Factors at work within a body of water are limiting, that is, each has a maximum and minimum value, above or below which life for many species cannnot continue. The pH of water is one such factor. pH is measured on a scale from 0 to 14, with 7 being neutral. below 7 is considered acidic and above 7 is considered basic. The pH range controls the types of organisms that can survive in a particular environment. Many species cannot tolerate a change of even 1 pH unit, and only a few species can tolerate pH values over 9 or under 5. Remember the nearer to 7 (neutral) the more types of species can live in the environment.