Christmas Island is renowned for its tropical seabird rookeries and a wealth of forest bird species.
A seabird mecca
Around 80,000 seabirds nest here annually, with 23 breeding or resident species.
More than 100 vagrant and migratory bird species have been recorded here, including eight breeding species. The most numerous is the wide-ranging red-footed booby, which nests in colonies in trees on many parts of the coastal shore terraces. The endangered Christmas Island frigatebird soars in the skies – it’s the world’s rarest frigatebird and nests only on the island. The golden form of the white-tailed tropic bird is an endemic sub species unique to Christmas Island. Known locally as the golden bosun, this stunning bird is graceful in flight and has been adopted as the island’s fauna emblem
The evergreen tall rainforests provide the world’s last remaining nesting habitat for the endangered Abbott’s booby. These birds nest exclusively in the branches of the trees that emerge above the forest canopy from which they fly out daily to their ocean feeding grounds.
The rainforests are also alive with a variety of forest birds — including seven endemic species, the Christmas Island hawk owl, thrush, goshawk, emerald dove, imperial pigeon, gossy swiftlet and white eye, four of which are listed as threatened species.
Bird ‘n Nature Week
Each September, Christmas Island National Park and the Christmas Island Tourism Association host a week long program of immersion into the Island’s wildlife. Researchers and natural history experts run a range of wildlife activities, such as studies of the nesting biology and foraging ecology of Christmas Island frigatebirds and red-tailed tropicbirds and surveys of distribution of rare birds such as the Christmas Island hawk owl survey and the Island thrush. On offer are seabird identification workshops and nightly seminars showcasing seabird research, the status of the endemic landbirds, and the island’s marine and terrestrial ecology.
Christmas Island is home to an enormous abundance and diversity of land crabs, not matched anywhere else in the world. More than 20 terrestrial and semi-terrestrial crab species have been found on the island. They include the endemic blue crabs that inhabit wetlands and the large robber crabs which are often seen in the forests and on the island’s roads. The most famous are the tens of millions of red crabs.
Did you know?
The extraordinary robber crabs are the biggest land crustaceans on earth. Christmas Island has the world’s largest and best protected population of these gentle giants – it is a major robber crab sanctuary. Robber crabs have exceptional climbing abilities. They earn the name robber crab as they will obsessively pick up and cart off any foreign items they may come across and are well known for stealing shiny objects such as pots and silverware from camps. Robber crabs are slow growing and there is strong evidence that they live to be more than 50 years old.
Robber crab - Birgus latro
Robber crabs are a close relative of hermit crabs, but do not (at least as adults) carry shells to protect their abdomen. They are found in most parts of the island, from the shore terrace to the highest plateau areas, generally sheltering during the day and venturing out at night or on overcast days. They forage on any vegetable material or carrion on the forest floor. The fruits of some trees and the pith of fallen Arenga palms are favourite foods. Though found through the tropical islands of the Pacific and Indian Oceans, Christmas Island now supports the world's largest population.
Road to recovery for robber crabs
Between 2010 and 2012, more than 2,000 robber crabs were killed by vehicles on island roads. National park staff drive the roads regularly to locate the carcasses of these gentle giants as part of a study of the frequency and distribution of robber crab road kills. They record the date, GPS coordinates and details such as sex and size, and paint a fluorescent pink circle with a cross to mark the fatality. It’s a reminder to all road users to slow down and take care when driving.
Hermit crabs - Coenobita sp.
There are three species of the genus Coenobita recorded on Christmas Island. When spawning, hermit crabs carry their eggs inside the shell attached to hairs on the pleopods on the left side of their body.
Red hermit crab - Coenobita perlatus
The red hermit crab is distinguished by flattened triangular eyestalks, a red carapace , and red about the joints of the legs on a white to pink background. It has a carapace length greater than 34 mm. Red hermit crabs are recorded on most beaches, although they prefer rubble beaches. Hermit crabs are shy and stay close to cliffs.
Purple hermit crab - Coenobita brevimanus
The largest of the island's hermit crabs (up to 45 mm carapace length), this is a distinctive purplish brown crab with cylindrical eyestalks. It is found mainly on the beaches and shore terraces.
Horn-eyed ghost crab - Ocypode ceratophthalmus
The horn-eyed ghost crab has a carapce greater than 45 mm. There are two colour forms on Christmas Island - one is deep olive green (common on Dolly Beach) and the other is greyish yellow to cream. It may be that these are two distinct species. The 'horned eyes' are shorter in females and half grown crabs and undeveloped in juveniles.
Blue crab - Discoplax celeste
In 2012, an international team of biologists discovered this new blue-colored species of land crab on Christmas Island. Its species name reflects the sky blue colour of the adult crab. The blue crab is restricted to parts of the island where there is surface fresh water. It was extensively harvested for food during the 1950s and has been fully protected since 1980.
Some 50 million red crabs live on Christmas Island – the only place in the world where they are found.
These stunning creatures are extremely important to the island’s rich biodiversity. They are a keystone species in the forest ecology, influencing the structure and function of the rainforest by selectively consuming seeds and seedlings, cleaning up leaf litter, turning over the soil, and fertilising it with their droppings.
At the beginning of every wet season millions of adult crabs begin a spectacular migration from the forest where they live down to the coast to breed. They release eggs into the sea, then return to the forest.
Where do they live?
Although most common in the moist environment of the rainforest, red crabs live in a variety of habitats including coastal shore terraces and even domestic gardens. They dig burrows in soil or live in deep crevices in rock outcrops. For most of the year, a crab will settle in one place, living in its burrow.
Red crabs are diurnal (active during the day) and almost inactive at night despite lower temperatures and higher humidity. Their sensitivity to moisture, combined with the seasonal climate on Christmas Island, create a distinct seasonal pattern of activity. Crabs retreat into the humid interior of their burrows during the dry season. They plug the burrow entrance with a loose wad of leaves to maintain a high humidity level, and effectively disappear from view for up to two to three months of the year.
What do they eat?
The red crabs’ diet consists mainly of fallen leaves, fruits, flowers and seedlings. They are not solely vegetarian and will eat other dead crabs, birds, the introduced giant African snail and palatable human refuse if the opportunity presents itself.
Did you know?
The beginning of the red crab breeding migration is difficult to predict. It starts with the first rains - usually in October or November, but sometimes as late as December or January. The spawning date – when the eggs are cast into the sea - is fixed to a particular lunar phase: a pre-dawn receding high tide during the last quarter of the moon. These fantastic creatures seem to know how to match the prevailing weather conditions with the best spawning date. If the rains arrive late, they may migrate rapidly and if they’re early they may meander down to the sea, often stopping to eat and drink on the way. If the rains arrive too late to meet the next lunar spawning date, they’ll stay in their burrows until the following month.
Red crab migration
At the beginning of the wet season (usually October - December), most adult red crabs suddenly begin a spectacular migration from the forest to the coast to breed and spawn. The migration is usually synchronised island-wide. The crabs need moist overcast conditions to sustain them during their long and difficult journey to the sea.
The breeding sequence
Males lead the first wave of the downward migration and are joined by females as they progress. Larger males arrive at the sea first (after about five to seven days) but are soon outnumbered by females. The crabs replenish moisture by dipping in the sea.
The males then retreat to the lower terraces to dig burrows. The density of burrows is high (one to two per square metre) and males fight each other for burrow possession. The females move to the terraces and mating occurs.
After mating, males dip again and begin returning inland.
The females produce eggs within three days of mating and remain in the moist burrows on the terraces for 12-13 days while they develop. The eggs are held in a brood pouch between their extended abdomen and thorax. A single female can brood up to 100,000 eggs.
In the morning and late afternoon around the last quarter of the moon, the egg-laden females descend from the terraces to the shoreline. They pack into shaded areas above the waterline, in densities of up to 100 per square metre in some places. The females usually release their eggs into the sea toward dawn, around the turn of the high tide. Release of eggs may occur on five or six consecutive nights during the main breeding migration.
Larvae grow to baby crabs
The eggs released by the females hatch immediately on contact with the sea water and clouds of young larvae swirl near the shore before being washed out to sea by waves and tides. Millions of the larvae are eaten by fish, plankton feeders such as manta rays and the enormous whale sharks which visit Christmas Island waters to feed.
After about a month in the ocean, and after growing through several larval stages, the surviving larvae have developed into prawn-like animals called megalopae. The megalopae gather in pools close to the shore for one to two days before changing into young crabs and leaving the water.
Although only five millimetres across, the baby crabs begin their march inland, taking about nine days to reach the plateau. Here they are rarely seen, disappearing into rocky outcrops and fallen tree branches and debris on the forest floor for the first three years of their life.
In many years, very few or no baby crabs emerge from the sea, but the occasional very successful year (perhaps only one or two every 10 years) is enough to maintain the red crab population to a high level.
Managing human impacts
Human activities have led to increased numbers of red crabs being killed during their annual migration. The crabs risk dehydration when they are forced to cross areas cleared of forest cover and many thousands of adults and young are crushed by vehicles as they cross the roads. Some have to negotiate up to three or four such hazards on their march to and from the sea each year.
To reduce this high death toll, a range of conservation measures is in place. To protect the crabs from being crushed by vehicles, park staff and the shire erect crab crossing signs so that drivers slow down. Some roads may be temporarily closed off with traffic detours. Park staff have built walls and plastic fencing along the roads, to funnel the crabs to 'crab crossings', 'crab bridges' and 'crab underpasses' where they may safely cross.
Park staff work actively with all employers on the island, so that all new staff are alerted to the need to drive slowly and drive around Christmas Island's red crabs.
Of the five native mammals, two have officially been listed as extinct since the arrival of humans. Maclear’s rat (Rattus macleari) and the bulldog rat (Rattus nativitatus) are believed to have become extinct within a few years of the introduction of exotic rodents by early human colonisers.
The Christmas Island shrew (Crocidura attenuata trichura) was thought to be extinct before two specimens were found in 1984 and 1985, and it is now listed as critically endangered. There have been no confirmed sightings since, although there has been some research to ascertain its current status.
The Christmas Island pipistrelle (Murray's pipistrelle bat, Pipistrellus murrayi), an endemic small insectivorous bat, was previously common and widespread on the island. However, in the 1990s and early 2000s, the bat declined markedly in distribution and abundance. Despite a rescue attempt in 2009-2010, no pipistrelles were found The bat is classfied as critically endangered but is believed to be extinct. It is not fully understood what has caused this rapid decline. For more information, see the report of the Expert Working Group on Christmas Island’s biodiversity.
The Christmas Island flying-fox (Pteropus melanotus natalis) is an endemic subspecies which was formerly widespread but has experienced significant population declines, the causes of which are still unknown. Surveys across the island in 2012 and 2013 confirmed that populations had declined since 2006. Park managers are now working with scientific experts, including the National Environmental Reserach Program (Environment Decisions Hub), to develop future management options.
Four exotic wild mammals have been introduced to Christmas Island: the black rat (Rattus rattus), the house mouse (Mus musculus) the feral cat (Felis catus) and wild dog (Canis familiaris).
Christmas Island is home to six native terrestrial reptiles. Five species are endemic: the blue-tailed skink Crytptoblepharus egeriae and the forest skink Emoia nativitatis; the giant gecko Cyrtodactylus sadleiri and Lister's gecko Lepidodactylus listeri; and a burrowing snake, the pink blind snake Ramphotyphlops exocoeti. The sixth native reptile, the foreshore skink Emoia atrocostata, is common on oceanic islands.
Declining reptiles in the wild
In the late 1990s and through the 2000s, there was a marked decline in the range of several species. The endemic blue tailed skink virtually disappeared from the north-east and eastern parts of Christmas Island. The nationally-vulnerable Lister’s gecko was thought extinct on Christmas Island for more than 20 years, until in 2009 a small population was discovered.
Introduced invasive species, particularly the yellow crazy ant, giant centipede, the Asian wolf snake, cats and rats, are thought to be the major reasons for the reptiles’ decline.
To bring back the reptiles at risk of extinction, in 2009 Christmas Island National Park embarked on a successful captive breeding program for the blue-tailed skink and Lister’s gecko.
Captive populations are contained within the national park, and as a safety measure against any on-island disasters, at Sydney’s Taronga Zoo. Populations continue to thrive. In mid 2013, there were 343 Lister’s geckos and 111 blue-tailed skinks housed on Christmas Island, with smaller numbers of 143 and 52 breeding at Taronga.
To house the growing reptile populations, the park has built a new reptile housing facility and will finalise the construction of eight predator-proof exclosures on the island in 2013,funded by a grant from the Foundation for National Parks and Wildlife.The exclosures will hold up to 1,000 skinks.
The park continues to work in the reptiles’ natural habitat to reduce threats, which also impact on a number of other native species.
To control cat and rat populations, the park works with the local shire to coordinate a multi-stakeholder program that includes several island agencies and the WA Department of Environment and Conservation.
A Christmas Island Reptile Advisory Panel provides advice on the captive breeding program and conservation activities.
Yellow crazy ants are the fiercest threat to the island’s biodiversity. A yellow crazy ant control program, advised by the Crazy Ant Scientific Advisory Panel, has gone from strength to strength with successful aerial baiting in 2012 and the signing of a contract with LaTrobe University to implement a biological control program from July 1 2013.
Further long term work will be needed to identify and address specific threats impacting on reptiles before captive populations can be released into their natural habitat.
Regionally abundant exotic reptiles associated with human colonisation are found in settled areas of Christmas Island and in the national park.
The house gecko Gehyra mutilata, the barking gecko Hemidactylus frenatus, and the black blind snake Ramphotyphlops braminus are found in the park. The wolf snake Lycodon aulicus capucinus appears to be moving further into the rainforest, and grass skinks Lygosoma bowringii have recently been seen on the fringe of the park rainforest.
Christmas Island’s invertebrates include 14 snails, 28 butterflies, some 70 moths, 90 beetles, 30 spiders, one scorpion, five false scorpions and one amphilicid. Several hundred species have been collected and await identification.
There are 28 species of butterfly on Christmas Island (2006) , including the endemic Christmas Island emperor, the Climena crow and the scalloped grass-yellow).
Some species feed only on introduced plants that have become established in cleared and disturbed habitats. Whether these butterflies arrived unassisted (that is, they flew in as adults from Asia) or came as larvae in shipments of supplies to the island is unknown.
Most of the resident butterfly species on the island are common. The Climena crow and scalloped grass-yellow are abundant in open habitats and also occur inside the forest. The Christmas Emperor is a rainforest canopy species and is therefore difficult to observe. It is sometimes seen floating over the canopy along the roads and tracks or the plateau.