A native prairie is a closed community, composed primarily of a wide variety of perennial plants that come back every year. It's very difficult for newcomers, such as weeds, to break into such a tight-knit society. But even in an established prairie, there is disturbance and change. Wind and water can move piles of silt or gravel from place to place.
Fires clean up the debris between plants, providing small areas of bare ground.
Gophers, ants, and other animals dig in the ground, creating piles of black soil just waiting to be colonized. And, of course, humans dig, plow, bulldoze, and generally rearrange the ground for farms, roads, and buildings. These plants put much of their energy into producing large quantities of seed, which they distribute widely. An example is the common plantain. But it is not only weeds that depend on seed for their success.