History of invasion and anticipated harm as well as  climate models (to match basic climate requirements of a species in its native and known invasive ranges with similar climates in the United States to assess risk of establishment) have been used to assess ecological risk and produce screening summaries for high, low, and uncertain risk fishes, crustaceans, mollusks, plants, vertebrates, and invertebrates (USFWS). Although the screening summaries are intended primarily for considering species used in trade, they are helpful when evaluating potential effects of species upon introduction or establishment. For example, high-risk mollusks include New Zealand mudsnails as well as quagga and zebra mussels, all of which have been demonstrated to have significant ecological, economic, and social effects upon establishment in freshwater ecosystems in the United States.

Many states have conducted risk analyses to determine the top 25 or 50 worst invasive species based on ecological, economic, and social effects. State invasive species coordinators can provide updated listed and information to local municipalities. The goal of a risk assessment process is to produce a risk assessment on a specific organisms, or to evaluate organisms associated with specific pathways. Documenting uncertainty is key to describing potential risk. The graphic (adapted from the Commission for Environmental Cooperation, www.cec.org) illustrates a commonly used risk assessment framework to evaluate organisms and pathways.

  • Determine the origin of the organisms associated with the pathway.

  • Determine the number of organisms traveling within the pathway.

  • Determine intended use, or disposition, of pathway.

  • Determine mechanism and history of pathway.

  • Review history of past experiences and previous risk assessments (including foreign countries) on pathway or related pathways.

  • Review past and present mitigating actions related to the pathway.


EXAMPLE: Trailered watercraft are key pathways of introduction for dreissenids (quagga and zebra mussels). The bivalves were introduced to the Great Lakes in the 1980s via ship ballast water, and have since spread throughout most of North America, with the exception of the far southeastern United States and Pacific Northwest. Understanding the role of trailered watercraft in the spread of dreissenids has informed prevention efforts and minimized widespread introduction and establishment.


Creating a list of non-indigenous species of concern:

  • Determine what organisms are associated with the pathway. 

  • Determine which of these organisms qualify for further evaluation using the table below.

  • Produce a list of the organisms of concern including:

    • non-indigenous species not present in country

    • non-indigenous species present in the country and capable of further expansion

    • non-indigenous species present in the country that has reached the probable limits of its range, but is genetically different enough to warrant concern, or is capable of harboring another non-indigenous pest and/or introduce the risk of hybridization

    • indigenous species genetically different enough to warrant concern and/or able to harbor another non-indigenous pest, and/or capable of fruther expansion and/or introduce risk of hybridization.

Taxonomic confusion or uncertainty should also be noted on the list.

  • Conduct organism risk assessments from the list of organisms developed in the previous step.


1. Estimate probability of organism being on, with, or in the pathway.

2. Estimate probability of organism surviving in transit.

3. Estimate probability of organism colonizing and establishing a reproductively viable population (access to adequate food, abiotic and biotic environmental resistance factors, the number of individuals likely to be introduced via the pathway, and the ability to reproduce or hybridize). Estimate whether environmental factors are suitable for establishment.

4. Estimate probability of organism spreading beyond the colonized area (ability for natural dispersal, ability to use human activity for dispersal, ability to develop races or strains, estimated range of probable spread based on availability of suitable habitat conditions).


5. Estimate economic impact if established.

6. Estimate environmental impacts if established.

7. Estimate impacts to social and cultural practices.

The risk assessment can then be characterized using three levels:

  1. A risk estimate on each of the 7 elements listed above.

  2. A combination of the 7 risk element estimates into an Organism Risk Potential - which represents overall risk of organism.

  3. Linking the Organism Risk Potential into a Pathway Risk Potential, which represents the combined risk associated with the pathway.