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.