I saw an old man shearing the tassels off switch grass in the "pine parks" of my neighborhood. I don't think he was with the landscaping company that the HOA employs, a vigilante environmentalist.
Many of the species of vegetation we see around us were not here 150 years ago. The seeds ride in on livestock, food shipments, and vehicles. Many have been stupidly introduced by humans intentionally. It seems to me that bad ideas spread around humanity in much the same way. Good ideas spread too though.