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

Davis, Andrew K.

doi:10.3390/insects13010063

Cited by 4 publications

(2 citation statements)

References 30 publications

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“…We also note that the NABA data showed a small region in central California where summer numbers are declining, which is consistent with other long‐term surveys from that same region (Espeset et al, 2016), and this consistency provides confidence in the NABA data. Further, reasons for declines in wintering monarchs in California have been the subject of ongoing debate, with some speculating that western monarchs may be transitioning to a less migratory lifestyle in California, which is being fueled by homeowner plantings of non‐native milkweed that thrives year‐round (Davis, 2022; James et al, 2022). Regardless of the reason, the discrepancy between wintering numbers and breeding abundance in the west, like that of the east, argues that overall population assessments should be based on multiple sources, and from across life stages.…”

Section: Discussionmentioning

confidence: 99%

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

Crossley

Meehan

Moran

et al. 2022

Global Change Biology

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Many insects are in clear decline, with monarch butterflies (Danaus plexippus) drawing particular attention as a flagship species. It is well documented that, among migratory populations, numbers of overwintering monarchs have been falling across several decades, but trends among breeding monarchs are less clear. Here, we compile >135,000 monarch observations between 1993 and 2018 from the North American Butterfly Association's annual butterfly count to examine spatiotemporal patterns and potential drivers of adult monarch relative abundance trends across the entire breeding range in eastern and western North America. While the data revealed declines at some sites, particularly the US Northeast and parts of the Midwest, numbers in other areas, notably the US Southeast and Northwest, were unchanged or increasing, yielding a slightly positive overall trend across the species range. Negative impacts of agricultural glyphosate use appeared to be counterbalanced by positive effects of annual temperature, particularly in the US Midwest. Overall, our results suggest that population growth in summer is compensating for losses during the winter and that changing environmental variables have offsetting effects on mortality and/or reproduction. We suggest that density-dependent reproductive compensation when lower numbers arrive each spring is currently able to maintain relatively stable breeding monarch numbers. However, we caution against complacency since accelerating climate change may bring growing threats. In addition, increases of summer monarchs in some regions, especially in California and in the south, may reflect replacement of migratory with resident populations. Nonetheless, it is perhaps reassuring that ubiquitous downward trends in summer monarch abundance are not evident.

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Section: Discussionmentioning

confidence: 99%

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

Crossley

Meehan

Moran

et al. 2022

Global Change Biology

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“…Finally, year-round resident populations of monarchs appear to be increasingly common and growing along the western and southern edges of the species range (29). Monarch migration leads to a natural culling of monarchs weakened by OE infection or other flaws, and abandonment of migration by these year-round residents is associated with dramatically higher OE infection levels (34,35), smaller wing and body sizes (41), and weakened dispersal abilities (42). Interbreeding between migratory and non-migratory monarchs, in turn, likely dilutes genes associated with migratory ability (37).…”

Section: Discussionmentioning

confidence: 99%

Dramatic Recent Declines in the Size of Monarch Butterfly (Danaus plexippus) Roosts During Fall Migration

Davis,

Croy,

Snyder

2024

Preprint

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The conservation status of monarch butterflies in North America is a topic of intense scrutiny and debate. It is clear that winter colonies in Mexico are declining, yet some recent studies suggest that summer breeding populations are relatively stable and similar to historical abundances. One possible explanation for these discordant patterns is that fall migration success has been recently disrupted. Here, we use a relatively unexplored citizen-scientist dataset on the size of monarch “roosts,” which are resting aggregations on vegetation, to infer changes in monarch abundance along the fall migration route over the last 17 years. We found that the timing of migration remained relatively unchanged while the flyway has generally become warmer and greener. Warmer and greener conditions were associated with larger roosts, yet we found steady, dramatic declines in roost sizes through time that were independent of climate and landscape factors. Roost sizes have declined as much as 80%, with losses increasing from north to south along the migration route. These findings suggest that failure during the fall migration could explain the apparent drop in monarch numbers from summer breeding to overwintering populations. This in turn suggests that conservation efforts that support fall migration success are most needed, such as limiting the planting of non-native milkweeds that enhance parasite loads and perhaps dampen migration success. Overall, it appears the fall migration of monarch butterflies is under imminent threat, even if the species’ overall survival is not.

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Complexity, humility, and action: a current perspective on monarchs in Western North America

Yang

2023

Current Opinion in Insect Science

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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

Abstract: A recent study in this journal aimed to understand certain changes in the wintering behavior of monarch butterflies, specifically in the western subpopulation of North America [...]

Cited by 4 publications

References 30 publications

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

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

Dramatic Recent Declines in the Size of Monarch Butterfly (Danaus plexippus) Roosts During Fall Migration

Complexity, humility, and action: a current perspective on monarchs in Western North America

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