Another interesting note with human infestation is that we tend to destroy trees and spread trees, as they are so useful. In open plains human settlement will tend to bring trees with it, if adequately advanced to do so, to use for wind breaks and other things. Hardy fast growing non-indigenous trees can spread to aid in the destruction of natural prairie. Cutting little cedar trees is often necessary to preserve natural prairie, as an example. A tree can become a weed... and spread that elven empire, heh heh.