How Are Isotopes of Carbon and Nitrogen Used in Food Web Studies?

Isotopes of carbon and nitrogen are used to determine where an animal belongs relative to potential food sources and other animals in its ecosystem. The soft tissue of living organisms is composed of roughly 40% carbon and 10% nitrogen, which are obtained through diet.

Animals take on the isotopic composition of the food they eat with a small isotopic enrichment -- the isotopic ratios become higher in both carbon and nitrogen compared to diet, as shown in Figure 1. In this way, we are able to determine at what level an animal is feeding in relation to its ecosystem.

Fig. 1. The nitrogen (d15N) and carbon (d13C) isotopic composition of tissues are integrated measures of diet that can be used to distinguish the relative food chain (food web) positions of biota.  View larger image

* Laboratory and field studies demonstrate increases in d¹⁵N (about 2 to 3 “permil” units) and d¹³C (about 0 to 1 “permil” units) between consumers and their diet, as shown in the figure above. So “you are not exactly what you eat”. Isotopers refer to this as “you are what you eat plus 2 permil”.