The annual count of Monarch butterflies overwintering in Mexico shows declines from last year’s numbers—a 15 percent decrease –according to figures from an official Mexican government count in the winter of 2017. These numbers underscore how at risk the iconic animal is, with a possible collapse of migration if populations are critically low.

Monarch butterflies (also known as Eastern Monarchs) embark on an impressive migration every year. Roughly 99 percent of all North American monarchs migrate each winter to oyamel fir forests on 12 mountaintops in central Mexico. Scientists estimate the population size by measuring the area of trees turned orange by the clustering butterflies. But for the second year in a row, its numbers are declining — 2.48 hectares of occupied winter habitat is down from 2.91 hectares last winter. Apart from partial rebounds in the winters of 2001 and 2003, numbers have gone down steadily since 1996. Overall monarchs have declined by more than 80 percent over the past two decades.