"For the first few centuries after the arrival of Europeans in North America, plants and animals of many species were sent between the two continents. The transfer of non-natives consisted not only of intentional westbound species ranging from pigs to dandelions but also of intentional eastbound species, such as gray squirrels and tomatoes. And for those centuries, the remaining non-native species crossing the Atlantic, uninvited and often unwelcome, were ignored if they were noticed at all. They were joined by various species arriving deliberately or accidentally from Asia and Africa. The national focus on invasive species arose in the 19th century, primarily owing to losses in agriculture (due to weeds or plant diseases), the leading industry of the time. A few recently arrived invasive species, and estimates of adverse economic impacts exceeding $100 billion annually, have sharpened that focus. Very broadly, the unanswered question regarding invasive species concerns whose responsibility it is to ensure economic integrity and ecological stability in response to the actual or potential impacts of invasive species. As this report shows, the current answer is not simple. It may depend on answers to many other questions: Is the introduction deliberate or accidental? Does it affect agriculture? By what pathway does the new species arrive? Is the potential harm from the species already known? Is the species already established in one area of the country? Finally, if the answers to any of these questions are unsatisfactory, what changes should be made?"
CRS Report for Congress, R44049