Alien species (those moved outside of their native ranges by humans) have large ecological and economic effects around the world. Most studies of alien species are done soon after the species appears, when scientific and public interest is highest. However, the effects of an alien species may change over time, either increasing or decreasing, as the alien species and the invaded ecosystem accommodate to one another, or as the effects of the alien species interact with other controlling factors (e.g., climate) that change over time. Results from our long-term study of the effects of the zebra mussel (a European invader) on the Hudson River ecosystem suggest that the effects of this invasive species may be changing over the long term. Such evidence includes partial recovery of some native species, changes in the size and geographical distribution of the zebra mussel population, development of transitory "hot spots" where zebra mussel effects are diminished, and strong interactions between consumption of algae by zebra mussels and freshwater flow, another important control on the Hudson's ecosystem. We propose to continue our long-term study of the Hudson to test several specific hypotheses about how interactions between the zebra mussel and the Hudson's ecosystem might change over time. This information will help better predict the true long-term effects of zebra mussels on North America's ecosystems, better understand the controls on aquatic ecosystems, and better judge how alien species interact with the ecosystems they invade.
