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.