Monarchs on PBS’s Nova Tonight

The PBS series Nova traces the epic migration of Monarch butterflies from Canada to Mexico in an episode to be broadcast tonight. And the news on Monarchs this winter isn’t good. The overwintering population appears to be smallest since the colonies became known to North American biologists in 1975. A number of factors explain the situation, mostly that Monarchs in 2009 found inhospitable breeding conditions throughout North America. First, high temperatures in Texas in March and early April of 2009 limited the reproductive success of that year’s first-generation returning from Mexico. This generation, the first offspring of the overwintering population, recolonizes much of North America. So its success can set the stage for the population throughout the breeding season.

A Monarch population that started low last spring went on to encounter cold and rainy conditions in many areas as it moved north. The summer of 2009 turned into a big chill for Monarch breeding success. When it’s cold and wet, adult butterflies aren’t flying as much (and finding each other to, you know, make more butterflies) and larvae don’t develop as well. The result was a depleted late-summer and fall generation of Monarchs. This is the population that makes the journey to Mexico. And most anyone paying attention noticed very few Monarchs last fall.

Here’s the report from Mexico this winter from my friends at Monarch Watch.

Only 7 colonies were found and the total area occupied by monarchs at the overwintering sites in December was 1.92 hectares (ha). … The numbers this year appear to be lower than observed for any year since the overwintering colonies became known to science in 1975. The lowest previous total, 2.19 hectares, was reported in 2004. This decline continues a trend that started in the late 1990s. In the decade of the 90s the mean area occupied by monarch colonies was close to 9 hectares. The mean for the last 10 years, through the 2009 migration, is now below 5 hectares per year; the three lowest monarch overwintering populations were reported in this decade.

Monarchs are losing ground. Monarch wintering sites in stands of Oyamel Fir in Mexico’s transvolcanic range are shrinking. Population growth in Mexico creates high demand for open land and wood. Farmers and loggers are moving higher into the mountains, and into Monarch habitat. In many ways, you can’t blame them. We all need land and trees, particularly those in developing economies. But opening the Oyamel Fir canopy makes the remaining trees, and their overwintering Monarchs, more vulnerable to freezing and strong winds. It can result in a big die-off of at wintering sites after rains and freezing temperatures. First they get wet, then they freeze to death. Although the Monarch is among the most widespread North American butterfly species, and apparently abundant, it is most vulnerable when concentrated at these overwintering sites. Thinks about it. You have most of the world’s population of this insect packed into just a few hectares of habitat.

The Monarch population can rebound. A lot depends on the number of Monarchs surviving the winter. If at least 1 hectare of Monarchs survives to move north, and if they encounter normal conditions as they move north through Mexico and Texas, the population can rebound in one breeding season. We’ve seen it before. Even so, the long-term trend for Monarchs, arguably the continent’s most favorite insect, is worrisome.