> Birds will drop seeds, and those seeds will germinate beneath the pioneer canopy, letting the larger species come through and eventually replace the pioneer species.

FYI This can actually be a huge problem if you live in Washington State, because Himalayan Blackberry spreads through bird droppings. You have to aggressively weed it or it will choke out everything else in the area within a matter of years, including saplings, mature brush, grass, native blackberry, and other invasives like Scotch Broom.