Update on the Monarch Butterfly

By Jan Chaiken and Marcia Chaiken

Mexico plays an important role in the life cycle of Mariposa monarca, or monarch butterfly, a species that is rapidly dwindling due to climate change. Every year monarchs migrate thousands of miles from northeastern US and Canada southward for the winter, and then northward for the summer. The southbound destination for about 70 percent of all these butterflies is in a forest between Michoacán and Estado de Mexico that has been set aside by Mexico as the Monarch Butterfly Biosphere Reserve. This 56,000-hectare (140,000-acre) reserve was established in 1980, at which time the number of butterflies migrating there was estimated in the hundreds of millions, approaching a billion. This was well before any significant level of concern about climate change.

Monarchs are known to have migrated to this area since pre-Hispanic times, centuries ago. Studies of the legends of pre-Columbian indigenous people in Michoacán found descriptions of swarms of butterflies flying high overhead in November. The legends depicted them as protectors of the souls of deceased relatives who were returning for Día de los Muertos (Day of the Dead), which is celebrated around the same time.

As the Climate Warms, Monarchs Disappear

The population of migratory monarchs is estimated annually by measuring the area in Mexico’s Biosphere reserve that is covered with butterflies in mid-winter. Analogous measurements are made for the western monarch butterfly, which overwinters in California, including at a reserve near our US home. A few decades ago, there were so many butterflies that the sound of their wings in the trees was like a rippling stream or a rainstorm. Now visitors or scientists have to stand quietly still and stare carefully to observe any butterflies.

The decline in the number of butterflies overwintering in Mexico has been so precipitous (estimated at up to 99 percent in this century, and currently averaging 22 percent per year) that in July 2022 monarchs were placed on the threatened species list of the International Union for Conservation of Nature (IUCN), meaning that they are in danger of worldwide extinction unless there is major intervention.

Climate change has impacted the migratory pattern of the monarchs, both in the US and Canada where they breed and in Mexico where they overwinter and become dormant. The temperatures where the butterflies become dormant need to stay cool enough so the butterflies’ metabolism is suppressed and they don’t need to eat nectar (which is nonexistent in the winter) to survive. As temperatures rise in the overwinter destinations, the butterflies become more active but do not have the food they need for survival.

The butterflies actually have developed an adaptation to address this problem. Researchers who take measurements annually observe that the monarchs adjust upward the elevation of resting places they choose in the forests of Mexico. However, the adaptation (around a meter upward a year) has not been adequate to counteract all effects of climate change. For example, climate change has also produced unpredictable fluctuations between too hot and too cold for the butterflies, or between too rainy and too dry.

As Habitat Disappears, So Do Monarchs

Another effect of climate change particularly important to monarchs is the gradual disappearance of milkweed in fields of the US and Canada. Milkweed plants are the only location where female monarchs lay their eggs, so their absence leads to an interruption of the reproductive purpose of the northward portion of migration. In addition to climate change’s detrimental effect on milkweed plants, grasslands containing milkweed and nectar-producing wildflowers in the areas on the butterflies’ migration routes are being converted to cornfields to produce cattle feed and to ranches where the herds can range. The more corn and cattle, the more methane produced by the cattle, the more climate change, the fewer wildflowers and milkweed plants, and thus fewer monarchs.

So what, aside from eschewing steak and hamburgers, should be done to help prevent extinction of the monarchs? The World Wildlife Foundation has a simple recommendation that can be carried out by individual families on the migratory routes. Their motto for this recommendation is “all it takes is one square foot.” By planting native local wildflowers in a garden or flower box, you can assist all kinds of pollinators – not only monarchs but bees and hummingbirds, which are also experiencing declining populations.

You may be rewarded by the sight of monarchs coming to sip nectar from your minigarden – not the erstwhile millions, but in sufficient numbers to know we haven’t entirely wiped these beautiful beings from the face of the earth.