BOZEMAN – From 2003 to 2019, amateur butterfly enthusiast Harlan Radcliff spent his lunch breaks observing butterflies on the grounds of the Camp Dodge military installation in central Iowa.
Over those years, between the months of April and November, Radcliff meticulously recorded the dates, times, locations and numbers of the butterflies he spotted, creating a dataset that made it possible for a Montana State University ecologist to pinpoint how climate variations have affected the migratory patterns of Camp Dodge's monarch butterfly population in the 21st century. The results were published this week in the Journal of Animal Ecology.
Diane Debinski, professor and head of MSU's Department of Ecology in the College of Letters and Science , is the lead author of the paper, which details the findings of research she led at Camp Dodge in the core of the eastern monarchs' summer breeding range. The work was funded by the U.S. Department of Defense as part of a larger study of the effects of climate change on three ecologically significant species of butterflies that use DoD lands and whose populations are declining.
Debinski, one of four principal investigators for the project, led the study of eastern monarchs, who winter in Mexico and spend summers east of the Rockies. Monarch butterflies – including western monarchs, who winter in California and spend summers west of the Rockies – were recently proposed for listing as threatened under the Endangered Species Act. The paper's co-authors include three other butterfly ecologists: Elizabeth Crone of the University of California, Davis; Sonia Altizer of the University of Georgia; and Norah Warchola of Iowa State University.
Debinski said her team expected to find that environmental variation and climate change were affecting the phenology, or timing, of important events for the Camp Dodge monarch population, including arrival and departure dates and the timing of their peak summer abundance in the field.
"We wanted to look at phenology of butterflies and test whether it was shifting, and if it was, to ask how that might affect the viability of the populations over the long term," she said.
Similarly, the team looked at phenological changes of the Camp Dodge monarchs' host plant, common milkweed, where females lay their eggs, and which serve as a major source of food for the camp's population of monarch caterpillars. The researchers wanted to know whether any phenological changes in the milkweed were occurring in tandem with those of the butterflies.
"If one shifts and the other doesn't, you could have a situation called asynchrony, where the butterflies arrive and the milkweed isn't growing yet, or maybe the plant has grown too early and dried out," Debinski explained. For historic information about Midwestern milkweed growth, the researchers consulted data kept by herbariums in Iowa, Minnesota and Wisconsin.
The field component of the study was done in 2020. Monarch eggs were placed on milkweed plants at staggered intervals to assess whether a shift in monarch spring arrival times – if one existed – affected how the larvae developed. One group was placed two weeks earlier than the monarchs' historic recorded arrival date, another at the expected date, and a third about two weeks later than usual.
The researchers designed the test according to the effect they expected to see: an earlier spring arrival date of the monarchs at Camp Dodge. Instead, Debinski said, they found that the timing of the monarchs' arrival had not changed but that by 2019, the butterflies were staying for nine days longer in the fall than they had in 2003. The observed shift for milkweed was similar, with the plant blooming later into the season, also by about nine days.
"What we saw was that milkweed herbarium specimens were collected later over time, indicating that they were blooming longer over time, and monarchs were in the field as adults longer over time, which means that the fall season was actually indicating a change," Debinski said. "This means that we need to look more at the phenology of the fall season, given that it could have implications for population dynamics over time. A longer season could have benefits, and it could also have costs."
The costs could include longer exposure for the insects to predators or parasites, or the creation of what ecologists call a "development trap," which occurs when a new generation embarks on a phase of seasonal development that it doesn't have time to complete. For example, eggs could be laid so late that they are unable to develop into adults, or adults could emerge from their chrysalises when conditions are no longer suitable for fall migration.
Though the field survey showed that monarch arrival times did not change between 2003 and 2019, the experiment indicated that such a change would be costly. In the Camp Dodge experiment, early-season larval survival was higher when the timing of the hatch matched that of the natural monarchs, while survival was lowest when egg hatching shifted two weeks earlier. That finding might be relevant for monarch populations in other areas of the eastern range, Debinski said, especially if the species of milkweed they depend on is undergoing different phenological shifts than common milkweed.
Though Radcliff, the enthusiast who gathered the original Camp Dodge data, has since passed away, Debinski said the legacy of the notes he kept for 17 seasons were "an amazing gift" to the study, which also revealed that monarch population growth at Camp Dodge declined between 2003 and 2019 – a finding consistent with U.S. Fish and Wildlife Service statistics showing significant decline over past decades.
"Discussions about phenology often exclude migratory butterflies, because they experience climates in multiple places," she said. "Our research showed that even migratory species can show shifts in phenology. The more information we have about phenology, population sizes and population growth rates, the better we will be able to conserve monarch butterflies and many other species of conservation concern."
