Populations overwintering in Mexico may be on the decline.
The area of Mexican forest patches covered by overwintering butterflies has been shrinking.
Researchers say they see a long-term decline in the butterfly's numbers.
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.
Several menaces, including habitat loss, confront monarchs but the researchers focused on assessing trends in the populations instead of confirming the causes.
Within the overall downward trend, seven of the 10 below-average years in the study followed one another in a worrisome streak through the winter of 2010–11, the researchers say. The downward trend did not appear to be a fluke based on a couple of good or bad years; it still showed up when researchers removed the largest area (20.97 hectares in 1996–97) or the smallest (1.92 hectares in 2009–10) from the data, Williams and his colleagues report online March 21 in Insect Conservation and Diversity.