Invasive species are a massive ecological problem, and they are primarily caused by humanity - either intentionally or unintentionally. In the case of the Bradford Pear, it was 100% intentional. This article by Adrian Higgins in The Washington Post Magazine details each stage of human decision-making that led to the Bradford Pear menace.
The Washington Post
The Washington Post recently covered this topic with some details I didn't know about before - like Denmark considering a tax on all red meat and the fact that of all the nations studied, the US could reduce its health care budget the most by cutting out American consumption of meat due to our country's ridiculous average diet. Check it out!