Steep regional population declines are leading the species toward the brink
While there has been some recent good news about short-term prospects for monarch butterflies, a new study led by scientists with theU.S. Geological Survey and the Scripps Institution of Oceanography at UC San Diego warns that the eastern population of the species could become extinct within a few decades.
The new study, published in the journal Scientific Reports, concluded that there was huge drop in the number of migrating monarchs. The study calculated the decline at 84 percent between 1997 and 2015. Based on that trend, there is a chance that the Eastern migratory monarch population could drop low enough to make extinction inevitable.
“Because monarch numbers vary dramatically from year to year depending on weather and other factors, increasing the average population size is the single-most important way to provide these iconic butterflies with a much-needed buffer against extinction,” said Brice Semmens, the lead author of the study and a scientist at Scripps.
Semmens said that as an example of this variability, just after the analysis concluded, the World Wildlife Fund Mexico and partners reported a large increase in monarch numbers since last year. However, this increase was followed by a recent winter storm that may have adversely affected the population. The authors emphasized that although one good winter – as occurred this year – is positive news, higher average monarch numbers are necessary for reducing the long-term risk of quasi-extinction.
Since scientists can’t count individual butterflies, their assessments of population health are based on the geographic area occupied by overwintering populations in Mexico. Conservation scientists are targeting a goal of about 15 acres. In recent years, that figure has dropped as low 1.7 acres during the winter of 2013-2014.
The new study found that, if the Eastern population reaches the six-hectare goal announced in last year’s national pollinator strategy, the quasi-extinction risk over 20 years would decrease by more than half.
“Previously published research suggested that the most effective way to increase monarch numbers is to focus on the restoration of their breeding habitat,” said USGS scientist Darius Semmens, a coauthor of the report. “Over the previous two winters, Eastern monarch populations were very low, indicating a higher risk of losing the species. If their numbers continue to grow, as they did this year, the risk will decrease.”
Scripps and the USGS collaborated with scientists from the University of Arizona, Iowa State University, University of Minnesota and the University of Kansas on the study. The research was conducted as part of the Monarch Conservation Science Partnership, a team of scientists and resource managers working together to help inform the management of monarch butterflies. The partnership was hosted by the USGS Powell Center for Analysis and Synthesis in Fort Collins, Colorado.
Scientists aren’t completely certain about the cause of the monarch decline, but loss of breeding habitat and impacts from pesticides are suspected as primary factors.
There are two main populations of monarch butterflies in North America: the Western, which winters and migrates west of the Rocky Mountains, and the more abundant Eastern population, which is the subject of the new study and ranges east of the Rockies from central Mexico to southern Canada. Eastern monarchs breed in the United States and Canada, and migrate to Mexico for the winter. Western monarchs migrate inland north and east from colonies along the California coast to states ranging from Washington to Arizona. The U.S. Fish and Wildlife Service is considering a petition to protect monarchs under the Endangered Species Act.