Two students were exploring near a creek in a forest. They could see fish swimming in the cold water, rising to eat insects at the water’s surface. Buried in the sandy soil under the shade of a cottonwood tree was a rock covered with leaves and mushrooms. One student carefully lifted the rock to reveal a small tunnel. Rice-sized feces were scattered around the opening, and a small pile of tiny grass seeds was just inside the tunnel. They carefully replaced the rock and left.

List three examples of populations in the story.