North American monarch butterflies living east of the Rocky Mountains could be on the way to extinction as soon as 20 years from now, said a new study.
The research is based partly on long-term declines in the butterfly's overwintering population, and was conducted by a team from the U.S. Geological Survey and Scripps Institution of Oceanography at the University of California, San Diego. For example, in the time between the winter of 1996-1997 and the winter of 2014-2015, the Eastern migratory monarch population fell by 84 percent.
Knowing about that decrease, the team showed that there is an 11 to 57 percent chance that that population could go quasi-extinct over the next two decades. Quasi-extinct means that so few individuals live that recovery cannot occur. In this "last monarchs standing" scenario, individuals could survive for a short while, but the population would inevitably become 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," noted Brice Semmens, the study lead author and a Scripps scientist.
Backing up this idea of variability, a recent report from the World Wildlife Fund Mexico and its partners noted a significant increase in monarch numbers since last year. But that jump in population was quickly followed by a winter storm that may have had a negative effect on the population. Good winters for the population are positive news, but it's still necessary to gain higher average monarch numbers in order to decrease the risk of quasi-extinction.
In the study, the scientists looked at the geographic area covered by a monarch colony while wintering in Mexico in order to measure population size. The aim of researchers in Mexico, the United States and Canada is to ramp up Eastern monarch numbers wintering in Mexico so that the geographic area takes up about 15 acres by 2020. As of this year, the population size has increased and reached nearly 10 acres. It was as low as 2.8 acres in the winter of 2014-2015, and even lower the winter before that at 1.7 acres.
The team learned that if the Eastern monarch population can reach the 15-acre goal, the risk of quasi-extinction over 20 years will fall by greater 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," noted USGS scientist Darius Semmens, a report co-author. "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."
The findings were published in the journal Scientific Reports.
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