In Which I Anger Environmentalists Everywhere

What's the big deal with invasive species? If a species is introduced to a new environment and thrives there, what's the problem?

There are a couple usual answers that come to mind:

1. It is killing off the native species
2. It is creating a situation in which there is little genetic diversity

Are these legitimate concerns? Returning to my original tenor, what's the big deal if native species are being killed off? We tend to mourn the loss of a species that has gone extinct - but such is life. Natural selection is, well, natural, isn't it?

As for the second concern, it seems to deal with two issues: the genetic diversity of the few native species that remain; and the genetic diversity of the environment as a whole, which is now composed much more substantially of the invasive species. As for the native species, I think I've already shown my lack of compassion for them. Concern for the genetic diversity of the invasive species seems to make even less sense. I would think environmentalists would feel that justice had been well served if some threat were to come along and devastate the evil invasive species.

In fact, we wouldn't be here if it weren't for an extinction-level event (and we're talking about the whole planet here), so can extinction (besides for ours) really be that bad?

I think we should only be concerned about invasive species if there appears to be some negative impact to us humanfolk. For example, if there's a threat to crops that we need, or trees that we think are pretty. But I don't think we should just try to maintain environmental "purity." In many cases, we could better use our resources elsewhere.