Good news for monarch butterflies?

WILLIAMSBURG, Va. - Aug. 22, 2023 - PRLog -- The migratory monarch butterfly might not be as endangered as previously thought, according to a new study published in Current Biology.

Humans may have artificially inflated the monarch population by making changes to the habitat of Eastern and North America. The numbers we see now may reflect an expanded population from precolonial sizes.

"This doesn't mean we shouldn't strive to have more pollinator-friendly habitats for monarchs and beneficial insects," said senior author Joshua Puzey, an associate professor in the William & Mary biology department.

Previous research from Boyle, Dalgleish and Puzey had disproved that genetically modified crops were the main culprits for the decline of the monarch butterfly (Danaus plexippus) and its dominant food source, the common milkweed (Asclepias syriaca).

In this new paper, researchers widened their focus to the past 25,000 years. They sequenced DNA from milkweeds and monarchs and tested several hypotheses using Approximate Bayesian Computation via Random Forests, a machine learning method for reconstructing demographic histories.

The researchers found that the numbers of milkweeds and monarchs increased after the last glacial period.

Another increase was detected in the 18th and 19th centuries, when deforestation and expanded agricultural fields created beneficial conditions for the common milkweed in Eastern North America.

What the study did not detect was an effective population decline in both monarchs and milkweeds over the past 75 years, a period that corresponds to an expanded use of chemicals in agriculture. This reduction may have been either too small or too recent to leave a signal in the researchers' data set, but the easiest explanation, as the study suggests, is that it simply did not happen.

The decline observed over the past 40 years may reflect a mismatch between the monarch population that overwinters in Mexico and the species' effective population size. However, the authors reiterate that their study should not directly impact current conservation efforts.

"What I really like about the monarch is that it's a signal to the world that insects are a vital part of our ecosystem," said Puzey. "Their population can be fragile, and we need to be concerned with not only the macroscopic organisms but also the smallest living organisms."

According to Puzey, the study's hypothesis demonstrates that humans have a massive impact on the landscape.

"What we see now is not necessarily what has always been, but we should focus our efforts in keeping things healthy and stable," he said.

Link to the study: https://doi.org/10.1016/j.cub.2023.07.057