The Monarch Butterfly Biosphere Reserve is a UNSECO-listed world heritage site in Michoacán, Mexico.
The area covered by monarch butterflies overwintering in Mexico decreased 53 per cent this year, new government data shows. And while cooler weather and the effect of pesticides are prominent factors in the drop off, the decline also highlights ongoing habitat loss due to illegal deforestation.
Mexico’s Commission for Natural Protected Areas said that monarchs covered an area of 2.83 hectares (ha) between November 2019 and March 2020 in 11 colonies studied in the states of Michoacán and Mexico.
This was a decrease of 53 per cent from the 6.05ha observed the previous year.
Given the density of butterflies grouped together, a measurement is taken from a collective area rather than as an individual count.
“The last season, 2018-19, was very good, with 6.05ha of forest cover, but it was certainly atypical, thanks to the fact that the first generation of butterflies in the spring of 2018 encountered favourable weather conditions to reproduce,” Jorge Rickards, general director of WWF Mexico, said.
Colder weather in southern US states during the last Spring period meant butterflies were reproducing less and thus led to a decline in population, according to WWF.
The use of herbicides on US crops is said to be another reason behind the decrease.
“Monarch caterpillars only eat milkweed, but the plant has been devastated by increased herbicide spraying in conjunction with corn and soybean crops that have been genetically engineered to tolerate direct spraying with herbicides,” the Center for Biological Diversity (CBD) in the US said in a statement.
Millions of monarch butterflies make the 4,000km journey from the US and Canada to hibernate in Mexico each year.
Homero Aridjis, a famed Mexican environmentalist and poet, is quoted by the Associated Press as saying that climate change also played a significant role and that crime and deforestation remained a “cause for alarm”.
“The decline of over 53 per cent of populations in the butterfly reserve is worrisome, above all because of the effects of climate change on the migration route and on the wintering grounds in Mexico,” Aridjis said.
Tierra Curry, a senior scientist at the CBD, added: “Scientists were expecting the count to be down slightly, but this level of decrease is heartbreaking.
“Monarchs unite us, and more protections are clearly needed for these migratory wonders and their habitat.”
Land clearances for crops including avocado in Michoacán and surrounding areas have become a growing concern in recent years.
Illegal avocado crops at the Monarch Butterfly Biosphere Reserve, a UNSECO-listed site in Michoacán, were discovered in 2018.
Global Forest Watch data indicates that Michoacán lost 10,000ha of tree cover in 2018, almost double that lost in 2017.
Attention fell on the reserve in 2019 following the death of two activists who campaigned for the butterfly’s conservation and against illegal logging in the reserve.
Campaigns by conservationists to end illegal logging operations, often started to enable crop production, has led to a sharp decline in deforestation within the reserve’s core area in recent years.