Figure Legends
Figure 1. Change in status of western monarch butterflies. (A.)
Breeding, overwintering, and resident range of western monarch
butterflies in the Western USA. (B.) Trends in the migratory population
through time, based on historical records and the Western Monarch
Thanksgiving Count. The black line shows estimates from a state-space
population viability analysis (PVA; Schultz & Crone 2017) with 95%
confidence intervals. The orange stars are raw counts from the Western
Monarch Thanksgiving count, showing the steep decline in recent years.
(C.) Increase of sightings of immature monarch butterflies (eggs,
larvae, and pupae) near the coast, as a rough indicator of increasing
abundance of a resident urban population of monarch butterflies. Point
area is proportional to the number of sightings during each time period.
The black line shows predictions from analyses of year-round breeding,
shown for summer months (June-August) because migratory monarch
butterflies do not breed in winter, with confidence intervals in gray.
The estimate of ~12,000 urban butterflies comes from
ground surveys in 2020-21. See further details in supplement S1
(analyses of sighting records) and S2 (estimating the resident
population size). The photo is of a monarch butterfly caterpillar on
tropical milkweed in a garden in San Luis Obispo, CA (photo credit:
Nancy Starry).
Figure 2 . Effects of infection by the protozoan parasiteOphryocystis elektroscirrha (OE) on monarch butterfly rates of
increase during breeding generations. Black lines indicate estimated
growth rates (see Supplement S4), with 95% confidence intervals in
gray. Points represent values for the estimated proportion of
OE-infected monarch butterflies in migratory and resident populations
(from Satterfield et al. 2018). Dotted lines identify (1) the minimum
growth rate for persistence, set at exact replacement (growth rate of 1
per generation), and (2) an estimate of the minimum growth rate for
range expansion during breeding season (3-fold increase per generation).
The range expansion minimum is a rough estimate, justified in at least
two ways: First, migratory monarch butterflies have low survival during
fall migration, on the order of 2-5% (Supplement S3). If a population
increases 3-fold per generation for four breeding generations, the
resulting 27-fold increase approximately balances migration mortality.
Second, the land area of core western states (California, Nevada, Idaho,
Oregon, and Washington) is about 527,733 square miles (136,682,847 Ha).
If a population starts at 1,000,000 overwintering butterflies (a
reasonable historic estimate) and increases to 27,000,000, there would
be ~1 monarch butterfly per 5 Ha in its breeding range,
which is broadly consistent with its former status as a common species.
The photo is of an OE-infected monarch butterfly emerging in the lab
(photo credit: Christopher Jason, Washington State University).