Watching a movie, we don’t need a narrator to tell us whether the action is set in a tropical rain forest, the African savanna, or Arctic tundra. The plants in the scene make it obvious. Plants can be linked closely with environment—palm trees with the tropics, for example, or cacti with the desert—because the environmental distributions of different species reflect the sum of their biological features. Palms cannot tolerate freezing and so are confined to warmer environments; the cactus can store water in its tissues, enabling it to withstand prolonged drought. But the geographic ranges of palms and cacti reflect more than just their physical tolerances. They are also strongly influenced by interactions with other species, including other plants that compete for the limited resources available for growth, animals that feed on them or spread their pollen, and microorganisms that infest their tissues. Ecology is the study of how organisms interact with one another and with their physical environment in nature.