The knee-jerk reaction against "foreign" species is disturbing. Many naturalists seem to suffer the same xenophobic delusions caused by misplaced patriotism. There's no doubt that the introduction of some species into a biome may cause a sudden and catastrophic change, and that the biome may never return to its previous state.
There's no doubt that the caulerpa taxifolia introduced into the Mediterranean has resulted in a gray (or rather green) goo tragedy. But to think all invasive species that disrupt the native fauna and flora are bad is simply shallow romanticism. Not all things native are good simply for the fact of being "native". Many species, including ourselves, are "invasive" by nature. We spread out over ever larger areas and alter the biomes we "invade".
Yes, sustainability is key. This is true for all species! Mormon crickets don't devour everything in their way. In partially decimated fields they turn on each other, sustaining their need for a protein rich-diet through cannibalism. But if they were to devour everything and each other, well, as a species they would be no more.
Our green monk parrots (Myiopsitta monachus) here in Brooklyn are a great example of a foreign species I welcome with open arms. They are not the green goo that taxophelia is to the Mediterranean. Monk parrots are smart social birds that deserve our respect even if they may displace other "native" species. I guess I should disclose that I myself (along with 99% of my fellow Brooklynites) am an invasive organism as far as Long Island goes.
Some, especially Argentinian farmers, could accuse me of romanticising the monk parrot. I think there is nothing romantic about respecting creatures that demonstrate human-like intelligence. It's the positive side of tribalism. Misplaced patriotism is patriotism which focuses on surface rather than substance. Monk parrots are complex survivalists that enrich the environments where they establish themselves. I'd rather have biome that, though less rich in color and surface variety, is much richer in culture.
Biomes are not static dioramas at the American Museum of Natural History. They are constantly changing as some species arrive, some go extinct and some new physiological and behavioural variations arise. Perhaps the fear is often that life itself cannot survive the tempestuous changes that human globalization is causing. I think this is both overestimating our impact and underestimating life's ability to adapt.
This doesn't mean we shouldn't try to counteract some effects (such as global warming) that we cause to biomes. Our ability to recognize that something is wrong and devise countermeasures is part of our ability to adapt and survive. But a simplistic nativist mind-set is not a healthy frame for deciding how we should treat foreign elements.