Re: GM Foods
Is there a need? Won't these plants just die on their arse when challenged by the natural variety in their usual habitat?
Often, yes, in which case there's no problem. But also often, no, because they find a niche to which they're already well-adapted; or no, because the ecological processes that formerly prevented species with similar adaptations from establishing themselves have been disrupted.
The last is what happened with prairie grasses in most of the US grasslands, for example. They are typically tall, slow-growing plants with very deep root systems. When established, they block most of the sun from fast-growing invasives; the ones that aren't controlled that way generally were done in either by drought (didn't have the roots to survive it) or burn-off. Burn-off, caused by lightning strikes, was particularly important - the native grasses could burn to the ground and regrow quickly from their root systems, while the burn would wipe out other plants.
When people started building permanent dwellings, cutting down the tall grasses, irrigating fields, and suppressing wildfires, the European invasive plants they brought (because guess which people these were) easily found niches in the modified ecosystem.
Invasive-species ecology is a whole complex field of study. Many people have made careers out of a single species (Asian carp, kudzu, Formosan termites ...).