While monarchs are native to North America, they've spread throughout much of the world during the past 200 years (Nail et al., 2019). They currently inhabit over 90 countries, islands, and island groups (pictured above) but differ in their morphology, migration and overwintering patterns, natural enemies, larval diet, and genetics.
Monarchs arrived in North America from a migratory ancestor, common to both Danaus plexippus and D. erippus. As the last ice-age receded 20,000 years ago, the monarch population occupying the southern USA and northern Mexico began to grow and expand their range and migration annually. These expansions were stimulated by the abundance of milkweed that was growing, exploiting the novel habitat uncovered by the glacial recession. The population underwent three separate dispersions into South America, westwards to Oceania and Australia, and east across the Atlantic. Upon dispersal, the Atlantic and Pacific populations underwent a signiifcant reduction in population size. These results, as reported in Kronforst et al. (2014), are supported by the fact that monarch populations outside of North America display high levels of linkage-disequilibrium, consistent with expectations of recent population dispersal.
Researchers are still unsure exactly how and why the monarch made its journey across the ocean. A few hypotheses have been formulated. It is possible that they were transported by the numerous ships that made the long voyage across the Pacific Ocean. Because larvae have to move around a lot to find suitable pupation sites, and since milkweed can be found around shipyards, it is possible that the larvae were transferred onto ships where they could travel a long distance before emerging. It is also possible that overwintering adult monarchs landed on ships and then were carried across during the winter. Even these hypotheses seem a little far-fetched, however. It is most likely that humans were involved in the process, but it is not known to what extent. Monarchs in North America can fly over 2,200 km during their migration, so it is possible that some made the journey on their own!
Monarch dispersal across the Atlantic was closely tied with the availability of milkweed larval host plants. Both Gomphocarpus fruticosus and Asclepias currassavica were prominently established in the northern Africa and Iberian Peninsula before initial monarch sightings east of the Atlantic Ocean. More recent anthropogenically introduced host plants in Southern Spain are thriving and seem to be expanding into the Mediterranean region where a favorable climate and growing conditions only appear to be encouraging the milkweed spread (specifically Gomphocarpus physocarpus). With established host plants, monarch butterflies that were blown off of their migratory pathway from North America to Mexico could still successfully colonize areas where host plants were available, like southern Spain and Portugal. While larvae or butterflies could have hitchhiked across the Atlantic, rare monarch sightings on the coast of the UK, likely caused by winds blowing vagrant monarchs across the sea , show that human interference is not necessary for monarchs to cross the Atlantic Ocean. The Mediterranean basin’s environment and host plant distribution is favorable for monarchs, and an increase in resident monarchs around the Iberian Peninsula is not farfetched.
References:
Nail KR, Drizd L, and Voorhies KJ. (2019) Butterflies Across the Globe: A Synthesis of the Current Status and Characteristics of Monarch (Danaus plexippus) Populations Worldwide. Front. Ecol. Evol. 7:362. doi: 10.3389/fevo.2019.00362
Pierce, A.A., Zalucki, M.P., Bangura, M., Udawatta, M., Kronforst, M.R., Altizer, S., Haeger, J.F., and de Roode, J.C. (2014). Serial founder effects and genetic differentiation during worldwide range expansion of monarch butterflies. Proc. R. Soc. B. doi.org/10.1098/rspb.2014.2230
Zhan S, Zhang W, Niitepõld K, Hsu J, Haeger JF, Zalucki MP, Altizer S, de Roode JC, Reppert SM, Kronforst MR. (2014). The genetics of monarch butterfly migration and warning colouration. Nature, 514(7522) :317-21. doi: 10.1038/nature13812