A Primer in Stable Isotope Ecology

Over the last 30 years stable isotope ecology has expanded from a fringe science to a core component of ecological research. In the first lecture, I will lay out the basics of stable isotopes and their applications to ecology. I will explain what isotope are, why some isotopes are stable and why this is useful to ecologists. I will outline the uses of the five stable isotopes most commonly applied to ecology: nitrogen (¹⁵N), carbon (¹³C), sulphur (³⁴S), hydrogen (²H) and oxygen (¹⁸O). During the lecture I will outline how to collect, preserve and prepare sample for stable isotope analysis. The lecture will cover key concepts of trophic fractionation and tissue turnover rates, which are integral to the accurate ecological interpretation of stable isotope data.

Most ecologists using stable isotopes will, at some point, want to run a mixing model. Essentially, stable isotope mixing models are a comparison of the stable isotope ratios of a consumer and its prey. They allow researchers to determine the resource use of an organism or the trophic interactions within a community. However, like all ecological models they are based on a series of assumptions and using data that violate these assumptions can lead to erroneous or misleading results. In this lecture, I lay out the most common flaws associated with the application of mixing models to stable isotope data and how to avoid them. The last 10 years has seen a proliferation of isotope mixing models (e.g. IsoSource, MixSIR, SIAR). A recent collaboration between the developers of two of the most commonly used models has lead to the creation of MixSIAR, now considered the ‘industry standard’ for mixing models of ecological data. This lecture will include demonstration of this package using an example dataset.

The application of mixing models to stable isotope data has spearheaded the advance of the technique in recent decades. However stable isotopes can also be used to infer other aspects of a trophic ecology across populations, communities and even whole food webs. In this lecture I will discuss the development and application of a the Layman metrics, series of measurements which summarise food web size and complexity, and standard ellipses, a quantification of population niche width based on stable isotope ratio values.