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published in the 2002 State of the Nation’s Ecosystems Report as well as the 2003 and 2005 Web-Only Updates have been superseded by the 2008 Report and thus should be used with caution.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.

The Indicator

This indicator reports the percentage of all hydrologic units (simplified here to represent watersheds; see below) having one of several ranges of established non-native species. Introduced species are those that are not native to the watershed in which they are found. These species may be from outside North America, or they may be from another part of this continent. Established species are those that have established persistent breeding colonies. In general, watersheds with higher numbers of non-native species are subject to higher levels of ecological and economic disruption.

Some non-native species become established at low population levels; other species are “invasive”—that is, they spread aggressively, creating ecological and economic disruption. Ideally, this indicator would track only invasive species, perhaps by reporting on a selected group of problematic or potentially problematic species, as identified by recognized experts. However, it is not now possible to identify potentially problematic species, and thus we have chosen to report on all non-native species. But changes can signal the emergence of an invasive species. Some become invasive quickly; others do so only after long lag times.

It is important to note that hydrologic units, which are represented by hydrologic unit codes (HUCs), can be loosely thought of as watersheds. However, only at the finest resolution is this accurate. Thus, the HUCs shown in the figure may include multiple watersheds in whole or in part, or they may actually represent a single watershed.

The Data

Data Source: Nonindigenous Aquatic Species Database, Biological Resources Division (BRD), U.S. Geological Survey (USGS). Roughly 90% of the data are derived from the published literature. Data are collected for the most part by federal and state biologists, although the public does contribute by reporting sightings.

Data Manipulation: Data for introduced species are maintained in a database whose units are 6-digit HUCs (there are 352 6-digit HUCs across the 50 states). The only necessary manipulation was to compute the indices as described above.

Data Quality/Caveats: Although the BRD database (Web site listed below) is widely known throughout the professional community, in some cases new discoveries are not reported by state and federal biologists.

Data Access: While these types of data are available on BRD’s Nonindigenous Aquatic Species (NAS) Web site (http://nas.er.usgs.gov/), the actual data presented here were prepared for this report by USGS.

The Data Gap

NAS includes information on a host of vertebrates, invertebrates, algae, and plants. At this time, however, the database managers do not feel that these data have matured adequately to be presented at the national level.