Can new residents replace old migrants?
One of the most obvious first questions is whether the increase in resident monarch butterflies can replace the decline in the migratory population. To get a more quantitative estimate of the number of monarch butterflies in urban gardens, we surveyed their density in gardens in Berkeley, CA (Supplement S2). If we scale this estimate to all of northern and central California, there would be ~12,000 butterflies in urban gardens. This is less than 1% of the number of migratory monarch butterflies that overwintered in this part of California in the 20th century. Furthermore, at most about 5% of monarch butterflies survive migration (Supplement S3). Therefore, at the peak of their summer breeding, there were probably at least 20 times as many monarch butterflies throughout the western states as we see in the overwintering groves. In other words, even if we added these urban monarch butterflies to our overwintering counts, and even if we have underestimated the abundance of urban monarch butterflies by an order of magnitude, we still have far fewer monarch butterflies in the West now than we did three or four years ago.
Another possibility is that the monarch butterfly populations from these urban gardens could themselves be a source of butterflies that would colonize the western states during summer. Again, it seems unlikely. Resident populations of monarch butterflies build up high levels of a protozoan parasite, Ophryocystis elektroscirrha (OE). In California, about 8% of migratory monarch butterflies are infected with OE, compared to about 75% of residents (Satterfield et al. 2016). Uninfected monarch butterflies have a tremendous capacity for population increase – a single female monarch can produce 12 adult daughters if milkweed host plants are not limiting (Flockhart et al. 2015) and perhaps 3 or 4 daughters under realistic conditions (Supplement S4, Figure 2). This capacity allows monarch butterfly populations to increase rapidly in size during the summer breeding generations. OE-infected monarch butterflies experience lower survival, lower egg-laying rates, and produce about 0.8 adult daughters per female (Supplement S4). If 75% of monarch butterflies have OE, the average rate of increase (0.75 x 0.8 + 0.25 x 3) is about 1.35 adult daughters per female. This rate of increase is enough for resident monarch butterfly populations to persist in urban areas, but it does not give them the ability to rapidly colonize the other western states. Furthermore, traits associated with migration can evolve rapidly in monarch butterflies (Freedman et al. 2020, Tenger-Trolander et al. 2019). To date, no one has looked for such changes in resident California populations, but they could lose the genetic tendency to migrate, as well as the demographic capacity to do so.