Christmas Island, Naturally

Main Club October 2018 Meeting

Long-term Club member Kevin Coate spoke to us about his visits to Christmas Island dating back to 1990.

The island lies 2600km north-west of Perth and is formed by an extinct volcano which rises more than 4000m from the ocean floor, reaching a height of approximately 360m above sea level. The island is surrounded by cliffs, with a plateau on the top. The only boat access to the island is at Flying Fish Cove toward the northern end, where the Malays reside today at the Kampong. To begin his presentation, Kevin showed images of what living on the Island was like in the 1960s—when the lifestyle of the people living there was more relaxed. The population at that time was mostly Asian, engaged in phosphate mining—which began in the 1890s and finished for a while in the late 1980s before resuming in the 1990s. A number of shrines representing the ethnic diversity of the inhabitants can still be seen from those days, although some are now hard to find. Christmas Island was occupied by the Japanese in WWII and became an Australian Territory in 1958. A National Park was declared in 1980 to protect vulnerable species such as Abbotts Booby and now covers about 63 per cent of the island.

In November or December, after the first rains, millions of Christmas Island Red Crabs (above, K Coate)(Gecarcoidea natalis)  migrate from inland areas to the ocean. They dip in the ocean then go to the first land terraces and mate. The males then return to the plateau and the females remain in burrows while their eggs develop. In the last quarter of the moon a month later the females return to the sea and release their eggs, which can number about 100,000 per female. As soon as the eggs touch the water they hatch. The larvae swim out to sea and return in their millions about 25 days later, crowding on the rocks along the shoreline for two to three days before changing into their final form and returning to the plateau, moulting several times before adulthood. Islanders keep their doors well shut during the migrations to avoid having crabs scuttling around their living room.

In recent times the accidentally introduced Yellow Crazy Ants (Anoplolepis gracilipes) have expanded in huge numbers due mainly to a symbiotic relationship they have with another invasive species, Yellow Lac Scale Insects (Tachardina aurantiaca), that secrete honeydew harvested by the ants for food. The ants have killed millions of crabs by spraying formic acid in their eyes. Since becoming aware of the ant problem in the 1990s National Park authorities have tried eliminating them with poison baits with some success. However they are now moving away from chemicals and trying biological control methods by releasing thousands of micro wasps (Tachardiaephagus somervillei) that parasitize female scale insects and kill them.

Other species of land crab such as Blue Crabs (Discoplax celeste) prefer areas of freshwater seepage where they shelter under logs and amongst the tangled roots of Tahitian Chestnut trees (Inocarpus fagifer). Apart from their delicate blue colour they are distinguished by a dense mat of bristles or ‘moustache’ on either side of their mouth, which is kept moist to aid respiration. Robber Crabs (Birgus latro) (below right, K Coate) are widespread over the island and one of their favourite foods is the Arenga Palm (Arenga listeria) which they climb for its fruit. If the tree falls over or the trunk is damaged they strip the bark off for its pithy centre.

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Kevin described other rainforest trees such as Syzygium nervosum (above left, K Coate), which have large buttressed trunks and the Strangler Fig (Ficus macrocarpa) which starts as an epiphyte then completely surrounds its host sending roots from the top down to the ground. There are many showy fungi in the rainforest and we were shown a very large bracket fungus about a metre wide.

Eleven reptiles were on the island during Kevin’s visits, five of which were introduced. Since then, the endemic pretty Blue-tailed Skink (Cryptoblepharus egeriae) has been almost eliminated by a combination of cats, Yellow Crazy Ants and the introduced Asian Wolf Snake (Lycodon aulicus) which feeds on other skinks and geckos. A gecko species that was once easily found, the Giant Gecko (Cyrtodactylus sadleiri) is also under threat and now seldom seen.

The Abbotts Booby is an endangered seabird endemic to Christmas Island. They nest in tall trees and lay one egg. It takes 16 months before the young are able to fend for themselves. During that time they are at risk of being blown from their nest by cyclones or strong winds. A few hours after a cyclone passed the island in 1995, Kevin and his intrepid group visited an area where the Abbotts Booby bred and found several young birds that had been blown from their nests. As there was no hope of them surviving where they were, they took them to National Park Headquarters. Park staff initiated further searches and a total of about 16 young birds were saved and successfully hand reared (below, K Coate) over many months. Fish to feed them were flown in from Jakarta.

Many birdwatchers visit Christmas Island to see the beautiful Golden Bosunbird, Christmas Island Frigatebird and other endemic species. Golden Bosunbirds usually nest in hollow trees and crevices in cliffs but for several years one pair nested in town at the base of a street tree next to a busy footpath, in full view of people passing by (below, K Coate). Red-footed Boobys are widespread, breeding in January, when it is comical to see them fight over nest building materials.

Often seen flying over forested areas is the Christmas Island Glossy Swiftlet, which nests in caves. We were shown some spectacular formations in Daniel Roux Cave, where hundreds of their nests were clustered high up on the roof.

Feral cats have been a major problem on the island, being partly responsible for a decline in the wildlife, and cat eradication programs have been operating to eliminate them. Kevin and his group were watching Common Noddys flying about a metre above the golf course searching for nesting material one day, when to their surprise a cat leaped up from a clump of grass where it had been lying unseen in front of them and caught a Noddy in full flight before making off with it. From its manner, it was not the first time.

National Parks established a nursery to rehabilitate mined areas in 1989. About 30 different species of native tree are used to revegetate. A clever ploy to aid diversity over the planted areas is the introduction of an occasional Jamaican Cherry (Muntingia calabura), an introduced species that has become naturalized. This small, fast-growing tree produces lots of fruit attractive to fruit eating birds such as the Imperial Pigeon and Christmas Island White Eye. The birds flying in from surrounding forest then drop seeds from other species they have been feeding on.

Joan Sharpe

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