Changes in Community Structure and Ecosystem Function Following Spartina alterniflora Invasion of Pacific Estuaries

One of the most pervasive human impacts to salt marshes around the world is the introduction of nonnative species. Plant introductions to salt marsh systems have resulted in significant changes ranging from species replacement to broad-scale alteration of ecosystem properties. In this chapter, we examine the changes produced by the invasive Atlantic smooth cordgrass Spartina alterniflora in two wetland ecosystems, one in San Francisco Bay, California, and one in Willapa Bay, Washington. We compare and contrast the impacts of Spartina invasion on a range of processes, focusing primarily on benthic invertebrate communities and the sediment environment. Our work shows that the structure of the plant itself and the changes it produces in the physical and chemical environment strongly influence benthic communities. The aboveground structure shades the substrate, reducing photosynthesis of benthic microalgae and restricts water flow, which contributes to decreased recruitment and slower growth of benthic suspension feeders. At the same time, the plant structure increases sedimentation of fine-grained particles and leads to the accumulation of detritus and low-quality organic matter. These changes, together with increased belowground plant biomass and peat production, influence sediment chemistry and metabolism. In addition, belowground plant biomass can preempt substantial amounts of belowground habitat, directly reducing benthic abundance and diversity. These physical and chemical changes can result in dramatic shifts in benthic communities including substantial reductions in larger, surfacefeeding taxa concurrent with increases in smaller, subsurface detritivores. Such shifts in the functional identity of benthic species may negatively affect feeding of both invertebrate and vertebrate predators. The magnitude and direction of these various changes depend on the habitat invaded and are generally more substantial where Spartina has invaded open mudflats and less where Spartina invades areas of native vegetation. We discuss the consequences of these changes in the context of plant effects in other coastal systems in order to develop a set of predictions regarding future plant invasions. The ultimate consequences of plant invasion on benthic communities involve a dynamic balance of positive and negative effects. 2