Biomes are major types of vegetation that are characteristic of broad geographic systems.
A more common situation involves food webs. In nitrogen cycling in intact, relatively undisturbed ecosystems, little nitrogen is lost from the system. There is solid data indicating that the temperature of the earth is increasing and that the percentage of carbon dioxide in the earth's atmosphere is increasing. This kind of community will function as an integrated unit. The movement of water through plants is often explained by the TACT hypothesis.