I saw an Australian wetland invaded by a south Florida native tree, pond apple or Annona glabra. The Australian government was spending lots of money to eradicate it. It was not far from a spot where I saw Australia's native Melaleuca quinquinervia growing naturally. It is a serious pest plant in south Florida. I was told by an Australian botanist that there are more American weeds in Australia.

Ethiopians call our native mesquite tree the devil's tree.

Here is a link that might be useful: mesquite: the devils tree

This was an interesting thread on the Trees forum.

Here is a link that might be useful: Does our stuff invade the Orient?

I didn't wade through all the content in the Tree Forum discussion, but I noticed a point made there that I wanted to bring up here.

There is a school of thought that North (and South?) America is more prone to new invaders (plants and animals) than Eurasia, because we have been relativel isolated from the large pool of potential invaders in the Old World. Also, the large east-west expanse of Eurasia may be more conducive to the free spread of species from east to west, therefore Eurasian plant communities have had to deal with a greater frequency of invasions over the millenia. (I got this idea from Jared Diamond's book: Guns, Germs, and Steel.)

This might explain why there are RELATIVELY few plant invaders in Europe and China from the U.S., despite many opportunities. Of course, there is also the cultural aspect that many ornamental plants were intentionally imported to North America in attempts to re-create 'Old World' landscapes.

This would also be consistent with the reports of North American invaders in Australia, which has been even more isolated from plant and animal invasions.

john MO - I found Guns, Germs, and steel to be very interesting as well. It certainly seems like Eurasia has tended to send more invasive plants (and animals and microbes) to other parts of the world than have gone in the opposite direction.

I've read in other places that the larger the landmass, the more cases there are of various species "invading" each other's territory, and the more the native species are resilient against new competitors. As the world's largest single landmass, this would mean that Eurasia would tend to have the most resilient or adaptive species on average, more resistant to other species and also more capable of out-competing other species on their own territory.

There are still quite a few invasive species from other continents besides Eurasia, though. I've heard that common grey squirrels from the eastern US and Canada were introduced to Britain and have driven the native red squirrels almost to endangered status. There is some type of freshwater fish from the southeastern US that was introduced into southern China to try and control insect larvae, but it has also almost wiped out several species of native fish in some regions. There is a type of jellyfish from the eastern coastal waters of the US that has become a menace in the Black Sea and has undermined much of the anchovy-fishing industry in that area. The common prickly pear cactus from North America became massively invasive in parts of Australia until they found an insect that helped control it.

From Australia, the melaleuca has become a well-known invasive species in Florida. A number of different species of Eucalyptus were introduced to California in the late 19th-early 20th century and some of them grew so well they became invasive pests. In more recent years, several species of eucalyptus have been grown around the Mediterranean and some threatened to become invasive there as well, especially in Spain.

There was a website with a URL that I've unfortunately forgotten that listed 15 or so plant and animal species from the US and Canada or their coastal waters that have become serious invasive pests in other parts of the world. I wish I knew where to find it now.

Some species invasive in various other countries include

Lodgepole pine
Coast redwood
Black locust
California poppy
Monterey cypress
Monterey pine

Thank You, for your Follow-Ups .