The population of the Eastern monarch butterfly declined 84 percent in less than two decades, says a team of researchers. They calculate that “the population has a substantial probability of quasi-extinction, from 11–57 percent over 20 years, although uncertainty in these estimates is large.”
A quasi-extinction means the population drops so low that recovery is impossible and extinction is inevitable, says the U.S. Geological Survey, whose scientists took part in the study. Brice Semmens, lead author of the study and a scientist at Scripps Institution of Oceanography, said “increasing the average population size is the single-most important way to provide these iconic butterflies with a much-needed buffer against extinction.”
A five-fold increase in population size, compared to the winter of 2014-15, “is necessary to halve the current risk of quasi-extinction,” say the scientists in their paper. “Conserving the monarch migration thus requires active management to reverse population declines, and the establishment of an ambitious target population size goal to buffer against future environmentally driven variability.”
The United States, Canada and Mexico have a goal of increasing the population of the butterfly so it occupies six hectares, or about 15 acres, of wintering territory by 2020. The butterflies occupied 1.13 hectares, or 2.8 acres, during the winter of 2014-15, and a scant 0.67 hectares (1.7 acres) at their low point in 2013-14. A recent survey said the population this winter was four hectares (10 acres), an encouraging report that was followed by a winter storm.
On March 10, two conservation groups sued the Interior Department to force it to decide whether to protect the monarch as an endangered species. The groups say population decline has been driven in large part by expansion of plantings of field crops, such as corn and soybeans, and the widespread use of herbicides to the detriment of milkweed, where monarchs feed.