Introducing exotic species is risky. Freed from their natural predators, pathogens, parasites, and competitors, exotic species have the ability to, threaten the diversity and abundance of native species, and disrupt human social and economic activities. Understanding the effect of introduced species on ecosystems is important since the global mixing of organisms has contributed to a worldwide decline in diversity, in both aquatic and terrestrial ecosystems. In the Great Lakes about 10% of introduced species have had serious impacts. Other exotics appear to have had minor repercussions, but the impact of most species is unstudied.