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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