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

1. Strontium isotope ratios (Sr-87/Sr-86) 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 Sr-87/Sr-86 for tracking migratory animals using the eastern North American population of monarch butterflies Danaus plexippus as a case study. 2. To enable continuous-surface geographic assignment using Sr-87/Sr-86, 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 Sr-87/Sr-86 dataset (n = 400), geospatial data and a machine learning approach, we first calibrate a regional, high-resolution Sr-87/Sr-86 isoscape (i.e. a baseline for Sr-87/Sr-86 assignment) over their eastern North American summer breeding range. We then use the Sr-87/Sr-86 isoscape to estimate the posterior probability surface of natal origin for 100 monarchs of unknown origin. 3. Our results demonstrate that Sr-87/Sr-86 can greatly improve the precision of isotope-based geographic assignment. Furthermore, combining delta H-2 and Sr-87/Sr-86 into a dual assignment provides the most constrained area of natal origin. 4. We provide a framework for ecologists and palaeoecologists to apply Sr-87/Sr-86-based geographic assignments for animal movement studies using contemporary or archived samples. The addition of the Sr-87/Sr-86 assignment tool will enhance our ability to study migration and dispersal in a wide variety of animals.

Automated summary

The study introduces a detailed framework for tracking migratory animals using strontium isotope ratios, specifically focusing on the eastern North American population of monarch butterflies. The results demonstrate the significant improvement in precision of isotope-based geographic assignment with the use of Sr-87/Sr-86, and combining it with δ2H provides the most constrained area of natal origin. The framework provided can enhance the study of migration and dispersal in various animals.