9610353 Levin Human introductions of species outside their natural range are rapidly homogenizing the earth's biota, and are a major threat to biodiversity in terrestrial, aquatic and marine habitats. Although the potential impact of biological invasions on biodiversity has been recognized for decades, until recently, most research has focused on terrestrial or freshwater rather than marine habitats. Anthropogenic introductions of marine species are numerous and have had a dramatic impact in some locals. While much has been learned about introduced species over the last 15 years, generalizations relative to marine systems are elusive. This study will address the effects of introduced species on kelp bed communities in the northwest Atlantic Ocean. Specifically, this research will examine the interactive effects of herbivory and competition on the introduced alga Codium fragile and the native kelp community of the northwest Atlantic, the indirect effects of a bryozoan on kelps in these communities, and the impact of increasing Codium and decreasing kelp biomass on the demography of fishes and decapods that use seaweed beds as nursery habitats. The kelp bed communities of New England shift between two alternate states: a species-rich community dominated by laminarian kelps, and a species-poor community dominated by sea urchins and crustose coralline algae. Recently, a new community consisting of a monoculture of the introduced green alga, Codium fragile tomentosoides, is now prevalent in the region. In addition the introduction of a bryozoan also appears to have severely impacted kelp bed communities. Using a series of multi-factor experiments and descriptive studies, this research will elucidate the causes of this shift in community structure, and therefore explain the mechanisms underlying the dramatic, human-induced loss of biodiversity in these kelp communities.
