A Saturday essay by Charles Mann (Wall Street Journal, August 6, 2011) provides a fascinating story of the impact of stowaways as a party of global trade. He describes how earthworms made their way as part of the English dirt used to grow tobacco in Virginia (itself imported from the Amazon). The earthworms had disppeared from the US Midwest and New Englad during the Ice Age. These newly imported eathworms flourished and changed the nutrient structure of the soil, wiping out some native plants at the time (wild oats for example). As a result of the nutrient rich soil, the eathworms changed US agricultural productivity. He describes how potatoes that we imported into Ireland enabled a jump in productivity of farms and enabled it to supplant grains.  But centuries later, potato blight wiped out these firms and, in Ireland, killed two million people in two years.  Global trade has unforeseen impacts over time, both positive and negative. How should individual companies and nations plan their trade choices or regulations to aniticipate such impacts ? How much of the liability for such accidental traffic should be imposed on the individual company supply chain ?