Question

An invasive species is introduced to an ecosystem through a

Answer 1

vector

A vector is a way in which an invasive species enters an ecosystem. Plato confirmed.

Answer 2

An invasive species, is referred that way when it's non-native to a certain ecosystem. This species was introduced to that new ecosystem through:
human activity (either deliberate or accidental).
The reason we call it "invasive" it's because it becomes established in that ecosystem and spread beyond the place it was introduced, causing damage to other species.