Complex sentence: An Adventive species is one that has arrived in the specified geographic area from somewhere else by any means (i.e. through imported plants), but it is not self-sustaining and whose numbers are only increased through non-reproductive means unlike a naturalized species.
Split into simpler sentences: An Adventive species is one that has arrived in the specified geographic area from somewhere else and by any means (i.e. through imported plants), but it is not self-sustaining. Their numbers are only increased through non-reproductive means unlike a naturalized species.