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Tropical Butterflies Spread as Monarchs Dwindle in East Asia


FILE - Butterflies land on branches at Monarch Grove Sanctuary in Pacific Grove, Calif., on Nov. 10, 2021.

Sparked by global warming and other forms of climate change, tropical butterflies are starting to arrive in Hong Kong and Taiwan in greater numbers, while temperate-zone species like the monarch appear to be dwindling in the region, conservationists told RFA.

"Seven new butterfly species were discovered in Hong Kong in 2021, including swallowtails, gray butterflies, and nymphs; most of them were tropical species," Gary Chan, project officer at Hong Kong's Fengyuan Butterfly Reserve, told RFA.

"Breeding records were found in Hong Kong for several of these species, which indicates that these weren't just strays arriving in Hong Kong with horticultural imports or the monsoon," Chan said.

According to Chan, Neptis cartica and Ancema blanka were both found in Hong Kong for the first time in 2021, along with Zeltus amasa, which is usually native to Malaysia, Thailand, India, Myanmar, Borneo and other points south of Hong Kong.

Meanwhile, tropical migrants are also being spotted in Taiwan, according to Hsu Yu-feng, a butterfly expert at Taiwan National Normal University.

Between 1985 and 2008, at least seven new species of tropical butterfly were found to have settled on the island, including Appias olfern peducaea, which traveled north from the Philippines to settle in the southern port city of Kaohsiung in 2000.

Even butterflies once found only in southern Taiwan are now found across the island, Hsu told RFA.

"When I was an undergraduate student in the 1980s, I went to Kenting [on the southern coast] to see Graphium agamemnon," Hsu recalled. "Then, I saw it for the first time on this university college campus last year, and it was breeding here."

Southeast Asia warming faster

Troides aeacus kaguya is another example of a butterfly that once only lived in southern Taiwan, and can now be found all over the island, he said.

The changes come as temperatures in East and Southeast Asia have risen more rapidly than the global average in recent decades, Chan said.

"There are many more places where tropical butterflies and other insects can breed, so that's why we're seeing this northward migration, or dispersal behavior," he said.

Hsu said the butterflies didn't actually migrate, however; rather, their habitats are expanding due to rising temperatures.

"Once upon a time, the more northerly areas were colder, and not suitable for them to settle in, but they are suitable now, because temperatures have risen," Hsu said.

"The north is warming at a higher rate than the south, meaning the difference in temperatures between north and south has been reduced," he said. "That's why southern butterflies are now living in the north."

But the changes are forcing out butterflies that need a temperate climate to breed in, experts said.

Few monarchs now

The Siu Lang Shui conservation site in Hong Kong's Tuen Mun district once saw tens of thousands of monarch butterflies spending the winter, as recently as 2013 and 2014, Chan said.

But numbers have fallen sharply in recent years, he said.

In Taiwan, the purple variegated butterflies that once overwintered in their millions in the Maolin valley outside Kaohsiung have also been dwindling in recent years, preferring to move north to seek out colder temperatures earlier in the year.

The warming environment is also becoming more hostile to temperate tree species some butterflies call home, Hsu told RFA.

"The Taiwan-endemic butterfly Sibataniozephyrus kuafui uses the Taiwan beech as a host plant ... so in contrast to the expansion of tropical butterflies, temperate species are being threatened," he said.

The changes in East Asia come after a study published in the journal Science in 2021 found that populations of most butterfly species in western North America have declined by nearly 50% over the past 40 years.

"California is the place with the most endemic species of butterflies in western North America," Hsu said. "California butterflies are most vulnerable to drought, because this is a Mediterranean climate zone, with dry summers and rainy winters."

"If climate change causes droughts in winter, plants will grow poorly, and the larvae of butterflies will have nothing to eat," he said.

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