The number of monarch butterflies wintering in Mexico dropped by 27 per cent this year, reversing last year's recovery from historically low numbers, according to a new study.
The experts say the decline could be due to late winter storms last year that blew down more than 40 hectares of forests where migrating monarch butterflies spend the winter in central Mexico.
Millions of monarchs make the 5,500-km migration from the United States and Canada each year, and they cluster tightly in the pine and fir forests west of Mexico City. They are counted not by individuals, but by the area they cover.
"The reduction in the area of forest they occupied this year is most probably due to the high mortality caused by storms and cold weather last year," head of the Mexico office of the World Wildlife Fund Omar Vidal said.
"It is a clear reminder for the three countries that they must step up actions to protect breeding, feeding and migratory habitat."
Officials estimate the storms in March killed about 6.2 million butterflies, almost 7.4 per cent of the estimated 84 million that wintered in Mexico, said Alejandro Del Mazo, Mexico's commissioner for protected areas.
The monarchs were preparing to fly back to the US and Canada at the time the storm hit.
While no butterfly lives to make the round trip, a reduction in the number making it out of the wintering grounds often results in a decline among those who return the next year.
A major cause in the drop of butterfly numbers is also illegal logging.
Last week, authorities detained a man trying to truck about a dozen huge tree trunks out of the butterfly reserve.
The butterfly itself is not endangered, but the migration does appear to be. Not all monarchs migrate.
Politics may also cause a problem for the butterflies. Environmental cooperation was close with the administration of former US president Barack Obama, but it is unclear whether that will continue under President Donald Trump, who has pledged to build a wall along the US-Mexico border.
"We are worried that there could be an environmental wall, and that its first direct victim might be the monarch butterfly," said writer and environmentalist Homero Aridjis.
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