I'd like to go back to your previous question.
In most cases you see a tight coupling of the number of invaders in a country and the size of that country's economy. The more you trade, the more invaders you get.
Many years ago, Herb Gray was the member of Parliament in my area. He asked an interesting question about zebra mussels. He asked why we are only getting zebra mussels now and not 50 or 80 years ago, because we've been trading with Europe for a long time. What you have to realize is that these species are not only spreading here, but they're spreading there. In this case, once these key invaders got into the northern European ports with which we have a lot of ship traffic, we had them come over. If you look at both terrestrial and aquatic ecosystems around the world, you'll generally see a curve like this. I have seen very few cases where it levels off, but that's, of course, what we're trying for.