Here in the northeast we saw a lot of monarch butterflies, both adults and caterpillars, in the summer and fall of 2019, more than have been seen for many years.
That produced surprise and enthusiasm among observers, and the many sightings raised hopes that the 20-year decline in monarch numbers had slowed or even reversed.
But World Wildlife Fund-Mexico recently released the official count for this winter, and it showed a significant decline, not an increase; less than half as many butterflies were seen in Mexico this winter compared to the previous winter. What happened? Why didn’t the numerous monarchs we saw throughout our region increase the abundance of the overall population?
First of all, the thrilling abundance of monarchs in 2019 was unusual. Monarchs that survive the winter in Mexico and migrate northward lay their eggs in Texas and other southern states, and the next generation develops from these eggs and continues the migration northward. Additional generations develop during the summer in the northern part of the range, expanding the size of the population. It just happened that at each stage in 2019 weather conditions were ideal for monarch growth and reproduction; excellent weather in 2019 – a rare occurrence – allowed for an unusual buildup of the number of adults.
We know that their numbers surged in 2019, but what happened to all those monarchs when they left the northeast? Though abundant in our region, monarchs were not equally numerous everywhere else. The regional differences are important because, based on recovery rates in Mexico of tagged individuals, monarchs from the northeast reach the Mexican overwintering grounds at a much lower rate than do monarchs from the Midwest. In short, our northeastern monarchs contribute less to the surviving and overwintering North American population than do monarchs from the Midwest. And in the Midwest, reproduction of monarchs has been harmed by loss of habitat, especially the decline in milkweed abundance because of industrialized agriculture and pesticide usage. Habitat in the northeast is less affected by these factors.
Climate change plays a significant role, too. Last fall’s warmth delayed the southward migration of monarchs, and that delay decreased their survivorship through the long journey. Importantly, drought along the migratory route in Texas and northern Mexico eliminated many nectar sources, right where monarchs must feed heavily to build up their energy (fat) supplies for the coming winter.
Sadly, the long-term decline in monarch abundance continues, despite an occasional good year such as we saw in 2019. Their overall numbers are determined by availability and quality of habitat (with milkweeds), freedom from pesticides, and weather conditions (influenced by climate change). Year 2019 was a fluke for the northeast, but it serves as a reminder that long term trends are better indicators of their status than single good or bad years and that critical determinants of monarch abundance are available habitat and weather throughout the butterfly’s full annual cycle.
Ernest Williams is an ecologist who taught at Hamilton College for 34 years. He has a summer camp on Twitchell Lake. He also provided the photos for this post.