“I see the potential problems with reintroducing species to a habitat and we constantly hear about the problems of introducing new species. But what about the non-native, so-called "invasive" species? Why is everyone bent on eradicating them? They are now part of the ecosystem — like it or not they have weaseled their way in. Don't they deserve a shot at living? After all, that's what humans do."
Your question is one that philosophers battle with regularly.
The reason that invasive species concern conservationists is that when they are introduced by humans, they may displace the native species. This is a particular concern if the native species are specialists with very small ranges. The most harmful invasives can completely transform ecosystems.
For example, invasive annual grasses from Europe can change the frequency that fires sweep through an area. The marvelous Sonoran Desert is being changed from cactus gardens into savannas as a result, and many species of plants and animals are at risk of perishing as the Sonoran Desert ecosystem is burnt away. These grasses are fine in their native lands, but not fine in the United States or Mexico.
Similarly, several species of trees from the U.S. have been introduced to New Zealand and South America and are displacing native species there!
On the other hand, some invasive organisms seem to have little impact. Many invasives — like the dandelion — rarely cause any damage to the native biodiversity. Scientists study the behavior of non-native species and determine which are harmful to biodiversity and which are not.
I agree that all organisms deserve a shot at living. Yet I also think that we must take responsibility for our actions. If we do something like pollute a habitat or introduce non-natives, we should work to clean up our messes.
The best solution is to not make a mess in the first place!
I've written more about this at http://www.invasive.org/gist/common.html — you might find it an interesting read.
Barry Rice is a Conservancy invasive species expert.