All biologically active elements exist in a number of different isotopic forms, of which two or more are stable. For example, most carbon is present as C, with approximately 1% being C. The ratio of the two isotopes may be altered by biological and geophysical processes, and these differences can be utilized in a number of ways by ecologists. 
The main elements used in isotope ecology are carbon, nitrogen, oxygen, hydrogen and sulfur, but also include silicon, iron, and strontium.