State of the monarch butterflies in the West
Until now, monarch butterflies bred in North America west of the Rocky Mountains during the summer (Figure 1A). In fall, these western monarchs would migrate to the California coast and spend the winter clustering in a partly-dormant state in groves of pine and eucalyptus trees. In the 20th century, millions of monarch butterflies clustered in these overwintering groves. However, by the 2010’s the number of overwintering butterflies in the West had dropped from millions to two or three hundred thousand (Schultz et al. 2017). In 2018 and 2019, this number dropped to about thirty thousand (Pelton et al. 2019). In the fall of 2020, only about two thousand butterflies showed up (Xerces Society 2021). These declines have been attributed to degradation of wintering groves and summer breeding habitat, expansion of pesticide use and interacting effects of climate warming (Pelton et al. 2019, Crone et al 2020). Although monarch butterfly migration persists east of the Rocky Mountains, the phenomenon of monarch migration is on the brink of disappearing from the West (Figure 1B).
At the same time, monarch butterflies are abundant in gardens in cities on the California coast. No one has been counting these urban butterflies, but people living in coastal cities say they may be getting more abundant through time. One rough measure of a non-migratory urban population is reports of monarch breeding in summer near the coast, which have increased dramatically in recent years (Figure 1C, Supplemental analysis S1). This number is not a perfect estimate of the urban monarch butterfly population – for example, it could mean that more people on the coast have become interested in reporting butterflies – but it is generally consistent with the notion that monarch butterflies are becoming year-round residents in coastal cities, at the same time as the migratory population is disappearing.