In the western United States, the population of migratory monarch butterflies is on the brink of collapse, having dropped from several million butterflies at coastal overwintering sites in the 1980’s to about 2000 butterflies in the winter of 2020-21. At the same time, a resident (non-migratory) monarch butterfly population in urban gardens seems to be expanding northward. If anything, this urban population has been growing in recent years. We explore the meaning of these changes. The new resident population is not sufficient to make up for the loss of the migratory population; there are still orders of magnitude fewer butterflies now than in the recent past. The resident population also probably lacks the demographic capacity to expand its range inland during summer months, due to higher levels of infection by a protozoan parasite, and subsequently lower survival and fecundity. Nonetheless, the resident population may have the capacity to persist. This sudden change emphasizes the extent to which environmental change can have unexpected consequences. It also demonstrates how quickly these changes can happen. We hope it will provoke discussion about how we define resilience and viability in changing environments.
