The correct answer is - accident.
In general, the invasive species are introduced into a new environment accidentally.This means that they have not been introduced on purpose, or in order to cause any harm to the ecosystem, though there are some exceptions.
The invasive species are usually very dangerous to the local ecosystem. The reason for that is that they usually do not have predators in the new ecosystem, or if they are the predators, than the local flora and fauna do not have defense mechanisms to cope with it. They also act very aggressively, have more offspring, as well as having the offspring on much shorter periods of time, which ends in occupying the space of the local organisms and even leading to their extinction.
