This article in interesting and helps to highlight how one small change can potentially have a small or catastrophic change on the environment. One invasive species that was mentioned was the Gypsy Moth. Over the last two years we have seen outbreaks of them, which has in turn shown how damaging they can be to oak trees, willow, birch, and apple trees. They also eat over 500 types of plants, leaving many to be vulnerable to their invasion.
Also? Super annoying when they swarm!
Some of the oak trees along my street have been heavily affected by these moths. The result being one side of the tree is much "thinner" than the other, and an overall weaker appearance to the tree.