Dietary data can be misleading

Posted on December 18, 2015

Information on the structure and function of food webs is commonly collected by using gut content analysis to reconstruct the diets of a wide range of animal species. However, this approach can be very time consuming and expensive, and gives no information on how much ingested food is actually assimilated into the body. A supplementary technique – stable isotope analysis – is increasingly used to trace the pathways taken by nutrients (especially nitrogen and carbon) through the food web. While the stable isotope approach is relatively cost-effective and potentially powerful, it’s now clear that there are some drawbacks associated with its use. For example, a substantial meta-analysis (by 22 investigators!) of 21 stable isotope tracer experiments in freshwater streams included model simulations that showed that consumers near the base of the food chain assimilated only 33–50% of the nitrogen available in food sources such as decomposing leaves and fine detritus. Therefore, methods of sampling food sources consumed by animals in streams only poorly reflect the pool of nitrogen they assimilate. Invertebrates near the base of the food chain consume a mixture of fine organic particles, including hard-to-assimilate material such as detritus as well as more assimilable material like algae, bacteria and microfauna. So food web studies that rely on an accurate description of consumer food composition are prone to errors if they don’t take into account the fact that assimilable food is only a fraction of the apparent food source. Omnivores pose a particular challenge for isotope studies, since their body composition can’t be compared to a single food type. The authors recommend further research into methods for separating assimilable from non-assimilable foods – such methods include density gradient centrifugation and compound-specific isotope analyses.

Reference: Dodds, W.K. et al. 2014. You are not always what we think you eat: selective assimilation across multiple whole-stream isotopic tracer studies. Ecology 95(10), 2757–2767.