Why are invasive species dangerous to the environment?
they pollute the environment.
they are voracious predators.
they threaten biodiversity.
they are nonrenewable.

I actually remember this question from two years ago. The answer would be they threaten biodiversity and that is because when a species that is NOT part of an ecosystem enters an ecosystem they unbalance it and threaten the biodiversity that's already there. They are, after all, invasive.