Download Risk Assessment: A cornerstone of early intervention strategies for

Risk Assessment: A cornerstone of early intervention strategies for invasive species management
Doria R. Gordon, The Nature Conservancy
Robert C. Venette, USDA Forest Service
Early intervention is widely recognized as the most cost-effective strategy to manage biological
invasions. Early intervention typically involves prediction and prevention prior to introduction or early
detection and rapid response to manage establishment of a likely invader before widespread harm can
occur. Responses include actions that will eliminate or confine the species. Because prevention efforts
have the potential to conflict with other social values (e.g., limits to freedom of personal movement or
trade), a reliable, scientifically-credible forecast about the likelihood that a non-native species is likely to
cause harm is needed to determine whether the benefits of a preventative measure outweigh its costs.
Risk assessment (RA) is broadly defined as a process for determining the probability that a specified
negative event will occur and the magnitude of that effect. For invasive species, the FAO (2010) defines
pest risk analysis as “the process of evaluating biological or other scientific and economic evidence to
determine whether an organism is a pest, whether it should be regulated, and the strength of any
phytosanitary measures to be taken against it.”
Ideally, RA is conducted within a preventative approach for screening species and their hosts prior to
introduction to a new country or region (“pre-border”). As a history of harmful invasion elsewhere is the
most consistently accurate predictor of invasion in a new region, cost-effective risk management could
start with this single question (see, for example, the USDA APHIS PPQ Not Authorized Pending Pest Risk
Analysis list). However, as movement of new species is increasing with global trade, we have no history
of introduction for many species to use as a guide. Additionally, as assessment of all new species or
potential host-pest combinations is required by few countries, RA is also conducted post-border to
prioritize management efforts.
Invasion RA methodology fundamentally addresses the probability that a species will arrive, establish,
spread, and cause harm. The specific assessment method depends on both the mode(s) of potential
entry into the region of interest and the type of species involved. Unintentional introductions are most
likely for insects, pathogens, and marine organisms that are transported in ballast water. Conversely,
plants, pets and livestock, and biological control agents are often deliberately introduced. While preborder pathway analysis is required for assessing the probability that a species will unintentionally
arrive, that probability can be assumed to be 1.0 for deliberate introductions. Additionally,
environmental variables (climate, substrate, etc.) are critical for understanding factors that may limit
establishment and spread of unintentionally introduced species, while cultural efforts will overcome
many environmental constraints for intentional introductions. However, assessment of establishment,
spread, and impact is required for both groups. Fortunately, we have increasing numbers of tools that
address specific biological traits that increase the probability of establishment and spread for groups
such as plants, insects, fish, and birds.
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Pathway analysis, risk maps, and the models that underlie them are critical tools for prevention and
management of unintentional introductions. These tools attempt to describe how the probabilities of
arrival, establishment, and spread by a non-native species, and the magnitude of its impact, might vary
spatially within an area of concern. These projections are often developed based on our understanding
of the population ecology of well-studied species or on statistical associations between the event of
interest (e.g., establishment) and environmental covariates. Pest risk maps are extremely useful for
determining whether quarantine restrictions might be warranted if the non-native species is not known
to be present in the area of concern, to structure an early detection survey if the species might be
present, or to describe the potential extent of impact if the species is not managed effectively. Each pest
risk map has some degree of uncertainty, and ongoing research is attempting to incorporate that
uncertainty into the decision-making processes that the maps support (Venette 2015).
One commonly used RA for intentional introductions of plants is based on the “Weed Risk Assessment”
system developed in Australia (Pheloung et al. 1999). This tool has now been demonstrated to
accurately identify over 90% of harmful plant invaders, mis-identify fewer than 10% of non-invaders as
invasive, and require further evaluation (biased toward non-invaders) for fewer than 15% of the species.
This accuracy is consistent across temperate and tropical and island and continental tests (Gordon et al.
2008). The USDA APHIS PPQ Plant Epidemiology and Risk Analysis Laboratory’s RA for pest plants is
based on this WRA tool (Koop et al. 2012).
These tools were originally designed for pre-border use. As a result, they generally define broad
environmental conditions found across the country of interest, particularly when large latitude or
elevation ranges exist. Climate, habitat, and soil mapping is generally unnecessary for analysis at this
scale. However, when WRA is instead applied for regional predictions post-border, specification of
narrower environmental conditions are often appropriate. More specific climate and risk mapping may
then be applied.
Risk assessment results for both unintentionally and deliberately introduced species can productively be
used at the landscape scale to distinguish or prioritize species that are already present and require
management from those that have naturalized but are unlikely to have significant negative impacts.
Where species are already present, field invasiveness and impacts should inform those prioritization
efforts. Management focus may be warranted for any high risk species for local environmental
conditions that: 1) are not invasive but have been introduced relatively recently; 2) are present at low
levels but have not been prioritized for management; or 3) are not yet present in the area of interest but
a likely pathway exists. Assessments and risk mapping can further help identify species of high invasion
risk that should be the focus of early detection / rapid response programs.
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Literature Cited
Food and Agriculture Organisation (FAO). 2010. International standards for phytosanitary measures
publication no. 5: Glossary of phytosanitary terms. Secretariat of the International Plant Protection
Convention, Food and Agriculture Organisation of the United Nations, Rome.
Gordon DR, CA Gantz, CL Jerde, WL Chadderton, RP Keller, and PD Champion. 2012. Weed Risk
Assessment for Aquatic Plants: Modification of a New Zealand system for the United States. PLoS
ONE 7(7): e40031. doi:10.1371/journal.pone.0040031.
Gordon DR, DA Onderdonk, AM Fox, and RK Stocker. 2008. Consistent accuracy of the Australian Weed
Risk Assessment system across varied geographies. Diversity and Distributions 14: 234-242.
Koop A, L Fowler, L Newton, and B Caton. 2012. Development and validation of a weed screening tool
for the United States. Biological Invasions 14:273–294
Pheloung PC, PA Williams, and SR Halloy. 1999. A weed risk assessment model for use as a biosecurity
tool evaluating plant introductions. Journal of Environmental Management 57: 239-251.
Venette RC. 2015. Pest risk modelling and mapping for invasive alien species. CAB International,
Wallingford, UK.
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