Opposing global change drivers counterbalance trends in breeding North American monarch butterflies
Crossley MS, Meehan TD, Moran MD and others. Global change biology, June 2022.
Read the paper Free to read · doi:10.1111/gcb.16282
What this paper is about
More than 135,000 monarch records from the North American Butterfly Association's annual count, covering 1993 to 2018 across the whole breeding range.
Overwintering numbers have clearly fallen; summer breeding numbers are harder to read. The authors find opposing pressures that roughly cancel out in the breeding data.
Summarised here from the abstract below. Not the authors’ words.
Abstract
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.
Abstract reproduced from the published paper, which is licensed CC BY. Copyright remains with the authors.
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@misc{milkweedusa_researchopposingglobalchange,
title = {Opposing global change drivers counterbalance trends in breeding North American monarch butterflies (listing)},
author = {{Milkweed USA}},
year = {2026},
url = {https://milkweedusa.com/research/opposing-global-change-drivers-counterbalance-trends-in-breeding-north-2022/},
note = {Accessed DATE}
}