Assessing the Relationship Between Propagule Pressure and Invasion Risk in Ballast Water (2011)Water Science and Technology Board
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The release of invasive species from the ballast water of ships to U.S. waterways has caused extensive environmental and economic impacts (on the order of $1 billion since 1989 for zebra and quagga mussels alone). To limit the spread of invasive species, the Environmental Protection Agency and the U.S. Coast Guard are developing plans to regulate the concentration of living organisms discharged from ballast water. These plans assume that there is a relationship between organism concentration in ballast water and the establishment of invasive species (known as the risk-release relationship). Establishing an initial benchmark to reduce concentrations of organisms in ballast water below current levels, and then selecting various risk-release models to analyze much-needed experimental and field-based data, would help inform future decisions about ballast water discharge standards.
- The density of organisms in ballast water is just one of many variables that influence the probability that an invasive species will successfully establish a population in U.S. waters. Therefore, any model that attempts to predict invasion outcomes based on this factor alone is likely to suffer from a high level of uncertainty.
- When controlling for all other variables, there is evidence that significantly reducing the density of organisms in ballast water will reduce the probability of invasions. Thus, establishing a benchmark discharge standard to reduce concentrations of coastal organisms below current levels is a logical first step.
- In the short term, mechanistic single-species models could help identify the species most likely to be invasive—for example, those with life histories that favor establishment with the smallest inoculum density, including fast-growth, parthenogenetic or other asexual reproductive abilities, and lecithotrophic larvae.
- Developing a robust statistical model of the risk-release relationship would help inform ballast water discharge standards. Currently, the greatest challenge in this approach is the insufficient scope and scale of the available data. The existing data on invasion risk and organism density discharged from ballast (from historical invasion records and recent ballast surveys) are substantially mismatched in time, and patchy in time, space, and taxonomy. Furthermore, there is no evidence that any proxy variable used thus far is a reliable stand-in for organism density.
- To obtain more informative data, a two-track approach should be pursued to gather both experimental and field-based descriptive data. Experiments to evaluate the risk–release relationship are expected to deliver results over the next three to five years, and collecting and analyzing field-based data, expected to take about ten years, would help strengthen the models and provide real-world validation of experimental data.
- The statutes that guide the Environmental Protection Agency and the U.S. Coast Guard regulatory programs appear to provide the essential considerations and scope needed to develop scientifically based numeric standards. Both statutes allow the implementing agency to be sensitive to critical risk factors such as voyage patterns and frequencies.