Reducing the potential harm from non-native species requires collaboration to develop a balanced course of action that begins, not ends, with risk assessment methods.
What do raccoon dogs and guppies have in common? Java sparrows and plecos? A mongoose and a black acara? At the moment, their commonality is limited to membership in the animal kingdom. However, a petition before the U.S. Fish and Wildlife Service hopes to expand that relationship by ushering guppies, three types of plecos, black acara, koi and 37 other aquatic species into the realm of federally listed “injurious wildlife,” a designation that, with few exceptions, prohibits importation into the U.S. and interstate transport.
What exactly is injurious? Technically, injurious wildlife is a legal subset of the broader category of so-called “invasive species”—species that may cause economic or environmental harm. In reality, “exactly” and “injurious” don’t belong in the same sentence, because the process of assessing whether a species may be harmful in a new environment is an inexact science, at best. Making predictions about nature is perilous. Projecting the aftermath of a species introduced to a new place involves sorting through a host of climatic, biological, ecological and human factors, all subject to change without warning.
With so much riding on listing decisions, it’s worth looking at the process through which a species is deemed injurious or invasive—biological risk assessment. In simplest terms, assessing the risks of the introduction of a species to a new location requires answers to three questions:
- How likely is it that a non-native species will be introduced to a new location?
- What is the chance that an introduced species will survive, reproduce, persist and possibly expand in its new environment?
- If the species hangs in there and maintains a population, what adverse impacts might result?
To reach these answers, scientists have developed various risk assessment tools. One such tool, developed by the Aquatic Nuisance Species Task Force (anstaskforce.gov), has been used by federal agencies as guidance in considering whether to list species as injurious. Typically, a comprehensive report on a species’ biology, ecology and geography has been assembled as a reference in assigning likelihoods to the questions of introduction, survival and impacts. These estimates are cobbled together to provide a single assessment of risk: low (acceptable) versus medium or high (both unacceptable without risk mitigation).
Sifting through literature on foreign species can be a daunting task. In many instances, little has been reported about species in their native habitats. Some reports are difficult to obtain, while others are quite old, with data of questionable value. Critics argue that these labor intensive and time-consuming reports limit the capacity to evaluate many species in trade.
In response to these concerns, risk screens have been developed as an alternative. Where a full risk assessment may send a scientist on a long and winding search taking weeks or months, screens offer a shortcut to estimating risk, relying on information accessible in online databases and other readily available electronic reports.
Risk screening methods vary somewhat, but they generally assign the likelihood of survival and expansion by examining the similarity of the climate in a species’ native country (or countries) with that in the U.S. Estimating the possibility of adverse consequences involves evaluating invasion history—in effect, has the species been introduced elsewhere and caused negative impacts? Some screens compile this information via a questionnaire with numerical scoring (such as the Fish Invasiveness Screening Kit), while others assemble the data qualitatively (e.g. Ecological Risk Screening Summary, which has been used by the U.S. Fish and Wildlife Service to assess many species, including the 43 in the aforementioned petition).
Unfortunately, lunch is not free at the risk-screening counter. Notwithstanding their advantage in speed (many can be completed in an hour or less), rapid screening methods are not designed to peel back the layer of uncertainty associated with many non-native species. Although some animals have received great scrutiny, drawing conclusions about many others amounts to a semi-educated guess, and risk screens inevitably involve a measure of subjectivity by the assessor. Further complicating the matter are the vastly different climates and habitats throughout the United States. A tropical species that may be assessed as risky in Florida poses no threat in much of the rest of the country.
In the end, risk assessment alone—in its full or abbreviated form—is a useful tool, but not the sole fulcrum by which a decision on invasive status should be based. The risk of invasiveness is seldom a simple up-or-down decision on a national scale. Similarly, a uniform risk mitigation strategy (e.g., import bans and interstate movement) is rarely appropriate considering the complexity of environments and species across the U.S.
By using the results of risk assessment as the starting point rather than the final decision, managers and engaged stakeholders can collaborate to develop a balanced course of action to reduce the risk of harm from non-native species.
Scott Hardin is the Pet Industry Joint Advisory Council’s science advisor for exotic and invasive species.