Are North American monarch butterflies really dying out?

Migrating monarch butterflies in North America have attracted a lot of attention over the past few decades, as their winter population numbers have juddered downwards.

But a new study on their summer populations brings hope, suggesting that the monarchs are still breeding in numbers when it’s warm. This means that their overall population may not be declining at all.

“There’s this perception out there that monarch populations are in dire trouble, but we found that’s not at all the case,” says Dr Andy Davis, an assistant research scientist at the Odum School of Ecology in the University of Georgia, US, and corresponding author on a paper describing the research, published in Global Change Biology.

“There are some once-widespread butterfly species that now are in trouble,” says co-author Professor William Snyder, from the University of Georgia’s College of Agricultural and Environmental Sciences.

“So much attention is being paid to monarchs instead, and they seem to be in pretty good shape overall. It seems like a missed opportunity.

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“We don’t want to give the idea that insect conservation isn’t important because it is. It’s just that maybe this one particular insect isn’t in nearly as much trouble as we thought.”

The researchers examined 135,000 monarch observations, taken between 1993 and 2018 by the North American Butterfly Association.

This association conducts around 450 butterfly counts every summer, where citizen scientists patrol a 24-kilometre-diameter circle, tallying all the butterflies they see.

Monarch butterflies. Credit: U.S. Fish & Wildlife Service

The researchers found that monarch numbers were increasing at an average of 1.36% each year. This means that, while the winter population may be declining, summer breeding is still capable of making up for the loss.

Declining habitats along the east and west coastlines of North America may be preventing the southern butterfly migration.

“But when they come back north in the spring, they can really compensate for those losses,” says Davis.

“A single female can lay 500 eggs, so they’re capable of rebounding tremendously, given the right resources. What that means is that the winter-colony declines are almost like a red herring. They’re not really representative of the entire species’ population, and they’re kind of misleading. Even the recent increase in winter colony sizes in Mexico isn’t as important as some would like to think.”

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