In early March of 2016, a severe storm raged cold strong winds, rains, and even snow down on the Monarch Butterfly Biosphere Reserve in Mexico, a World Heritage Site since 2008. Thousands of trees were felled, thus devastating the Oyamel forest ecosystem where the monarchs overwinter. Studies conducted following the storm reported a 7.4% mortality of monarchs. This number has now been found to be ghastly underestimated. A new study published in the journal American Entomologist reports findings that mortality following the storm likely surpassed 40%.
Previous research from Williams and Brower (2015) showed the importance of the Oyamel forest ecosystem for the butterflies: the forest “provides microclimatic protection by acting as a blanket that holds heat beneath the forest canopy, as an umbrella that reduces wetting of the butterflies clustering on the tree boughs, and as a heat source from the tree trunks radiating warmth that protects the butterflies clustering on the trunks from freezing,” writes the recent study.
The March 2016 storm essentially wiped out this microclimatic protection by homogenizing sub-freezing temperatures throughout the Oyamel fir forest ecosystem, the study goes on to explain. This, in turn, caused the deaths of 31–38% of the butterflies in the Sierra Chincua and Cerro Pelón and perhaps more than 40% mortality of monarchs in the Rosario colony. This creates an alarm for concern when looking at the greater picture, in which monarch populations have declined by 90% since the 1996-1997 overwintering season.
A side effect of the storm was the tens of thousands of felled trees left in its wake. In an effort to avoid wildfire or insect damage, the Mexican government authorized a volume of 60,000 cubic meters of timber (several thousand trees) to be removed for salvage logging. The scientists fear that this environmental disturbance only hurt the monarch colonies more, by reducing the forest's resilience to return to mature canopy cover which the butterflies so depend on. “This loss of canopy cover will diminish the normal microclimatic protection provided by the intact forest. The unexpected effects of this storm may take place at a greater frequency in an era of changing climate,” elaborates Science Daily.
More investigation is needed in order to fully understand the impacts of the storm. Lincoln Brower, Ph.D., who led the study says: "We hope to carry out a drone study, photographing the colonies from a standard height above the butterflies and then using photometric analysis to estimate changing colony densities," he says. "We are concerned that the current methodology of reporting only total colony areas is underestimating the decline of monarchs overwintering in Mexico."