The Hawaiian islands are a great place to study evolution because the islands are very isolated, the islands have many small habitats on relatively small islands, and recent introductions to the islands are reasonably well documented. One of these introductions was that of the Hawaiian Monarch Danaus plexippus.
The Hawaiian Monarch is not a unique species but non other than Danaus plexippus, the same as is found on the rest of the American mainland. In the mid 1800’s the milkweed plant (one of the Monarch’s host plants), Calotropis gigantea, was introduced to the islands. Shortly thereafter the monarch was detected in the Hawaiian islands.
In the 1960’s a white morph of the monarch butterfly was detected on the Island of Oahu. It is called the nivosus morph and has been the only population of white morphs until a second population was detected in Vanuatu around 2000 (Moulds and Lachlan, 2001).
Dr. John Stimmson at the University of Hawaii has been studying the relationship of the white morph to the entire population since the mid 1980’s. In 1965 the white morph was less than 1% of the population. In 1984 it was 4 percent of the population and by 1988 it was 8% of the population. However, by 1996 the population had then decreased to about 1.7% of the population. (Stimson and Kasuya-Maiko, 2000).
Its difficult to know exactly the cause, but it has been hypothesized that the introduction of two species of bulbuls (Pycnonotus jacosus and Pcynonotus cafer) in 1965, which are the only insectivorous birds on butterflies in the monarchs range, are probably responsible for the natural selection. It is hypothesized that they preferred the orange morphs. The orange morphs are simply easier to see than the white ones on the crown-flower milkweed. This makes sense, but why the decline of white morphs in the 1990’s? Again, it has been suggested that the bulbuls are responsible. It is believed that they have switched their diet from the butterflies to the caterpillars, which consequently show no variation depending on the morph (Stimson and Kasuya-Maiko, 2000).
Text by Rob Nelson
Stimson and Kasuya-Maiko. 2000. Decline in the frequency of the white morph of the monarch butterfly (Danaus plexippus plexippus L., Nymphalidae) on Oahu, Hawaii. Journal-of-the-Lepidopterists'-Society. [print] 6 November, 2000; 54 (1): 29-32.
Moulds and Lachlan. 2001. First record of white monarchs, Danaus plexippus (L.) form nivosus (Lepidoptera: Nymphalidae: Danainae) from Vanuatu. Australian-Entomologist. [print] 28 March, 2001; 27 (4): 113-116..