I think it is mostly invasive species we never used to have either. They grow like weeds and they are a pain in the neck. There seem to be a lot of "pockets" of infestation. Some trails on public land have become impassible due to them, especially when a vine infested tree falls across the path. Between the vines & the prickers, there are some sections of public land you want to hunt that you just can't get to. No power equipment allowed, and you would be there all week with hand tools. I've resorted to packing a folding hand saw and clippers in my hunting vest so I don't get trapped.