This definition is often referred to in legal documents; however, in the scientific literature many terms are used, each of which has a slightly different connotation and meaning, depending on the context. Some of these terms include: introduced, foreign, exotic, alien, non-native, nonindigenous, immigrant, transplant, and naturalized. Whatever the term, they represent the addition of a new species to a natural system from which it has been historically absent.

Nonindigenous species may threaten the diversity or abundance of native species, alter the natural functioning of ecosystems, disrupt species interactions, and negatively impact commercial and recreational activities that rely on native marine resources. In addition to their ecological and economic impacts, some species represent significant human health hazards and diminish the social and aesthetic value of our natural resources.

In many cases, these species were moved intentionally. For example, some plants have been used by public works agencies to stabilize soils and enhance public areas. State agencies have also added numerous fish species to lakes and streams throughout the United States to bolster commercial and recreational fishing interests. And while the benefits and potential impacts of these intentional introductions are often thoroughly discussed and reviewed by scientists, there are several examples in which unforeseen outcomes make such decisions seem uninformed in hindsight.

More recently researchers have investigated the causes and consequences of accidental introductions, especially in marine ecosystems. For example, the role of ballast water exchange became a hot research topic in the early 1990s.

Once a large ship has unloaded its cargo, the ballast tanks are filled with water to provide balance and trim for safe transit to the next port of call. Along with the ballast water, thousands of organisms are pumped into the tank, where they reside until the ballast water is offloaded. In many cases these organisms survive the journey, and as new cargo is loaded, these organisms are dumped into a new environment, which could be in another part of the state or in another country.

San Francisco Bay, considered the most invaded aquatic ecosystem in North America, has at least 212 introduced species, consisting mostly of invertebrates such as clams and crabs. An additional 123 species are cryptogenic (not clearly native or introduced), which could drive the number of introduced species even higher.

Over the past 150 years, an average of one new species became established in the bay every thirty-six weeks. Since 1970 the rate has increased to one new species every twenty-four weeks, and this shows no sign of decreasing.

A recent study prepared by the California Department of Fish and Game and submitted to the California State Legislature (December 2002) found 360 introduced species in state waters. The study focused on seven major harbors and ports (e.g., San Francisco Bay; Los Angeles and Long Beach Harbors), but also included minor ports, bays, and estuaries.

In Monterey Harbor, 72 organisms were collected from the fouling community in the study. Of these, 12 were nonindigenous, 25 native, 9 cryptogenic, and 26 could not be identified to the species level.

Below are profiles on two of the invasive species found in the Monterey Bay National Marine Sanctuary.