caterpillars when other factors are not present is unknown. Additionally, milkweed grown at carbon dioxide levels of 760 parts per million was found to produce a different mix of the toxic cardenolides, one of which was less effective against monarch parasites. Conservation status On July 20, 2022, the International Union for Conservation of Nature added the migratory monarch butterfly (the subspecies common in North America) to its red list of endangered species. The monarch butterfly is not currently listed under the Convention on International Trade in Endangered Species of Wild Fauna and Flora or protected specifically under U.S. domestic laws. On August 14, 2014, the Center for Biological Diversity and the Center for Food Safety filed a legal petition requesting Endangered Species Act protection for the monarch and its habitat, based largely on the long-term trends observed at overwintering sites. The U.S. Fish and Wildlife Service (FWS) initiated a status review of the monarch butterfly under the Endangered Species Act with a due date for information submission of March 3, 2015, later extended to 2020. On December 15, 2020, the FWS ruled that adding the butterfly to the list of threatened and endangered species was "warranted-but-precluded" because it needed to devote its resources to 161 higher-priority species. The number of monarchs overwintering in Mexico has shown a long-term downward trend. Since 1995, coverage numbers have been as high as during the winter of 1996–1997, but on average about . Coverage declined to its lowest point to date () during the winter of 2013–2014, but rebounded to in 2015–2016. The average population of monarchs in 2016 was estimated at 200million. Historically, on average there are 300million monarchs. The 2016 increase was attributed to favorable breeding conditions in the summer of 2015. However, coverage declined by 27% to during the winter of 2016–2017. Some believe this was because of a storm that had occurred during March 2016 in the monarchs' previous overwintering season, though this seems unlikely since most current research shows that the overwintering colony sizes do not predict the size of the next summer breeding population. In Ontario, Canada, the monarch butterfly is listed as a species of special concern. In fall 2016, the Committee on the Status of Endangered Wildlife in Canada proposed that the monarch be listed as endangered in Canada, as opposed to its current listing as a "species of concern" in that country. This move, once