Populations overwintering in Mexico may be on the decline
North America’s beloved monarch butterfly may be sliding into a long-term decline. While monarch numbers have fluttered up and down over recent decades, one research group now says that there’s enough data to spot a downward trend.
During the past 17 years, the area of Mexican forest patches covered by overwintering butterflies has been shrinking overall, says conservation biologist Ernest Williams of Hamilton College in Clinton, N.Y. He and his colleagues use the area occupied, which has averaged 7.24 hectares since the end of 1994, as a rough index of winter monarch population size.