They havent got enough natural enemies, Thompson. That's what happens if you let an alien plant or animal get established. Rhododendron ponticum, the common one with mauve-purple flowers that you see everywhere, is beautiful for a month every year but is an absolute menace, smothering our native woodland flora, providing next to nothing for the native wildlife and harbouring a serious disease of our native oaks. Mink are beautiful animals, but they are not native and they have decimated the native water vole population. That's Ratty to me and you. Prickly Pears, aliens in Europe, grow like a plague all over the Mediterranean region, to the detriment of the native flora.