Not allowing species from outside the continent doesn't mean everything stays "as is". The whole eco-system there is evolving (like at any other place) and will

Not allowing species from outside the continent doesn't mean everything stays "as is". The whole eco-system there is evolving (like at any other place) and will take its own course. They customs folk are only trying to ensure no one brings a (potentially) disruptive element into the mix. There have been instances of invasive species wiping out local species. I'll leave it to you to find examples using your favourite search engine.