Oh he's definitely explained himself, it's his favorite topic to rant about when he's drinking too much.

Basically Native Americans spent thousands of years reshaping the forest in North America to basically become giant deer making factories. The native population didn't cultivate fame animals the same as the Europeans, but they did change the ecology of forest to sustain a very large population on wild game.

After the great dying in the Americas substantially reduced the number of people maintaining the ecology and moderating the deer population exploded. Which is part of the reason colonizers described America as some untouched and ever bountiful land full of wild game. Their expansion west and their hunting of what little natural predators were left just made a bad situation worse.

Now deer are super overpopulated and eating the forest floor clean of underbrush and new saplings, causing a cascading failure in the forests ability to sustain itself and other animals.