The natural and stable isotopes of carbon vary in systematic ways largely determined by the photosynthetic pathway (C₃ or C₄) which fixes atmospheric carbon dioxide into organic matter. Within both C₃ and C₄ species there is a genetic component to the variation in this ratio which may be as great as 3per thousand. A variety of environmental factors also affect this ratio in plants. Individual biochemicals differ from each other in their isotopic values. The ratio in bulk plants is transferred to higher trophic levels with reasonable fidelity however, the collagen-diet spacing is variable and needs to be understood. An appreciation of these sources of variation is necessary as researchers quantify diet interpretations.