- The biodiversity of most marine communities is more or less dependent on continuous invasions from sources with greater richness. These ongoing, natural invasions have become greatly augmented by ship traffic in numerous estuaries and harbors where the native biota has been diminished or lost due to habitat destruction and pollution. Some of the invaders proved to be pests, indicating that these kinds of human introductions need to be controlled, but the ultimate management problem is that the invaders have generally increased diversity at the lower trophic levels after the top-level predators were lost or diminished. During the past 20 yr, work on habitat improvement has been progressing within many estuaries. But so far there is little evidence of the final step in restoration, which should be the revival of a balanced, more productive ecosystem. The inflow of invasions can be lessened by the prevention of ballast water release, pollution control, and habitat improvement. These changes should be followed by steps to reintroduce apex-level predators in order to restore natural ecosystems.