Blog, Food webs, Methods, Science, SIA

How stable isotopes tell us who eats who

An essential part of understanding how any ecosystem works is understanding the food web: who eats what and in what amount. Determining who is eating who for dinner can be done directly by observation of prey capture or by examining stomach contents. In many cases however, these direct observations are next to impossible: we can’t follow an amphipod around the bottom of the Arctic ocean to see what it eats and cutting open its stomach may not tell us much about what food items are most important to its diet. In cases like this, stable isotope analysis can be used as a reliable, indirect way to map a food web.

The food web of Arctic estuaries. From Dunton et al. 2012.
The food web of Arctic estuaries. From Dunton et al. 2012.

To explain how stable isotope analysis (SIA) works, we’re going to have to dive into some elementary chemistry. Note that the following explanation is a simplified one. If you want a more detailed explanation of SIA, Dr. Google and Dr. Wikipedia are excellent resources.

All living things contain carbon (C) and nitrogen (N). Like all elements, C and N have a standard atomic number defined by the number of protons plus the number of neutrons an atom of that element has. For C, the atomic number is 12, for N it’s 14. However, not all C and N atoms have that exact atomic number, some have more or fewer neutrons. For example, 13C has one more neutron than 12C. We call these atoms with variations in the number of neutrons isotopes. Isotopes with more neutrons are referred to as “heavy” and ones with fewer are referred to as “light”. Now, back to biology.

Not only do all living things contain C and N, they contain them in different isotopes. The main isotopes are 12C and 14N, but there is also significant amounts of “heavy” 13C and 15N. Things at the base of the food web (e.g. plants) will contain a lower ratio of heavy:light C  and N than things at the top of the food web, because lighter forms of C and N are expelled at each step up the food web (see Figure 1).

A simple food chain showing how light C and N are exuded at each trophic level.
Figure 1. A simple food chain showing how light C and N are exuded at each trophic level.

The amount of light isotopes expelled at each step up the food web (at each “trophic level”) is relatively predictable: from plant to cow, the ratio of 13C:12C increases by around 1 and the ratio of 14N:15N increases by around 3. This is super handy because it allows us to plot species in a food web in “isotope space”, as in Figure 2. If we didn’t already know that people ate cows who ate flowers, we could infer this from our plot -hooray!

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Figure 2

Now, imagine that there are two initial food sources. In some cases, the food sources have different enough carbon isotope “signatures” that we can tell if a consumer is eating both of them (Figure 3). Things on the same trophic level have very similar nitrogen signatures. Therefore, carbon tells us the food source and nitrogen tells us the trophic level.

This cow eats both flowers and peaches, which have different carbon isotope signatures
Figure 3. This cow eats both flowers and peaches, which have different carbon isotope signatures

Now switch out flowers for phytoplankton and cows for Arctic Ocean animals, and you have how we use SIA to map Arctic Ocean food webs.

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