Mark notices that a tree in his yard is dying. A few weeks later, he notices that the flowers planted around the tree are dying. He notices that many leaves from the tree are piled around the flowers and concludes that the dead tree killed the flowers. His landscaper, however, tells him that the river near his house was recently polluted by a local factory and that the toxins in the water got into the soil, killing his tree and flowers. This is an example of