands. Friday Harbor Laboratories, Friday Harbor, WA.

Spread and impact of the introduced Japanese seaweed in native kelp forests of the San Juan Archipelago

Introduced species have the potential to radically alter the structure and function of native ecosystems and are a leading threat to biodiversity in the Georgia Basin/Puget Sound region. While it is always better to prevent the introduction of exotic species, once a species is introduced it is critical to know the impact of the species on native plants and animals and if necessary, to try and slow the spread of or eradicate the invasive species. The Japanese alga Sargassum muticum was introduced and began invading kelp forests in the San Juan Islands of Washington nearly fifty years ago, however very little was known about how this introduced alga affects native species. In 2001, 2002, and 2003, the SeaDoc Society funded a research project to assess how introduced Sargassum muticum impacted native kelp forests and what could be done if the effect was negative.

This research showed that by competing for light, Sargassum muticum displaces native algae and has a negative, indirect effect on the native green urchin, (Strongylocentrotus droebachiensis). This is an important finding and a big concern because kelps are an important source of food and provide critical habitat for a wide variety of species in nearshore ecosystems. Similarly green urchins have an influential role in structuring shallow subtidal communities in this region.

Small scale experiments showed after removing S. muticum, the native community recovered within about a year, suggesting that Sargassum-induced changes are reversible. Our experience with this seaweed indicates that complete eradication is labor intensive and difficult. But small-scale removals are possible and can be successful, particularly if focused on specific, limited target areas such as marine reserves. They need to be repeated every few years in order to stop incipient recolonization of the area.

Another focus of this research has been to study how native communities resist invasion by this species. Results suggest that native communities with all functional components intact (i.e. with high functional group diversity) will be more resistant to invasion. Because S. muticum recruitment is a space-limited, disturbance-dependent process, human activities that increase levels of disturbance in nearshore kelp communities are likely to facilitate the spread of S. muticum. What this all means is that human disturbance of nearshore algal communities should be minimized to help prevent further S. muticum spread and consequent damage to kelp and green urchins.