What does it mean for a species to be alien - and why is it a bad thing?

Invasive alien species are frequently discussed in academic literature by practitioners, government agencies, and popular media, but what does it mean for a species to be alien and why it this seen as a bad thing? To answer these questions, I have analysed texts about invasive alien species in academic journals and in communication from government agencies. The almost totally unanimous answer to the first question was that a species is alien if and only if it is introduced to an area outside its natural range by humans. I found three primary answers to the second question, namely that (1) alien species are more probable to behave invasively or that it is impossible to know for sure if an alien species will behave invasively, (2) being alien is bad in itself or at least that alien species have a lower value than native species and native environments, and (3) being moved by humans is unnatural and being unnatural has negative value. All three answers probably contribute to why being alien is considered a negative property in species but none of them seem like a satisfying answer to why being alien should be seen as a bad thing in a species.