The Hawaiian islands are great place to study evolution because of the islands are very isolated, the islands have many small habitats one relatively small islands, and 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 simply Danaus plexippus, the same as is found on the rest of the American mainland. In the mid 1800’s the milkweed plant (the Monarch’s host plant), Calotropis gigantea, was introduced 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. Its 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 its 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’s 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, its been suggested that the bulbuls
are responsible. Its 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.. |
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