4 years ago



Figure 2, Appendix A.

Figure 2, Appendix A. The spread of monarch butterflies, Danaus plexippus plexippus, across the Pacific in the 1800s. The map is generated by assuming that each new population was derived from the nearest neighboring population (in any direction) with a confirmed earlier arrival, unless an intervening island group was known to be free of the butterfly. Note that populations appear to stem from one or two incursion points in the South Pacific. Figure 1 from Zalucki and Clarke 2004, original legend. Based on the short amount of time since these introductions of D. p. plexippus outside of North America, the new populations are still considered part of the subspecies. However, genetic analyses show that they have less genetic diversity than monarchs in North America (Shephard et al. 2002, Lyons et al. 2012, Pierce et al. 2014), and most are now likely to be genetically isolated from the main North American population (Lyons et al. 2012). There is debate about how and why these dispersals occurred when they did, both east and west (Brower 1995, pp. 352 – 357). Host plants in the milkweed family had been introduced to the Pacific and Atlantic during this same time frame. Given sightings of vagrant monarchs far from North America over the years, it is plausible that some monarchs have always ventured far from their native habitat during migrations but would not have been able to establish breeding populations in the absence of suitable milkweeds. Such milkweeds were absent before colonial times. In both the Atlantic and Pacific islands and coastal areas, non-native tropical milkweeds were introduced by colonists and travelers, intentionally as garden flowers and for medicinal uses, and unintentionally in packing materials and as seed contaminants (Brower 1995, Zalucki and Clarke Monarch ESA Petition 148

2004). These milkweeds also have become naturalized to greater or lesser extents, usually in disturbed areas such as pastures and roadsides, or along watercourses, and are now considered to be pan-tropical. The most common are Asclepias curassavica (tropical milkweed, scarlet milkweed) (see:, native to South America; Gomphocarpus physocarpa (balloon plant, giant swan plant) and G. fruticosus (swan plant, cotton bush), native to South Africa; and Calotropis procera (apple of Sodom, giant milkweed) (see:, originally from Africa, India and Southeast Asia. In addition, C. gigantea (crown flower, tree calotrope), also from Asia, is found in many areas of the Pacific (see:; and the non-tropical A. incarnata (swamp milkweed) from North America is cultivated specifically to feed monarch larvae in New Zealand (Elliot et al. 2009). In some islands, naturalists report boom and bust cycles accompanying monarch introductions, as monarchs first thrive on and then decimate the introduced host plant populations: From the records of early naturalists we get a clue as to how the introductions and rapid spread may have proceeded. A number of commentators (Semper, 1873; Sturm, 1878; Walker, 1886; Collenette, 1925) point out that monarchs on some islands reached very high levels shortly after introduction. For Upolu, in the Samoan group, Semper (1873) wrote ‘. . . it was observed in 1869 for the first time. On Upolu the species became quickly very frequent and in 1870 it was one of the most common butterflies.’ On New Caledonia, one writer reported ‘millions of butterflies’ (Walker, 1914). Initial ‘boom’ commonly appears to have been followed by ‘bust’, however, as large caterpillar populations appear to have eaten out their host plants, e.g. ‘In New Caledonia, . . . it became very abundant some years ago, but is now comparatively scarce, owing, . . . to the destruction of nearly all the food-plant by the larvae’ (Walker, 1886). Collenette (1925) reported that this butterfly had changed from being common, to rare or absent, on Hiva-Oa, Tahuata and Nuka-Hiva Islands, in the Marquesas, on Papeete, Tahiti, and on Moorea Island in the Society Islands. Diggle (Marks, 1963) went so far as to use the recently introduced (to Australia) monarch to illustrate perhaps the first ever talk on biological control using herbivorous insects (Zalucki and Clark 2004, p. 114). Decimation of host plants results in cycles of monarch abundance, depending on the particular milkweed species and their capacity to rebound: Such variation in abundance still happens: on Oahu (Hawaiian archipelago) butterfly numbers fluctuate widely during the year, with periods when caterpillars are so abundant that host plants (Calotropis spp.) are defoliated, alternating with periods when numbers are low (M. P. Zalucki, pers. observ.; John Stimpson, University of Hawaii, pers. comm.). Thus, it appears likely that once monarchs successfully colonized an island, their populations increased rapidly until the local carrying capacity was exhausted. Subsequent outbreaks only appear to be possible with hosts that can recover relatively quickly from defoliation (e.g. Calotropis). Blakley & Dingle (1978) reported the virtual elimination of A. curassavica by monarchs on Barbados. Initial outbreaks following colonization would have resulted in high levels of non-directional local dispersion, probably resulting in high levels of population mortality, until the next island was chanced upon and the cycle repeated (Zalucki and Clarke 2004, p. 114). Monarch ESA Petition 149

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