July 16th 2010
"Magic" Island nature
Take the "net" out of Magnetic and we have "Magic" Island. Right now, Magnetic is displaying, along quiet creeks and gullies, some very special magic in the form of thousands of overwintering (mostly Blue Tiger) butterflies, resting and storing their energy until more food appears. And if you come across such a gathering keep that "net" hidden and move slowly. These critters are easily disturbed and, if visitors continue to set their lovely wings a-flutter, they will lose the precious energy they seek to store and may not make it through the winter to breed.
According to James Cook University expert, Associate Professor of Environmental Science, Peter Valentine, "Magnetic Island is a very special place for overwintering. There are very few butterflies (like the Blue Tigers, Tirumala hamata) that aggregate in such numbers," and, "There are quite a lot of overwintering sites on Magnetic Island."
Peter Valentine observed that some overwinterings - the term used to describe the gathering, for safety in numbers, of thousands of butterflies in relatively humid, sheltered, locations in tropical Australia - can number tens of thousands of butterflies.
Magnetic Island is one of the most accessible places to see overwinterings says Peter Valentine.
"Butterflies that overwinter are actually avoiding drought or dry conditions or cool temperatures. Every butterfly has to survive through the weather variations of the year. They pass through these unhospitable times by entering a diapause."
Diapause is when the butterflies "shut down". The hormones which trigger sexual activity turn off and the animals minimise their energy use, clinging to twigs, leaves and branches of trees to see out the winter. Magnetic's overwinterings are examples of butterflies in diapause.
Some of Magnetic's Magic in the bush
"Most butterflies do it as eggs, larvae or pupae but a few do it as adults. Due to the warmer temperature, there are many more diapause/overwinterings on Magnetic Island than Tasmania," says Peter who adds that some cold climate butterflies have their own form of anti-freeze!
Magnetic's Blue Tigers are "Wanderer" butterflies which are also found in North America where they are known as Monarchs. Annual Monarch migrations from North America to Mexico are major natural phenomena and Australia's wanderers also form massive migrations inland and south from the tropics to the subtropics. The migration was even noted by Captain Cook who observed the event off the Queensland coast.
As a dry tropics Island Magnetic is a very likely candidate for overwinterings. "In the tropics most butterflies continue reproducing but in the dry tropics there is not enough food so they have to shut down and no energy is wasted," says Peter who observed that there are other sites on Cape Upstart and Cape Cleveland he has visited.
"As soon as the hormones are switched on again the males chase the females which burns lots of energy."
According to Professor valentine, small butterflies may only live for a week or two but as they get larger they generally live longer as well. Our blue tigers can live for between six to nine months while sedentary (non-migrating) species may live only for three or four weeks.
"Let them rest"
As far as human interaction is concerned, Peter advises that people move slowly in a way that does not disturb them. Clearly the urge to see and take photos of the butterflies taking flight and filling the air is very strong but Peter says, "Let them rest because you are causing them to burn precious energy if you disturb them."
While Magnetic Times has identified at least one major overwintering site on Magnetic's west coast, we have decided to withhold the exact location, which also runs into private property, in order to avoid over exposure of the site to too many curious humans. But for those who are keen to discover an overwintering site and heed Professor Valentine's advice, the best places to look are relatively humid, still, shaded gullies with vine thicket or riparian vegetation which are free from wind as the butterflies have to conserve moisture.
Story and photos: George Hirst
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