Monarch butterfly population decline in North America: identifying the threatening processes

Due to dramatic declines of eastern migratory monarch butterflies over the last two decades, this study assessed a number of variables to determine long-term drivers of their decline. The variables studied fit within the broad categories of survival, reproduction and habitat availability.  The researchers recognized the challenges of discerning causal relationships due to the complexity of ecological process. Still, they were able to find trends and associations that help clarify drivers in monarch decline to inform restoration efforts.

Slow moving variables relating to habitat amount and quality were found to be the long-term drivers for population declines. Within that, the use of glyphosate (a proxy for loss of host and nectar plants) was the stressor most highly associated with decline. Both neonicotinoid use in the breeding period and the loss of overwintering forest had weaker negative associations with population size. Annual monarch population changes were more strongly influenced by climate.  

In the discussion, the researchers postulated little additional loss of habitat around agricultural fields since the dominant crops, corn and soy, are 92% glyphosate resistant. The researchers went on to state that programs such as the U.S. Department of Agriculture’s Conservation Reserve Program (CRP) could help offset habitat loss by providing high-quality habitat for monarch butterflies.

Thogmartin, W. E., R. Wiederholt, K. Oberhauser, R. G. Drum, J. E. Diffendorfer, S. Altizer, O. R. Taylor, J. Pleasants, D. Semmens, B. Semmmens, R. Erickson, K. Libby, L. Lopez-Hoffman
Royal Society Open Science
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