SAN FRANCISCO (CBS SF) – The population of monarch butterflies east of the Rockies is facing significant risk of going extinct within the next two decades, according to research published Monday.

Researchers at the Scripps Institution of Oceanography at UC San Diego and the U.S. Geological Survey found the eastern butterfly population declined by 84 percent from the winter of 1996-1997 to the winter of 2014-2015.

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The study found that the eastern monarch butterfly population has an 11 to 57 percent risk in the next 20 years of reaching “quasi-extinction,” a population so low that it would be impossible to recover.

Researchers said the population must increase five-fold to reduce the extinction risk by half.

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“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,” USGS scientist and study co-author Darius Semmens said in a University of California statement.

Numerous factors could be behind the population decline, including a loss of breeding habitat, adverse weather, disease and chemical exposure.

Unlike the western monarch butterfly, which migrates to California every winter, the eastern population migrates to Mexico each winter from as far north as Canada.

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The research has been published in the journal Scientific Reports.