Erecting barriers against invastions
What can be done to stem the tide of bioinvasions? For one, before intentionally introducing an exotic, it would be helpful to thoroughly analyze potential risks and trade-offs of the introduction. However, biologists cannot predict with certainty the invasive potential of any given plant, animal, or microbe . For this reason, a few nations such as New Zealand – where 47 percent of the flora is already exotic  – have adopted the precautionary principle, banning importation of all exotic species except for a few clean-list species that are known to be benign. In contrast, most nations, if they have any import restrictions at all, use a dirty-list concept, only denying import of known problem pests or weeds .
In the case of unintentional introductions, the first line of defense is a system of quarantines and regulations designed to limit the free flow of species through trade, transport, aquaculture, agriculture, forestry, game farming, horticulture, the pet trade, recreation, tourism, and travel . Strengthening these barriers will not be easy in light of potential conflicts with treaties such as the General Agreement on Tariffs and Trade (GATT) that promote fewer, rather than more stringent, restrictions on international trade .
Yet, some steps are already underway. The 156-nation International Maritime Organization has been developing regulations to control ballast water discharge, which is the source of many exotic species in coastal estuaries . In the interim, several countries have acted individually to protect their own waters from invasions. In the Great Lakes bordering the United States and Canada, mandatory controls on ballast water releases from commercial shipping have been in place since 1990, in reaction to the invasion of the zebra mussel . More recently, the United States has extended voluntary ballast water controls to other U.S. ports, requesting that ships filter or exchange their ballast water at sea before entering port . Chile and the port of Haifa, Israel, have also instituted mandatory ballast water requirements, and Australia has a program to control ballast water releases as well