Imagine that you have a garden full of sunflowers in your backyard. Unfortunately, you don't have much time to tend to the garde
n. You eventually notice that two small birch trees sprout and begin to grow among the sunflowers. The birch trees are slower growers than the sunflowers, but by the fifth summer they start to shade the sunflowers. The progression of your garden from sunflowers to birch trees is an example of: