An impact is defined as an effect that a species has on an ecosystem, flora/fauna, human health, and/or economy in the area where it is introduced. Impacts are not based on value judgements, nor are they inclusive of normative language, and encompass all effects whether perceived as positive or negative. In the NAS Database, impacts are recorded globally where a species is introduced outside of its historic range, and does not include the effects a species has in its native range. The cost of mitigating the impacts of introduced species is a management decision and not considered a direct impact from that species.

For each impact, the following information is recorded (defined below):

  • Impact Type (what was the effect of the impact?)
  • Study Type (how was the impact documented?)
  • Study Location (where did the documented impact occur?)

Impact Type Definitions

Predation/Herbivory – the species consumes or is consumed by another species

Competition – the species shares a niche (ecological role and required environmental conditions) with another species where it is introduced, and they cause a measurable decrease in population size, growth, survival, or fecundity of the other species (such as reduced biomass, egg mass, or population size)

Habitat Alteration – the species modifies the physical, biotic, or abiotic environment to which it was introduced (for example, zebra mussels colonize soft, muddy sediments and change the substrate of a waterbody). This excludes water quality changes (see "Water Quality")

Water Quality – the species creates measurable changes in water chemistry/quality/parameters, such as pH, dissolved oxygen, or turbidity, as compared to pre-introduction of the species

Food Web – the species causes changes either directly or indirectly, at multiple trophic levels within an ecosystem (for example, the filter feeding of zebra mussels reduces plankton biomass and increases abundance of benthic invertebrates)

Genetic – the species hybridizes with another species, irrespective of the viability of offspring

Disease/Parasite/Toxicity – the species has brought a novel or rare pathogen or parasite to the introduced area and facilitated infection of non-human organism(s); this includes moving a native parasite/pathogen outside of its historic range, increasing the rate of disease or parasitism that was already present, or introducing an exotic parasite/pathogen. Alternatively, the species is toxic and impacts native species through envenomation and/or poisoning (such as the Cane Toad being poisonous when consumed by other animals, or the envenomation of native fish by Lionfish)

Infrastructure – the species has impaired the normal operation of human-made structures or facilities, such as zebra mussels clogging water intake pipes of factories or power plants

Recreation – the species has changed the economic utility of the waterbody by affecting or changing the entertainment activities in or near a waterbody that took place prior to introduction, such as recreational boating, swimming, or fishing

Aquaculture/Agriculture – the species affects, either directly or indirectly, the cultivation of crops, livestock, or aquatic organisms

Property Value – the species causes a change in value of any type of property (such as homes, apartments, office buildings, land, or water), whether the change was physical (such as reduced structural stability) or via economic utility (for example, the neighborhood or area is less enjoyable, such as from noise or odor)

Human Health – the species causes health impacts, directly or indirectly, to human well-being whether considered positive or negative

Harvest – the species is taken and used commercially as a result of its introduction, whether its introduction was purposeful or accidental (this includes commercial fishing, artisanal/subsistence fishing, and collection for resale, but excludes recreational fishing, even if the angler keeps the catch; see Recreation)

Commerce – the species causes an increase or decrease in profit, employment, production, trade, or any other business metric

Navigation – the species causes boat travel to be impossible or impaired, either through impacting boating equipment directly or through prevention of passage or travel

Other – any other impact that does not fit into another category

Study Type Definitions

Experimental – a study or reference with a claim that was supported by direct manipulation of variables

Observational – a study or reference with a claim that was found through examination, i.e., the claim in the study or report was not a result of manipulating variables

Anecdotal – a study or reference with a claim that is supported by theory, inference, or correlation, but not tested or observed

Study Location Definitions

Field – The study or impact occurred in the field (a natural setting)

Laboratory – The study or impact occurred in the laboratory (indoors or a manipulated setting)

N/A – The study or impact was not in a lab or field setting


The data represented on this site vary in accuracy, scale, completeness, extent of coverage and origin. It is the user's responsibility to use these data consistent with their intended purpose and within stated limitations. We highly recommend reviewing metadata files prior to interpreting these data.

Citation information: U.S. Geological Survey. [2021]. Nonindigenous Aquatic Species Database. Gainesville, Florida. Accessed [9/23/2021].

Contact us if you are using data from this site for a publication to make sure the data are being used appropriately and for potential co-authorship if warranted. For queries involving fish, please contact Matthew Neilson. For queries involving invertebrates, contact Amy Benson.