The monarch butterfly fluttered a step closer to extinction Thursday, as scientists put the iconic orange-and-black insect on the endangered list because of its fast dwindling numbers.

"It's just a devastating decline," said Stuart Pimm, an ecologist at Duke University who was not involved in the new listing. "This is one of the most recognisable butterflies in the world."

The International Union for the Conservation of Nature added the migrating monarch butterfly for the first time to its "red list" of threatened species and categorised it as "endangered" — two steps from extinct.

The group estimates that the population of monarch butterflies in North America has declined between 22 per cent and 72 per cent over 10 years, depending on the measurement method.

Monarch butterfly in Victoria Park, Auckland. Photo / Michael Craig, File

"What we're worried about is the rate of decline," said Nick Haddad, a conservation biologist at Michigan State University. "It's very easy to imagine how very quickly this butterfly could become even more imperiled."

Haddad, who was not directly involved in the listing, estimates that the population of monarch butterflies he studies in the eastern United States has declined between 85 per cent and 95 per cent since the 1990s.

In North America, millions of monarch butterflies undertake the longest migration of any insect species known to science.

After wintering in the mountains of central Mexico, the butterflies migrate to the north, breeding multiple generations along the way for thousands of kilometres. The offspring that reach southern Canada then begin the trip back to Mexico at the end of summer.

"It's a true spectacle and incites such awe," said Anna Walker, a conservation biologist at New Mexico BioPark Society, who was involved in determining the new listing.

A smaller group spends winters in coastal California, then disperses in spring and summer across several states west of the Rocky Mountains. This population has seen an even more precipitous decline than the eastern monarchs, although there was a small bounce back last winter.

Monarch butterflies in a Taradale tree. Photo / Paul Taylor, File

Emma Pelton of the nonprofit Xerces Society, which monitors the western butterflies, said the butterflies are imperiled by loss of habitat and increased use of herbicides and pesticides for agriculture, as well as climate change.

"There are things people can do to help," she said, including planting milkweed, a plant that the caterpillars depend upon.

Nonmigratory monarch butterflies in Central and South America were not designated as endangered.

The United States has not listed monarch butterflies under the Endangered Species Act, but several environmental groups believe it should be listed.

The international union also announced new estimates for the global population of tigers, which are 40 per cent higher than the most recent estimates from 2015.

The new figures, of between 3726 and 5578 wild tigers worldwide, reflect better methods for counting tigers and, potentially, an increase in their overall numbers, said Dale Miquelle, coordinator for the nonprofit Wildlife Conservation Society's tiger programme.

In the past decade, tiger populations have increased in Nepal, northern China and perhaps in India, while tigers have disappeared entirely from Cambodia, Laos and Vietnam, said Miquelle. They remain designated as endangered. - AP