Newton, J. 2015. BOU Proceedings – Birds in the entangled bank: advances in foodwebs theory and practice
http://www.bou.org.uk/bouproc-net/foodwebs/newton.pdf
Proceedings of the BOU’s 2014 Autumn Meeting
Birds in the entangled bank: advances in foodwebs theory and practice
View other papers from these proceedings at www.BOUPROC.net.
Stable isotopes and food webs: advantages and limitations
JASON NEWTON NERC Life Sciences Mass Spectrometry Facility, SUERC, Rankine Avenue, East Kilbride G75 0QF, UK [email protected] | Twitter: @JasonNewton19 Bulk stable isotope analysis can no longer be described as a new technique for elucidating food webs: following seminal work by DeNiro and Epstein looking at how carbon and nitrogen isotope compositions change from prey to consumer, there is over 30 years’ worth of publications using stable isotopes in an ecological context, and concomitant improvements in the precision and sample throughput of instrumentation to keep up with the need from the ecology user community. Even so, currently, stable isotope systematics are rarely a formal component of a life sciences undergraduate degree, and the technique remains largely isolated in specialist laboratories. Although stable isotopes can be extremely powerful as nutrient tracers, common pitfalls exist that render interpretation equivocal or, worse still, erroneous. Interpretation of an animal’s diet from its stable isotope composition is, at its simplest level, calculated from knowledge of (i) fractionation, i.e. knowledge of the change in isotope ratio from diet to consumer (often known as a trophic enrichment factor or TEF); and (ii) mixing models which attempt to elucidate diet from the isotope composition of a number of possible food items. However TEFs are known from only a few species/tissues (e.g. <30 species’ feathers), despite numerous calls for more laboratory experiments, so there is a level of uncertainty here. Many tissues (e.g. blood components) have a specific turnover time, which means that one has to be careful to match the time at which the tissue was synthesized to the question being asked. Other tissues are synthesized at one point in time and are then isotopically inert, so for feathers for example it is important to know the time of moult. The number of dietary items for which a mixing model has a unique solution is n + 1, where n is the number of isotope ratios. As the most useful isotope ratios are those of carbon and nitrogen, this generally equates to only three dietary items, which is rarely a realistic situation. Although advances have been made in statistical methods that elucidate the proportions of food items in a consumer’s diet from a larger number of sources, it is still just as critical to sample an adequate number of food items and to account for any spatial isotope heterogeneity in the diet and habitat. 1
Newton, J. 2015. BOU Proceedings – Birds in the entangled bank: advances in foodwebs theory and practice
http://www.bou.org.uk/bouproc-net/foodwebs/newton.pdf
Proceedings of the BOU’s 2014 Autumn Meeting
Birds in the entangled bank: advances in foodwebs theory and practice
View other papers from these proceedings at www.BOUPROC.net.
It is also important to understand the limitations of stable isotope analysis – although gut (or regurgitate) contents provide only snapshot analyses of a consumer’s diet, stable isotope analysis of tissue samples rarely provides such dietary detail, but gives a broad overview of diet over a longer timescale and includes all assimilated food. Combining both techniques provides a more robust short-­‐ and long-­‐term estimate of diet. Stable isotopes have already made a substantial contribution to foodweb analysis, and, when used carefully, remain a useful tool for providing insights into trophic interactions – in particular involving species, including many birds, that are difficult to study in their natural environment. 2