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On occasion, however, the region is hit with torrential rains, which cause a surge in plant life. The caterpillars gorge on the flora cornucopia, and the butterfly numbers balloon. There are also few parasites and predators to keep them in check, because painted ladies aren’t showing up in abundance from one year to the next, said Jason Chapman, a co-author of the paper and associate professor at the University of Exeter who specializes in animal migration. (Chapman suspects part of the reason they migrate is to avoid parasites and predators.)
Taken together, “that results in enormous numbers of painted ladies arriving in Europe — in the UK, in Sweden and Germany, and other countries that are so far away” from sub-Saharan Africa, he said.
By linking the population in Europe to sub-Saharan Africa, the study also reveals that these butterflies are expert migrators. Over several generations, they travel as much as 8,700 miles round-trip, and unlike many insects, they appear to be crossing one of the largest deserts in the world. “It’s the longest migratory cycle that has been described for an insect,” said Stefanescu. “It’s really exceptional.”
The researchers also found that wind patterns likely play a role in the migration. Unlike birds, butterflies are at the mercy of winds, and depend on them to cross the desert.
The far-reaching impacts of climate change
The study reveals how ecosystems in disparate parts of the world are connected. And that carries important implications for conservation and pest control.
Consider the fall armyworm, a pest native to tropical and subtropical regions of the Americas that feeds on corn. In 2016, it arrived in Africa, and now researchers fear that its adult moth form could make its way to Europe.
A big remaining question is whether and how it might cross the Sahara. According to Keith Cressman, who studies migratory pests at the Food and Agriculture Organization (FAO) of the UN, the fall armyworm’s flight abilities are not that unlike the painted lady’s. “The fall armyworm could do the same thing,” he said, following the same route to Europe.
The study also serves as a reminder that the impacts of climate change will be complicated and far-reaching. It might make a particular region drier or wetter and in doing so affect places thousands of miles away. “We need to think on a big scale, especially for migratory species,” Oberhauser said.
Climate change could, for example, make cyclones more common or severe. That’s frightening, in part because “historically, cyclones have been the originators of locust plagues,” said Cressman, a senior locust forecaster at FAO. “That’s very serious.” An ongoing locust outbreak in Africa and the Middle East, which began in 2018, started due to unusual rainstorms in part of eastern Saudi Arabia, Cressman said.
There are also migratory insects that conservationists are trying to save, not control, such as the monarch butterfly. The population of Eastern monarchs, the most common species in North America, has declined by more than 80 percent in the past 20 years, and Oberhauser said climate change is likely to make things worse. Understanding the migration of species helps scientists figure out which habitats are especially in need of protection.
As for the painted ladies, they seem to be doing just fine, even as the climate changes. In sub-Saharan Africa, rainstorms are becoming more erratic — but also more severe. “These changes will undoubtedly impact painted lady generations south of the Sahara and likely influence immigrations to Europe,” the study authors write. “But exactly how these changes will manifest is difficult to predict.”
A butterfly effect, indeed.
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