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
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.
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”.