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Scientists studying monarch butterflies traditionally focus on winter habitat loss in Mexico and fewer milkweed plants in the Midwest. But that misses a big part of the puzzle—the butterfly’s fall migration, according to a new study.

Focusing on this southerly trek, as well as changing the scale at which researchers examine winter populations, offers a wider, more-accurate spectrum of threats that contribute to the monarch population’s downward trend.

“Getting accurate monarch counts in the summer is tough,” says Sarah Saunders, a former integrative biologist at Michigan State University and lead author of the paper, which appears in the Proceedings of the National Academy of Sciences.

“Finding them in the fall, though, is nearly impossible as they’re moving hundreds of miles daily.”

Missing piece

Monarch butterfly numbers have been dropping precipitously for more than two decades—partly due to shrinking winter habitat and increased herbicide use in the Midwest that eliminates their host plants from summer breeding grounds. However, herbicide use and habitat loss have diminished over the past decade, yet monarchs continue to decline.

These key facts told scientists that a big part of the story was left out, says senior author Elise Zipkin, an integrative biologist.

“Migratory periods are missing from most research because they are the most difficult periods to investigate,” she says. “For monarchs, the fall is the least-studied season because the data are so sparse and are generally opportunistic.”

Multiple threats

For the study, Saunders and her team built a multi-scale model and included a summer population index to account for year-to-year variations. They then added environmental factors occurring during the fall migration, such as temperature and landscape greenness.

A brown, dry fall means fewer nectar resources—where monarchs get their energy—to fuel them on their journey.

OE (Ophryocystis elektroscirrha), a parasite that infects monarch and causes deformed or smaller wings, also pose a threat. Understanding the changing threats affecting animal migration is key to conservation, says Elizabeth Blood, a program director at the National Science Foundation, which funded the work.

“The survival of migratory animals has been an important area of investigation as climate changes spark ecosystem and migratory pattern changes,” she says. “This research boosts our understanding of larger processes—from regional to even continental scales—and the impact they have on migratory animals.”

Four seasons

The researchers re-evaluated the long-standing base on which scientists make monarch estimations. Rather than consider the winter population as a single entity, the new model used the numbers from all 19 known colonies individually.

“We’re the first to examine the winter colonies this way,” Zipkin says. “If you aggregate the winter colony data, you can get the wrong result because there are important differences in habitat quantity and quality at the individual colony sites.”

“Unsurprisingly, our model shows that all seasons are important; summer, fall, and winter factors are all connected,” says Saunders, who’s now a quantitative ecologist at the National Audubon Society.

“In particular, we found that landscape greenness during the fall migration, in addition to the peak summer population size and the amount of habitat at local winter colonies, were the key factors influencing the winter population size.”

Additional scientists from Georgetown University, Universidad Nacional Autónoma de México, Monarch Fund (Mexico), and World Wildlife Fund Mexico contributed to the study. The National Science Foundation funded the work.

Source: Michigan State University