The number of monarch butterflies in the Mexican colonies where the colourful orange and black migratory insects spend their winters has declined to the lowest on record.
The colony size totals only 1.92 hectares this winter, the equivalent of about 2½ soccer fields, compared with the previous low in 2004 of 2.19 hectares, according to the latest Mexican census.
Although the slippage between the two years is slight and is being attributed mainly to weather-related factors last year, biologists and butterfly watchers have been alarmed by the trend to significantly smaller colonies. In the 1990s, monarchs occupied an average of about nine hectares of forests each winter, but for the 10 years ended in 2009 the size had fallen to less than five hectares, according to figures issued by researchers at the University of Kansas.
"The trend has been downward for the last quite a number of years," observed Donald Davis, an Ontario-based board member of Monarch Butterfly Fund, a conservation advocacy group.
The main factor behind the decline in 2009 was the weather, with a mixture of drought and excessively high and low temperatures undermining the butterflies across the vast North American territory where they breed and migrate.
Spring temperatures in Texas, for instance, were very high last year, harming populations on the first leg of their migration north. But summer in Canada and elsewhere in the U.S. was too cold for the insects. It is rare for the butterflies to face adverse conditions across almost their entire range in a single season; in most years, poor breeding success in some regions is offset by better results elsewhere.
The latest census was conducted by WWF Mexico, a conservation advocacy group, and posted on its website.
Monarchs are found in many parts of Canada, but the largest populations are in the southern areas of Ontario and Quebec. The butterflies that migrate south from Central and Eastern Canada in the fall are the so-called Methuselah generation, long-lived insects that travel up to 2,500 kilometres to Mexico, where they winter in dense, tree-covering colonies first discovered by scientists in the 1970s.
These butterflies then begin the northward migration each spring to nearby areas in Texas, where they breed, producing progeny whose offspring eventually return to Canada each summer to repeat the cycle. The monarchs that winter in Mexico can live up to eight months, but those in the northward migration may live only a few weeks.
The number of monarchs in the colonies has been estimated at up to 60 million per hectare, but butterfly experts fear that populations will crash to such low levels that it will eliminate the annual mass migration of tens of millions.
"They're not in danger of extinction, but what we're really concerned about is preserving the migration because the migration is such a magnificent phenomenon," said Orley Taylor, professor of ecology at the University of Kansas and director of its Monarch Watch program.
Although weather can affect population numbers from year to year, Dr. Taylor said, the monarchs have been suffering from a loss of habitat. One problem is the massive expansion in the amount of genetically modified corn and soybeans planted by farmers. These crops have led to an increase in herbicide use, which has eliminated milkweed plants that the butterfly larvae depend on for food.
Rural land is also being converted to urban development, and once-idled farmland that may have hosted milkweed plants is being returning to production to take advantage of the demand for corn and soybean biofuels. Because of the key role of milkweed as food for the species, Dr. Taylor has been urging landowners to plant some of it to help the butterflies.