How do resident and migrant populations interact?
In responding to the current status of western monarch butterflies, one
of the greatest uncertainties is how much the resident and migratory
populations interact. It may be that the growth of the resident
population is independent of the collapse of the migratory one. Monarch
butterflies have numerous resident populations worldwide, as well as the
migratory ones in North America (Freedman et al. 2020). It could be that
climate is becoming suitable for monarchs to live year-round in northern
California. It could also be that urban monarch butterfly populations
are growing because more people are planting milkweed in their yards,
especially tropical milkweed (Asclepias currasavaca ). Tropical
milkweed is a popular horticultural plant, native to South America, that
provides year-round food for monarch butterflies.
Alternatively, resident and migratory populations could be
demographically connected. Perhaps migratory populations are declining
because some individuals are attracted to the urban gardens in Fall,
instead of migrating to coastal overwintering groves. Perhaps gardens
are attractive stopping places in Spring, essentially absorbing
butterflies that could have begun recolonizing inland sites. If our
estimates of population growth rates or infection rates are just a
little bit off, then the resident population may be a demographic sink
(sensu Dias 1996), in the sense of being sustained only by
immigration from the migratory population. In that case, we would expect
declines in the migratory population to be followed within a few years
by the loss of the resident population.
If populations do interact, a second concern is that the presence of an
OE-infected resident population may increase parasite levels in the
migratory population. OE is transmitted horizontally on milkweed leaves,
and tropical milkweed does not die back in winter. In other parts of the
southern United States, migratory monarch butterflies accumulate higher
parasite loads when they interact with resident populations on tropical
milkweed (Satterfield et al. 2018, Majewska et al. 2019). The potential
transmission of OE from resident to migratory monarch butterflies
creates a huge source of uncertainty. If the western monarch migration
is on the point of collapse, it seems sensible to keep as many
individual butterflies alive as possible, including the ones in gardens.
However, increasing survival of infected monarch butterflies in gardens
could increase parasite transmission to the migratory population (see
Ezenwa and Jolles 2015 for a similar example in a mammal population).
For monarch butterflies in California, this kind of negative interaction
is possible but by no means certain. It may be that helping monarch
butterflies in urban gardens is in fact the very best way to sustain
monarch butterflies in the West during this critical time. We simply
don’t know.