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Per capita invasion probabilities: A linear model to predict rates of invasion via ballast water
Citation:
REUSSER, D., H. LEE, II, M. R. FRAZIER, G. M. Ruiz, P. Fofonoff, M. S. Minton, AND W. Miller. Per capita invasion probabilities: A linear model to predict rates of invasion via ballast water . ECOLOGICAL APPLICATIONS. Ecological Society of America, Ithaca, NY, 23(2):321-330, (2013).
Impact/Purpose:
Scientists from USGS, WED and the Smithsonian collaboratively proposed a method of establishing ecologically meaningful ballast water discharge standards. Ballast water discharge is one of the primary vectors of nonindigenous species into coastal aquatic environments. Under the Clean Water Act the U.S. EPA’s Office of Water will establish standards for the concentration of organisms that can be in ballast water discharge. These standards should be biologically defensible, adequately protective of aquatic environments, and transparent in their assumptions. This research provides a framework for accomplishing these goals. This method directly relates the risk of invasion to the concentration of organisms in ballast discharge based on historic invasion rates, the estimated organism concentrations in ballast discharge, and the volume of foreign ballast water discharged in the United States. This approach will allow policy makers to establish biologically defensible discharge standards using various levels of risk and ballast water discharge volumes.
Description:
Ballast water discharges are a major source of species introductions into marine, estuarine, and freshwater ecosystems. To mitigate the introduction of new invaders into these ecosystems, many agencies are proposing discharge standards that establish upper concentration limits for organisms that may be present in ballast discharge. Ideally, ballast discharge standards will be biologically defensible and adequately protective of the marine environment. We propose a new technique that would allow managers to quantitatively evaluate the relative risk of different concentration based ballast water discharge standards calculated using the estimated per capita invasion probability (PCIP). PCIP represents the likelihood that a single discharged organism will become established as a new nonindigenous species within a waterbody or coast. This value is calculated by dividing the total number of ballast water invaders per year by the total propagule pressure from ballast discharge. To estimate propagule pressure, a randomization technique was used to obtain the likely range of propagules introduced into the waterbody based on two independent sources of data. Analysis was done at the individual estuary scale as well as a coast-wide scale for the Atlantic, Gulf and Pacific coasts. There was less uncertainty with the coast-wide scale analysis as it removes the unknown influence from secondary invasion vectors between estuaries on a single coast. Depending upon the assumptions used in the risk analysis, this approach predicts that approximately one new species will invade every 10 to 100 years with the current discharge rates proposed by the International Maritime Organization (IMO) discharge standard of <10 organisms with body size >50 microns per m3 of ballast. This approach resolves many of the limitations associated with other methods of establishing ecologically sound discharge standards, and it allows policy makers to use risk-based methodologies to establish biologically defensible discharge standards.
