Are isotopes representing reliably the structure of food webs?

Franck Jabot, Carolina Giraldo, Sébastien Lefebvre and Stanislas Dubois

Most atoms can be found in nature in various forms that are called isotopes. Primary producers (plants, algae, bacteria…) often have different isotopic compositions, notably in carbon isotopes, so that the isotopic composition of their consumers will reflect the food items it has consumed. But the isotopic composition of a consumer is not simply the average of the isotopic composition of its food. When an organism eats another organism, it discriminates the isotopes at different rates, notably the nitrogen isotopes. The ratio of the different isotopes is therefore modified between a prey and its predator and the shift (i.e. trophic discrimination) in isotopic composition is roughly constant. Many studies are determining the isotopic compositions of the different organisms encountered at a given place, generally by focusing on carbon and nitrogen isotopes. Their goal is to reconstruct the feeding relationships between organisms (the food web) based on their isotopic compositions. The difficulty is that different mixes of food items with different isotopic compositions can lead to similar isotopic composition of a consumer. This drawback has been recurrently pointed out in the scientific literature, with some theoretical illustrations, but its magnitude for realistic food webs is uncertain. We here assessed how reliable is the use of isotopes to represent food web structure based on a large set of simulated food webs with realistic characteristics. We simulated artificial food webs and the isotopic composition of the different organisms, starting from the base of the food web (primary producers) to the top of the food web (top carnivores) with realistic rules of isotopic trophic discrimination. We then compared the isotopic similarity between consumers and their diet similarity, to assess whether they are strongly correlated or not. We found that correlation levels were modest, meaning that isotopic compositions lead to severely blurred representations of real food webs. Still, we found that isotopic compositions do encapsulate with relatively good accuracy some general characteristics of the food webs, such as the diversity and the similarity of feeding regimes among consumers.

Image by Stanislas Dubois

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