Note that the data published in the 2002 State of the Nation’s Ecosystems Report as well as the 2003 and 2005 Web-Only Updates have been superseded by the 2008 Report and thus should be used with caution. For the most recent data, purchase the 2008 Report from Island Press.

Further refinement is required to produce an indicator that combines both number of species and the area they inhabit. However, even with such an indicator, assessment of the national situation would be impossible without a program of data collection and assessment. Monitoring activities will need to be conducted regularly in estuaries around the country. These activities must be supported by historical research to determine which species are native and which have been introduced.

Two definitions are appropriate for this indicator. “Invasive species” are defined in Executive Order 13112, “Invasive Species” (Feb. 3, 1999), as “alien species whose introduction does or is likely to cause economic or environmental harm or harm to human health” where “alien species” are, “with respect to a particular ecosystem, any species, including its seeds, eggs, spores or other biological material capable of propagating that species, that is not native to that ecosystem.”

According to an Office of Technology Assessment study (U.S. Congress 1993), just 79 non-native species had cost the American public some $97 billion in damages to natural resources and lost industrial productivity during the 20th century. Pimentel et al. (2000) recently published a more comprehensive estimate; they found that more than $120 billion is spent every year in the United States to deal with the effects of some 50,000 non-native species. Although these figures are not limited to coastal non-natives, they illustrate the scope of the issue.

The discharge of ballast water by oceangoing vessels is a major source of such introductions in coastal waters. With the high speed of modern vessels, organisms taken in with ballast water at one location have little difficulty surviving the trip to a distant destination, where the ballast water and its associated organisms are discharged. Other mechanisms, such as the escape of fish from aquaculture facilities or the intentional introduction of non-native species of shellfish to supplement dwindling native populations, can also contribute to the introduction and spread of non-native species.

The number of successful new invasions appears to have increased dramatically during the 1970s and 1980s, perhaps as a consequence of nutrient enrichment and over-fishing in coastal ecosystems. The list of recent invaders includes several species of benthic algae, submerged aquatic vegetation, toxic dinoflagellates (e.g., Alexandrium catenella in Australia) , bivalves (e.g., the zebra mussel in the Great Lakes and the Chinese clam in San Francisco Bay), polychaetes, ctenophores, copepods, crabs, zooplankton, and fish. Such invasions can profoundly alter the population and trophic dynamics of coastal ecosystems. For example, the introduction of the ctenophore Mnemiopsis leidyi caused the collapse of the anchovy fishery in the Black Sea by preying on the anchovy’s preferred food, copepods; the introduction of the macrobenthic green algae, Caulerpa taxifolia, displaced a diverse community of sponges, gorgonians, and other seaweeds on more than 10,000 acres of sea floor in the northern Mediterranean.

The Indicator

The indicator will report the degree of influence (low to high) of non-native species in major U.S. estuaries. The proposed approach does not evaluate the significance of non-native species (also called non-indigenous, exotic, introduced, or invasive species) to economic or ecological condition, but rather focuses on the degree to which non-natives occupy the system. It would report an index created by combining the percentage of all species in a region that are non-native with the percentage of habitat they occupy (see Table 1). For purposes of this indicator, non-native species fall into at least five categories: fishes, mollusks, crustaceans, higher aquatic plants, and macroalgae. Non-estuarine areas can also have a significant non-native presence; however, the assumption of this indicator is that estuaries are generally more negatively affected. Table 1 is a proposed framework for establishing the degree of significance of non-native species in a region. Both the number of species and the area they inhabit (or their biomass) are factors, so this measure proposes a combined ranking approach, in which both factors contribute to an overall score. The values presented in the table are arbitrary and are intended only to illustrate the utility of such a ranking system. The basis for judging significance will vary somewhat among the different categories of organism––for higher aquatic plants, mollusks, and macroalgae, the percentage of the total potential area inhabited would be measured, while for fish and crabs, the percentage of biomass accounted for by non-native species would be measured. The non-native rankings for selected major estuaries should be calculated periodically.

The Data Gap

The number and distribution of native species—not to mention non-natives—are not well documented in most coastal ecosystems, especially when considering species of bacteria, microalgae, and protozoa.

Species lists for the five categories of organism will need to be developed and maintained for each major U.S. estuary. The lists will be based on existing knowledge of the species in the target groupings in each estuary and on ongoing surveys of biological resources that are conducted in these regions for a variety of purposes. These surveys will also provide the data required to assess the ecological significance of the non-native species found in an estuary.

Species lists, data on ecological significance, and some monitoring data on various species are available from surveys and compilations produced by a variety of sources, including state living resource and environmental protection agencies, environmental impact statements, and academic research projects. However, such data are not available in any consistent fashion for many of the major estuaries, and there is no nationwide compilation of data.

The National Aquatic Nuisance Species Task Force (http://www.anstaskforce.gov/), an interagency group, is working to bring together information and data that will undoubtedly be useful for this indicator in the future. Also, the Smithsonian’s Environmental Research Center is actively creating databases on invasive species (see http://invasions.si.edu/).

References

Pimentel, D., L. Lach, R. Zuniga, and D. Morrison. 2000. Environmental and economic costs of nonindigenous species in the United States. BioScience 50:53–65.

U.S. Congress, Office of Technology Assessment. September 1993. Harmful non-indigenous species in the United States, OTA-F- 565. Washington, DC: U.S. Government Printing Office.