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.