Water plays a vital role in shaping life. You will learn more about different ecosystems in the biomes section of this week, but it should be clear to you that different parts of the Earth have different amounts of water. The amount of water available changes the kind of life present. Marine life, for example, requires vast amounts of water to survive. In the desert, life has adapted to use little or no water, or to store water for extended periods of time.

Beyond the amount of water as a factor in life, the kind of water also affects water. Most life is either suited to use fresh water or salt water; they rarely overlap. Contamination of water, especially from human sources, can have a significant impact on life.