Daniel, Yes, in today's world we see a lot of foreign plant / animal invaders. This would appear to be a problem caused by the extent of globalisation we currently have.
I have not read enough about Dr. Miyawaki's methods yet to know if he has addressed this problem but would imagine he has taken this into account. Maybe the 300 plants per 100 sq.m. density plays a major role in keeping the invaders out ....... just no room for them. They (the invaders) are opportunists after all ๐