Environment: What are your views on uninvited guests?

There are many cases where animals and plants have been introduced to an environment that is not naturally theirs, and are now spreading so fast that they become a threat to biological diversity. Some originally "non native" species such as the tomato or potato have historically been introduced without any problem. However, numerous other species including Canada geese, American bullfrogs, Japanese knotweed and Caulerpa seaweed are now spreading in our environment and threatening local fauna and flora and causing considerable damage to ecosystems and biodiversity.