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Perspective piece: Comparison of eastern and western North American monarch butterflies

Cite this dataset

Freedman, Micah (2021). Perspective piece: Comparison of eastern and western North American monarch butterflies [Dataset]. Dryad.


Monarch butterflies are a species of conservation priority due to declining overwintering populations in both eastern and western North America. Declines in western overwintering monarchs—more than 99.9% since monitoring began—are especially acute. However, the degree to which western monarchs are a distinct biological entity is uncertain. In this review, we focus on phenotypic and genetic differentiation between eastern and western monarchs, with the goal of informing researchers and policy-makers who are interested in monarch conservation. Eastern and western monarchs occupy distinct environments and show some evidence for phenotypic differentiation, particularly for migration-associated traits, though population genetic and genomic studies suggest that they are indistinguishable from one another. We suggest future studies that could improve our understanding of differences between eastern and western monarchs. We also discuss the concept of adaptive capacity in eastern and western monarchs as well as non-migratory populations outside of the monarch’s primary North American range. Finally, we discuss the prospect of completely losing migratory monarchs from western North America and what this entails for monarch conservation.  


All data analyzed in this manuscript come from publically available data sources.


Climate data consist of the following data types:

30-year climate normals (PRISM Climate Group, Oregon State University)

Bioclimatic variables (Worldclim 1)

Canadian historical climate data (


Monarch butterfly occurrence records come from:

All records: GBIF (dataset ID:

Larvae, pupae, egg records: iNaturalist (imported via the iNatTools package in R using the following URL and search terms:,6,7)


Population numbers come from:

Eastern North America: WWF Mexico

Western North America: Xerces Society Thanksgiving Day Counts

Usage notes

Scripts used for analysis can be found at the following URL: