Wildlife trade investigations benefit from multivariate stable isotope analyses

Abstract

The investigation of wildlife trade and crime has benefitted from advances in technology and scientific development in a variety of fields. Stable isotope analysis (SIA) represents one rapidly developing approach that has considerable potential to contribute to wildlife trade investigation, especially in complementing other methods including morphological, genetic, and elemental approaches. Here, we review recent progress in the application of SIA in wildlife trade research to highlight strengths, shortcomings, and areas for development in the future. SIA has shown success in species identification, determination of geographic provenance, and differentiating between captive-bred and wild individuals. There are also emerging applications of SIA in wildlife trade research including the use of labelling for traceability, more in-depth analyses such as compound specific isotope analysis (CSIA), the use of trace metal isotopes, and monitoring the health of individuals (e.g. dietary history and nutritional status). While these applications have shown the utility of SIA in wildlife trade investigations, there are a number of limitations and issues where standardisation of analytical procedures would improve the comparability and interpretation of results. First, there is high variation within many stable isotopes geographically and within tissues - this variation presents opportunities for tracking and monitoring but can also challenge detection of patterns when variation is high. Second, the choice of isotopes and tissues within an organism (and ideally, multiple isotopes and tissues) should be considered carefully as different isotopes and tissue types have variable strengths and weaknesses depending on the research question. Third, validation of SIA methods remains underutilised in the field but is critical for applying SIA broadly to wildlife trade investigations and, particularly, for applications in forensics and in court. Fourth, standards are essential for comparisons across studies. Fifth, while some reference databases exist for the use of SIA in wildlife trade research (e.g. ivory), there are still few comprehensive reference databases available. Development of robust reference databases should be a priority for advancing the use of SIA in wildlife trade research, and ecological study more broadly. Ultimately, further recognition of these primary challenges (and development of solutions) within wildlife SIA research will improve the potential for this technique in tackling the threat of overexploitation to global biodiversity - particularly in concert with the application of other investigative techniques such as genetics and elemental analysis.

Keywords: captive–wild; geographic origin; species identification; stable isotope analysis; validation; wildlife forensics; wildlife trade.

Fig. 1

General workflow for bulk stable isotope analysis (SIA). (1) Sample collection and preparation: sampling, drying (in an oven or freeze drier), and homogenisation (with a mortar and pestle, drill, or blender). (2) Weighing, followed by packing into tin/silver cups. (3) Mass spectrometry using, for example, a continuous flow isotope ratio mass spectrometer (CF‐IRMS), thermal ionization mass spectrometer (TIMS) or multicollector inductively coupled plasma mass spectrometer (MC‐ICP‐MS). (4) Data interpretation, e.g. data plotting and statistical analysis. (5) Application of SIA to wildlife trade research in three main areas (species ID, provenance and captive/wild). Stable isotopes that have been used most commonly for each area are shaded, stable isotopes that have been used to a lesser extent are not shaded (C: carbon; N: nitrogen; S: sulphur; O: oxygen; H: hydrogen; Sr: strontium; Pb: lead). Note that steps and methods for analysis may vary according to individual protocols and aims.

Fig. 2

Ranges of δ ¹³C and δ ¹⁵N values for captive‐bred and wild‐caught animals based on previous studies (Dempson & Power, ; van Schingen‐Khan et al., ; Castelli & Reed, ; Brandis et al., ; Andersson et al., ; Hopkins et al., 2022). Bars represent the ranges of the δ ¹³C and δ ¹⁵N values in each study. The δ ¹³C and δ ¹⁵N value ranges for the captive northern bobwhite samples were narrow and lie within the icon.