Populations in nearly every ecosystem have shifted in response to human introductions of formerly isolated species from other regions. The change really is dramatic - in the last two hundred years the northeastern US forests, which I've studied somewhat, have completely lost the dominant overstory plant, the American Chestnut (introduced fungi), and have lost a many-inches-thick layer of topsoil debris with resident moss & fungus populations (introduced earthworms). That's in the ones that have recovered from the agricultural/logging clearcuts and that weren't affected by the fire suppression.