A paper co-authored by Christian A. Johnson Professor of Biology Ernest Williams titled "Decline of monarch butterflies overwintering in Mexico: is the migratory phenomenon at risk?" is the subject of an article on the homepage of Science News dated April 4.
As described in the paper's abstract and summarized in the Science News article, the researchers found that "during the 2009–2010 overwintering season and following a 15-year downward trend, the total area in Mexico occupied by the eastern North American population of overwintering monarch butterflies reached an all-time low. Despite an increase, it remained low in 2010–2011. Although the data set is small, the decline in abundance is statistically significant using both linear and exponential regression models.
"Three factors appear to have contributed to reduce monarch abundance: degradation of the forest in the overwintering areas; the loss of breeding habitat in the United States due to the expansion of GM herbicide-resistant crops, with consequent loss of milkweed host plants, as well as continued land development; and severe weather. This decline calls into question the long-term survival of the monarchs’ migratory phenomenon." The research was published in Insect Conservation and Diversity.
In the Science News article, Williams noted that during the past 17 years, the area of Mexican forest patches covered by overwintering butterflies has been shrinking overall. He explained that, "For butterflies in Mexico and in their summer range, we see a confluence of threats.”
Winter monarch retreats are officially protected, but illegal logging chews away at Mexico’s forests. Plus, looming climate changes may bring more episodes of severe weather, which can hammer the butterflies. And monarchs across North America are finding less breeding habitat than they used to. Open land for milkweeds is falling to development, and researchers warn that a boom in genetically engineered crops is changing herbicide use patterns and thinning the ranks of milkweeds.