Are Eastern and Western Monarch Butterflies Distinct Populations? A Review of Evidence for Ecological, Phenotypic, and Genetic Differentiation and Implications for Conservation

Abstract

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% since monitoring began—are especially acute. However, the degree to which the western monarch is 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 genetically 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.

Population genetics of a recent range expansion and subsequent loss of migration in monarch butterflies

“…Endangered Species Act is “warranted but precluded” (USFWS, 2021). This decision will be re‐evaluated annually, and the retention of genetic diversity in nonmigratory monarch populations from outlying US states/territories (Hawaii, American Samoa, the Mariana Islands, Puerto Rico, US Virgin Islands) may be important in decisions regarding the adaptive capacity of the species (Freedman et al, 2021). Second, some recent evidence suggests that climate warming and planting of nonnative milkweed species might tip the scales in favour of year‐round breeding and loss of migratory behaviour in North American monarchs, particularly in western North America (Crone & Schultz, 2021; James, 2021; James et al, 2021).…”

Resilience or Catastrophe? A possible state change for monarch butterflies in western North America

“…Western monarch butterflies are distinct from the larger, eastern monarch population that overwinters in Mexico and breeds east of the Rocky Mountains (Freedman et al, in press), and they are in greater peril (Espeset et al, 2016; Pelton et al, 2019). Until now, western monarch butterflies bred throughout the western states during the summer (Figure 1a).…”

Lack of evidence for a fine‐scale magnetic map sense for fall migratory Eastern North American monarch butterflies (Danaus plexippus)

“…Monarchs caught, tagged, and released on the same day from the same location were found overwintering in either California or Mexico (Billings, 2019 ). Similarly, if monarchs possess an inherited magnetic map sense, there should also be genetic differentiation between Eastern and Western monarchs; however, Eastern and Western monarchs are genetically identical (Freedman et al, 2021 ). The patterns and observations found in our study provide compelling evidence that indicates that monarchs do not use genetically inherited geomagnetic map cues for migrating to and finding overwintering sites.…”

“…Fall monarchs use sensory-based compass mechanisms to maintain a southward flight orientation during fall migration (Guerra, 2020). The dominant mechanism used by monarchs is a time-compensated sun compass (Froy et al, 2003;Mouritsen & Frost, 2002;Perez et al, 1997). Monarchs use the sun as a visual cue to maintain a southward heading and their internal circadian clock to compensate for the sun's position in the sky throughout the day.…”