LA JOLLA (CNS) - The largest segment of the North American monarch butterfly population could be headed toward quasi-extinction over the next couple of decades, according to a study released Monday by the Scripps Institution of Oceanography at UC San Diego and U.S. Geological Survey.

The study, published in the journal Scientific Reports, found that the number of the orange butterflies that migrate into the area of the U.S. and Canada east of the Rocky Mountains declined by 84 percent between the winter of 1996-97 and the winter of 2014-15. The butterflies spend their winters in central Mexico.

The researchers determined the chance of quasi-extinction -- a point where the numbers become too few to recover a species -- is 11 percent to 57 percent over the next 20 years. According to Scripps Oceanography, remaining members of a species might survive for a short time in such a worst-case circumstance, but the population as a whole will inevitably go extinct.

"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 Scripps biologist Brice Semmens, who studies extinction risk in fish and other animal populations.

The World Wildlife Fund Mexico and partners detected a large increase in monarch numbers since last year, but the butterflies might have subsequently been adversely affected by a winter storm. The scientists said a one-year population increase was good news, though higher averages over time are needed to reduce the risk of quasi-extinction.

Since counting butterflies individually is all but impossible, researchers observe the number of acres in Mexico covered by wintering monarch colonies.

According to the researchers, the butterflies filled just over 1 1/2 acres at their lowest point in the winter of 2013-14. The next winter, it was nearly 2.8 acres, and the winter that just ended saw a huge increase to 10 acres.

Officials and scientists in the U.S., Mexico and Canada hope to bump up the population to cover 15 acres by 2020.

The eastern population studied by the researchers is one of two, and the most abundant. The western monarchs stay west of the Rocky Mountains, according to Scripps.