Monarch butterflies east of the Rocky Mountains have a significant chance of going extinct in the next two decades, said a San Diego researcher on Tuesday.
The eastern population of the iconic orange and black butterfly fell precipitously from 1996 to 2015. The monarchs lost about 84 percent of their population during that span, and that decline greatly increases chances of the migratory eastern butterflies going extinct.
The butterflies are suffering because milkweed is getting scarce.
"Recovery for the population really depends on the amount of breeding habitat," said Brice Semmens, a researcher at the Scripps Institution of Oceanography. "And breeding habitat means the amount of milkweed that's available for reproduction. Monarchs rely exclusively on milkweed to reproduce."
Semmens put together the population assessment with the U.S. Geological Survey.
Monarch Watch has developed a recovery plan for the species. It involves planting milkweed in the Midwest where agriculture has largely eradicated the plant. The recovery effort calls on large scale reintroduction of milkweed into the natural habitat.
There is an 11 percent to 57 percent chance that the migratory flock's population will fall below sustainable levels in the next 20 years.
The risk of extinction could be cut in half if the breeding area in Mexico's forests grows by a third.
Western monarchs have also suffered population drops in recent years, but the population hasn't been assessed by researchers.
"That doesn't mean that the western population is not imperiled. It just means that we haven't done a formal assessment to figure out to what extent the western monarchs are at risk," Semmens said.
The population assessment of the eastern monarch was published in the journal Scientific Reports.
