The Center for Biological Diversity announced on Wednesday the monarch butterfly population counted over-wintering in Mexico was 144% greater than last year, with the highest overall population since the winter of 2006–2007. Scientists cautioned against an excess of optimism for the species.
Remarked Center for Biological Diversity scientist Tierra Curry, "This reprieve from bad news on monarchs is a thank-you from the butterflies to all the people who planted native milkweeds and switched to organic corn and soy products[...] But one good weather year won't save the monarch in the long run, and more protections are needed for this migratory wonder and its summer and winter habitats." Milkweed is the sole food of the monarch caterpillar.
According to ecologist Chip Taylor of the University of Kansas, last year's relatively cool temperatures in the insects' migration corridor kept them in the right place at the right time to lay eggs with a good chance of survival. He also said this is not likely to happen again next year. Ecologist Ryan Norris of the University of Guelph in Ontario also said human actions had little to do with the increased population.
Almost all of the monarch butterflies ranging across the continent, as far north as Canada, return to the same wintering area in central Mexico's oyamel fir forests every year.  Scientists from the World Wildlife Fund Mexico estimate the population by observing the area turned orange by butterfly bodies on the trees.  This year, the butterflies occupied roughly 6.05 hectares (14.95 acres; .0605 square km), up from 2.48 (6.12 acres) last year.  The hibernation area was as large as 21 hectares in the 1990s and as low as under a hectare in 2014.  
Factors cited for the drop in the monarch's overall population include habitat loss, loss of milkweed to agricultural herbicides, and use of insecticides.
The Center for Biological Diversity and Center for Food Safety requested monarch butterflies be given Endangered Species Act protections from the United States government in 2014.  The decision is pending and expected in June of this year. George Kimbrell of the Center for Food Safety said "This year's count is a temporary reprieve that doesn't change what the law and science demands, which is that we protect monarchs under the Endangered Species Act before it's too late."

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Report indicates Mexican monarch butterfly population at ten-year high, reasons unclear