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 99.9% 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.

Monarchs Reared in Winter in California Are Not Large Enough to Be Migrants. Comment on James et al. First Population Study on Winter Breeding Monarch Butterflies, Danaus plexippus (Lepidoptera: Nymphalidae) in the Urban South Bay of San Francisco, California. Insects 2021, 12, 946

“…Finally, the reported wing sizes of the 500 winter-reared monarchs from James et al were included for comparison. When the wing sizes of all collections are plotted side by side (Figure 1), it becomes obvious that the traditionally migratory western North American monarchs have wings that are as large as those from the east, which is consistent with the current evidence that eastern and western monarchs are not dissimilar [24,25], and that the demands of long-distance migration select for large wing size [26,27]. Meanwhile, the monarchs reared by James et al had an average wing length that more closely matches those from nonmigratory populations (Figure 1).…”

Opposing global change drivers counterbalance trends in breeding North American monarch butterflies

“…Even though the most recent evidence indicates the monarchs west of the Rocky Mountains should not be considered a separate population (Freedman et al, 2021;Talla et al, 2020), the assessment of monarch abundance in the west has traditionally been via counts of wintering monarchs along the California coast (Espeset et al, 2016;Pelton et al, 2019). However, as we found with the larger cohort of monarchs east of the Rockies, the trend of diminishing wintering colonies in California does not appear to mirror long-term trends in breeding monarch abundance to the north or northeast, either in Oregon or Idaho (Figure 2a).…”

Do demographic processes change at extremely low population size in western monarch butterflies?

“…In North America, there are two migratory populations, a larger eastern monarch population that overwinters in Mexico and breeds in North America east of the Rocky Mountains and a smaller western population that overwinters on the California coast (the state of California in the USA and northern Baja California in Mexico) and breeds in North America west of the Rocky Mountains. These two migratory populations are not strongly genetically differentiated, but they are demographically distinct, in the sense that they fluctuate independently (Freedman et al 2021). Although both populations have experienced declines in recent years, the western population is both smaller and is declining at a faster rate, so is at greater risk of extirpation (Semmens et al 2016, Schultz et al 2017).…”