Experimental evidence shows no fractionation of strontium isotopes (⁸⁷Sr/⁸⁶Sr) among soil, plants, and herbivores: implications for tracking wildlife and forensic science

Abstract

Strontium isotopes ((87)Sr/(86)Sr) can be useful biological markers for a wide range of forensic science applications, including wildlife tracking. However, one of the main advantages of using (87)Sr/(86)Sr values, that there is no fractionation from geological bedrock sources through the food web, also happens to be a critical assumption that has never been tested experimentally. We test this assumption by measuring (87)Sr/(86)Sr values across three trophic levels in a controlled greenhouse experiment. Adult monarch butterflies were raised on obligate larval host milkweed plants that were, in turn, grown on seven different soil types collected across Canada. We found no significant differences between (87)Sr/(86)Sr values in leachable Sr from soil minerals, organic soil, milkweed leaves, and monarch butterfly wings. Our results suggest that strontium isoscapes developed from (87)Sr/(86)Sr values in bedrock or soil may serve as a reliable biological marker in forensic science for a range of taxa and across large geographic areas.

Intra‐ and inter‐individual variability of stable strontium isotope ratios in hard and soft body tissues of pigs

“…Meat has only been analyzed for oxygen and hydrogen isotopes . Feeding studies with a focus on stable strontium isotopes are extremely rare: Flockhart et al performed a controlled feeding study on 87 Sr/ 86 Sr with butterflies under laboratory conditions; and Lewis et al investigated the Sr concentration, Sr/Ca ratio, 87 Sr/ 86 Sr ratio, and δ 88 Sr systematics in a controlled feeding experiment in pigs, which was designed to test the effect of marine food resource. However, their focus was on δ 88 Sr values, and they only analyzed teeth, food, and feces, not bones or organs, and unfortunately the two food sources that they compared coincidentally had the same 87 Sr/ 86 Sr ratios.…”

Migration distance as a selective episode for wing morphology in a migratory insect

“…Stable isotope relative abundances (e.g. δ 2 H, δ 15 N and δ 13 C) are translated up the food chain [15, 17] and, if they vary in a spatially predicable manner (termed isoscapes ; [18]), reflect the geographic origin of where tissues are grown in herbivores and higher order consumers [15]. Since stable isotopes in the chitin of wing tissue of adult flying insects is metabolically inert after formation [15, 19, 20], stable isotope measurements can be used to identify the geographic natal origin of adults regardless of how far they have flown from the natal site [21].…”

Natal origin of the invasive biosecurity pest, brown marmorated stink bug (Halyomorpha halys: Penatomidae), determined by dual‐element stable isotope‐ratio mass spectrometry

“…Our understanding of this relationship is limited to a few species for which this has been directly quantified (e.g. monarch butterfly, Helicoverpa armigera (both Lepidoptera) and the hoverfly Episyrphus balteatus [Diptera] (δ 2 H only) or meaningfully extrapolated as provided in various dragonfly (Odonata) species) . Reliable knowledge of these ‘calibration’ relationships, including temporal or seasonal fluctuations if appropriate, will enable the generation of spatially explicit statistical methods appropriate to biosecurity and other ento‐forensic applications.…”

“…28 However, provenance resolution can be improved by considering 2 H and 18 O together, as they serve as semi-independent markers due to differences in evaporative response. 27,29,30 Additionally and importantly, data derived from separate isotope systems also permits the use of more powerful, multivariate statistical analyses. 31 Our choice of markers was also restricted by the biosecurity circumstance.…”