Monarch Migration Miracles and Challenges

“Absolutely spectacular. Nothing compares with it.” Effused Cecilia Peterson as she sat down to tell me about her three visits to the monarch butterfly overwintering sites in Mexico. At first glance, Cec (pronounced “cease”) – a retired elementary teacher and avid gardener – doesn’t have much in common with Eve Depew, a seven-year-old girl raising monarchs in Hayward, WI. But as Cec continued, describing the amazing experience of being in a forest with butterflies practically dripping from every trunk and branch, the enthusiasm and wonder that lit up her weathered face was exactly the same as Eve’s. They have yet to meet, but these girls are like two peas in a pod.

In fact, Cec can more than empathize with Eve’s efforts. Starting in the mid-1990s, all the fifth graders in Cec’s inner-city Duluth, MN, classroom raised a monarch from egg to adult in a plastic shoebox on their desk. “That experience with nature overrode any behavior problems,” reminisced Cec.

In the beginning, students like Cec’s would have only learned about the mystery of where monarchs go in the winter. While the overwintering sites have been known to locals for many years, science only discovered them in 1975.

Since then, the students all over the country have learned about the amazing migration that monarch’s undergo, and the remote oyamel fir tree forests in the mountains west of Mexico City where they rest from December to April. On steep, southwest-facing slopes, ten-thousand feet above sea level, the temperatures are just right. There, in the cool, moist microclimate in the trees, monarchs bide their time. The forest is both their umbrella and their blanket. The dampness helps keep them from dehydrating, while the canopy prevents dangerous wetness. Moderate temperatures under the forest cover prevent them from freezing to death, while still being low enough to slow their metabolism, and stretch out their limited fat reserves as long as possible. On warm days, they will fly nearby for a nectar snack and drink of water.

Dangers still abound in the forest, though. The canopy can’t totally eliminate freezing temperatures or ice storms, and especially when a cold snap follows rain, wet butterflies may freeze. One such storm happened the year that Cec returned to the forest for her second visit. Instead of ethereal beauty, she remembers the horrendous stench of rotting butterflies. As our climate changes and extreme weather becomes more common, scientists are worried that those dangerous storms and temperature fluctuations will happen more frequently.

Compounding the weather issues are pressures on the forests to provide useful products for local residents. Although the wintering areas have been set aside as Biosphere Reserves, monitoring shows illegal logging is taking its toll. “At night, in my hotel, I could hear HUGE logging trucks rumbling down the mountain from the butterfly groves,” lamented Cec. Because of that, many of the monarch conservation efforts have been aimed at protecting these winter refuges. While the large-scale illegal logging practices have largely been brought under control since Cec last visited, the thinned forests and continued subsistence harvest of the trees allows more moisture to get through to the butterflies, and they retain less heat.

Some natural predation occurs in the mountains as well, since a couple types of birds and one species of mouse have adapted to eating the sleepy butterflies. This is no mean feat, since the toxins that caterpillars acquire from milkweed plants survive metamorphosis and remain in the adults. The critters have adapted, though. One mouse can eat about thirty-seven monarchs a night in the oyamel forest.

With almost all of the continent’s monarchs concentrated in one region (another smaller population overwinters in California), they become vulnerable to a single storm, drought, fire, disease, or human transgression.

The concentration of monarchs also brings in a high volume of ecotourists, who mean well, and who can help support the local economy, but who can sometimes impact (literally, with their feet) the very beauty they came to revere.

Despite all this, many monarchs survive the winter. The warmth of spring triggers a mating frenzy, which also triggers their reproductive diapause to end. Females can mate before they are sexually mature, and both mating and the presence of milkweed seem to speed up their development.

In mid-March, these long-lived butterflies begin the last leg of their journey. Their destination is fresh, spring milkweed in the southern United States. Here, at the tail end of their 3,000-mile, 8-month journey, comes the most significant challenge they’ll face. Seven-year-old Eve knows about it. Cec gardens because of it. And there are things that you can do to help. More about that next week.

Overwintering monarch butterflies in a preserve outside of Angangueo, Mexico, completely cover some trees. Photo from Wikipedia.