Alien invaders?

It seems DEFRA are trying to come up with a scheme for controlling "invasive" species of flora and fauna coming into this country from elsewhere, and which are threatening to overwhelm native plants and animals. Why do I immediately get a mental picture of Canute commanding the tide not to come in? Isn't this the natural way of things - survival of the fittest and all that? Can't help but feel you're on a loser, mates.