Witchweed is a root parasite on corn and sorghum from Africa. It is also spread by raccoon, deer, and infested sod. If such invasions continue, the Earth will become simpler and poorer biologically instead of six bioregions, there will be one. Biological pollution is the single most underestimated threat to sustainability. We generally wait until things get out of hand before we do anything about them.
Prevention is always better and cheaper than a cure.
We need to use caution in the movement to free trade. If we become a true global village, the final chapter of biological invasion may come to pass. People who want to bring in nonnative species for profit should have to prove that the species has never been a problem as a weed or invader elsewhere. That is not enough though, because there are some species that don't seem to a problem right away but then become a problem later. We can work harder to determine the vectors of unintentional introductions, and exercise more control over intentional introductions.