Population and Trends. Monarch, Danaus plexippus plexippus (Linneaus, 1758), is a species of butterfly globally distributed throughout approximately 90 countries and island nations. These butterflies are well known for their phenomenal long-distance migration that occurs over multiple generations in North American populations. Descendants of these migratory monarch populations expanded from North America to other areas of the world where milkweed (their obligate host plant) was already present or introduced. With the year-round presence of milkweed and suitable temperatures, many of these new monarch populations no longer annually migrate. Currently, three known monarch populations occur within North America. Two North American populations are migratory and located east and west of the Rocky Mountains. Both have been monitored since the mid- to-late 1990s. The third population is a non-migratory population located in southern Florida. Although exact numbers of individuals are not known throughout all populations worldwide, the largest population of monarchs is the eastern North American population. Monitoring data for the eastern North American population dating back to 1994 estimated monarchs numbering consistently in the hundreds of millions throughout the 1990s and early 2000s (Figure 2-1). Data for the western North American population dating back to 1997, compared to historical data, documented approximate past population sizes of over 10 million (Monarch watch, 2018). The third population of monarchs are found in south Florida and live in areas where the climate permits year-round nectar resources and breeding, thereby negating the need to migrate. Other portions of the southern and western U.S. contain areas where year-round breeding and monarch presence may occur. These non-migratory monarchs show genetic differentiation from the migratory North American monarchs, even though there is an annual influx of individuals from the eastern monarch population (Zhan et al. 2014, Pfieler et al. 2016).
