This question has always puzzled me because it rest on some high moral and philosophical ground.
If you agree with evolution then all species are invasive - that is to say that at one point there wasn't the species. It's also the tendency of all species, if they thrive, to expand their range 'naturally', such as the many times salmon's territory either expanded or retreated with the ice ages. Most of the 'natural' ways of specie migration are caused by chance (flies and birds getting swept across the ocean to island), changing environment, population pressure, and hitch hiking (seeds and animals stowing away on migrating animals).
If the migration of species by hitch hiking is a natural way of distributing species why is it unnatural when humans do it? Which lead to the big question are we part of nature or above it?
If we are a part of nature then the movements of species by us is fine. We just happen to be facilitating the most viable species to push out those unfit. And soon we will have land environments that are much like the pelagic ocean, where there is the possibility of species traveling around the world and there is much commonality few species (I know the oceans have great diversity, however, pelagic fish like marlin, tuna, sword fish aren't genetically the same but are still the same spice, that is they can mate and have viable offspring weather in the Pacific, Indian or Atlantic - populations mix). The big downside is that we won't have as much biodiversity and many of our favorite animals will go the way of the doe doe. (I personally disagree with argument the biodiversity will be lost. I think the mechanize of evolution is not damaged by the loss a specie... but that another can of worms).
If we are above nature, it's our duty to try and keep everything the same. But how do we set the bar for what is natural and what is not? An example is the re-introduction of sea otters into Kylaquat (SP!) sound. They're return after more than a century of being gone changed the environment from a hugely successful environment for shell fish, which were the staple for the native there, to one dominated by kelp, great for rearing fish but the shell fish were decimated. The loss of the shell fish fishier caused a call by the native community for the killing of the sea otters. Who's right?
I don't know.
Have fun, Jason