I hate to tell you that often they do have natural enemies, and in many cases, if they didn't initially, then very shortly thereafter many native species will go after them. There are colossal disasters where humans have tried to introduce other things. We introduced the mongoose to try to control things like rats. Well, the mongoose is active at a different time of day than the rats are, so the mongoose ends up going after the wrong species. There are textbook examples in conservation biology of what not to do, and it's largely because.... I mean that's pretty basic. If I'm going to introduce a biological control agent, I want it to go after my target. Not knowing that one species is active during the day and the other is active at night, I mean, how much worse can you get than that? That's just totally incompetent.
On April 29th, 2019. See this statement in context.