We don't know that for sure though. All it takes is a single seed in an ideal condition. And that condition could've probably happened in a way that the pear wouldn't have the advantage that's caused some to name it "invasive".

But also, left alone, nature tends to rebalance on its own. Any species with a dominant advantage will eventually lose that advantage, given a few generations. Well, except for humans, who continually fight the natural rebalancing, and are only succeeding in increasingly destroying that which sustains life on this planet.