It is so abundant that biologists measure its population in terms of the area it covers, rather than numbers of individuals. But the iconic monarch butterfly is in sudden trouble. Numbers overwintering in Mexico are at their lowest since records began in the 1970s.
Every winter, monarchs head from North America to balmier climes, where they form massive congregations. This winter, though, numbers were a tenth of their 1996-97 peak (see Graph).
Mexico has set aside an area of land as a butterfly reserve. But according to research by WWF Mexico’s Eduardo Ramirez – who is the reserve’s director…
