Strontium isotope ratios (⁸⁷Sr/⁸⁶Sr) have shown promise for tracing the geographic origin of animal tissues because they have high-resolution and show discrete spatial patterns independent and complementary to those of light isotopes. In this study, we provide a complete quantitative framework to apply ⁸⁷Sr/⁸⁶Sr for tracking migratory animals using the eastern North American population of monarch butterflies Danaus plexippus as a case study.
To enable continuous- surface geographic assignment using ⁸⁷Sr/⁸⁶Sr, we recommend following five key steps:(5) assessing feasibility, (2) sample collection, (3) laboratory analysis, (4) modelling the isoscape and (5) geographic assignment. We provide a detailed outline of these steps and then focus on steps 3–5 for the case study. For monarchs, using an extensive plant ⁸⁷Sr/⁸⁶Sr dataset (n = 400), geospatial data and a machine learning approach, we first calibrate a regional, high- resolution ⁸⁷Sr/⁸⁶Sr isoscape (i.e. a baseline for ⁸⁷Sr/⁸⁶Sr assignment) over their eastern North American summer breeding range. We then use the ⁸⁷Sr/⁸⁶Sr isoscape to estimate the posterior probability surface of natal origin for 100 monarchs of unknown origin.
Our results demonstrate that ⁸⁷Sr/⁸⁶Sr can greatly improve the precision of isotope-based geographic assignment. Furthermore, combining δ2H and ⁸⁷Sr/⁸⁶Sr into a dual assignment provides the most constrained area of natal origin
We provide a framework for ecologists and palaeoecologists to apply ⁸⁷Sr/⁸⁶Srbased geographic assignments for animal movement studies using contemporary or archived samples. The addition of the ⁸⁷Sr/⁸⁶Sr assignment tool will enhance our ability to study migration and dispersal in a wide variety of animals.