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Continuous‐surface geographic assignment of migratory animals using strontium isotopes: A case study with monarch butterflies

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: (a) assessing feasibility, (b) sample collection, (c) laboratory analysis, (d) modelling the isoscape and (e) 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/⁸⁶Sr‐based 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.