Christmas Island is a classic example of a tectonically uplifted coral atoll with its characteristic steep series of rainforest-covered terraces and sheer limestone cliffs. The island's geological formations are significant in illustrating the evolution of the Christmas Rise due to tectonic and volcanic action and the collision of the Asian and Australian plates.
The evolutionary significance of Christmas Island is demonstrated both by its high level of endemism and by its unique assemblage of plant and animal species.
The dominance of the land crabs is a striking feature of the island's fauna. The island has thirteen of the twenty species known worldwide and one of the highest land crab densities known in the Indian Ocean. The land crabs of Christmas Island are remarkable for their variety and numbers and for the role they play in the ecology of the rainforest. The endemic red land crab (GECARCOIDEA NATALIS) is numerically the most notable of this crab assemblage with an estimated population of approximately 120 million crabs. The threatened robber or coconut crab (BIRGUS LATRO), with a population estimated at one million individuals is one of the largest remaining in the world.
Christmas Island is famous for its spectacular annual red crab migrations from the plateau rainforest to the sea during the wet season. The migrating population has been estimated at numbering 30-45 million adult crabs.
The rainforests of Christmas Island are biogeographically significant; species have evolved from being either shoreline forest or early rainforest succession species to those that fill a tall climax rainforest role. The Island contains unique plant communities of high conservation and scientific interest including a variety of elevated and relict cycad and back-mangrove communities of international significance.
The presence of seventeen endemic plant species in the climax rainforest community contributes to the place's significance for understanding evolutionary relationships. Notable examples include a rare fern ASPLENIUM LISTERI, a tall tree-like pandanus PANDANUS ELATUS and a palm ARENGA LISTERI. The island's rich endemic fauna includes three mammal species, ten bird species, five reptile species, one crab species, two insects, three marine fish species and several marine sponges species. The island is recognised as an internationally significant Endemic Bird Area. The well-developed karst landscape of Christmas Island contains an internationally significant cave fauna with twelve endemic invertebrate species.
The island is also one of the world's most significant seabird islands, both for the variety and numbers of sea-birds, with over one hundred species of bird having been recorded, including eight species which breed on the island. The island rainforest provides significant habitat for two endemics the nationally endangered Abbott's booby (PAPASULA ABBOTTI) and the nationally vulnerable Christmas Island frigate bird (FREGETA ANDREWSI).
The island's relatively simple fringing reefs and adjacent waters support a rich diversity of marine species typical of Indian Ocean tropical reefs. The island also provides habitat for two nationally vulnerable species of turtle, the green (CHELONIA MYDAS) and hawksbill (ERETOCHELYS IMBRICATA), which nest on two of the Island's beaches and two nationally vulnerable shark species.
Christmas Island is one of the most scientifically documented oceanic islands in the world. Island ecosystems have been historically critical in the development of evolutionary theory as they highlight natural selection, speciation and niche filling. Christmas Island correspondingly is a significant location for scientific research. The unique ecosystems of the Island present special opportunities for the study of evolution of species in relative isolation and the adaptation of migrant species to new environments. These species have often evolved to fit different ecological niches to which they are usually associated and the rainforests on the island exhibit species with many of these characteristics.
Christmas Island provides habitat for four nationally endangered and six nationally vulnerable fauna species, and one nationally vulnerable plant species.
There are a number of places of cultural heritage value included within or adjacent to this area that are included in the Register of the National Estate (see Register database). It is possible that additional cultural heritage values exist within the area that are yet to be identified.
Christmas Island is an isolated seamount of volcanic origin with a discontinuous capping of limestone. It rises from ocean depths of 4,500 metres and reaches a maximum altitude of 357 metres above sea level at Murray Hill. From the undulating central upper plateau the land descends to the sea in an alternating series of steep slopes or cliffs and relatively level terraces. A 10-20 metre wave-cut sea cliff is continuous around the island except for a few small beach areas and at Flying Fish Cove where the main settlement and port is located and acts a significant barrier to landing from the sea.
The island's phosphate-rich soils were historically thought to have been derived from the decomposition of avian guano over the centuries but are now thought to have resulted from weathering of marine lagoonal sediment deposits, which date from the time the island was still a coral atoll. However the geochemistry of their formation is still unclear.
The limestone covering of the island is riddled with karst formations including caves and sinkholes, the more extensive systems following fault lines. As the island is predominantly porous limestone there is little surface water except where it collects at the interface between the limestone and relatively impervious underlying basalt. These surface springs are significant ecologically for moisture loving species, which include relict mangrove stands and the island's distinct blue crabs. Also associated with the extensive cave system of the island is a group of internationally significant cave invertebrates, which live both in cave waters (stygofauna) and in the air filled passages (troglofauna).
Christmas Island is subject to the influence of northwest monsoons with strong prevailing winds and heavy rains and swells occurring during the summer months of December to April. Due to the seasonality of the island's rainfall, the wet season is the real period of productive plant growth. The island's animals are similarly adapted and an example includes the spectacular annual summer migration of the endemic red crab to breed.
In common with all oceanic islands, Christmas Island has a distinct biota, which include an unusual assemblage of plants and animals. These species have adapted to the island's seasonal climate, phosphate-rich soils and karst-dominated environment. This biota is both rich and diverse and includes a significant variety and number of seabirds. Because of the distance of the island from any landmass, there is notable endemism in the plants and animals, the result of evolution in isolation. Christmas Island is situated within the overlap of the Australian and Malaysian regions and has representatives of the flora and fauna from both regions. A significant example is the Christmas Island shrew (CROCIDURA ATTENUATTA TRICHURA), which is Australia's only shrew.
As a forested oceanic island with access to a rich nutrient oceanic upwelling near Java it provides an important breeding environment for many sea birds. The island is relatively undisturbed when compared to other Indian Ocean islands. The island has three notable endemic seabirds, which include the endangered Abbott's booby (PAPASULA ABBOTTI), which is now recognised as the oldest of the sulids and belongs to its own genus. The other two notable endemic seabirds are the vulnerable Christmas Island frigate bird (FREGETA ANDREWSI) and a sub-species the golden bosun or white-tailed tropicbird (PHAETHON LEPTURUS FULVUS). The island also has seven endemic land birds, which fill a variety ecological niches on the island, including two raptors, two rainforest fruit eaters and a cave dweller. Many migratory birds also live on or visit the Island, including birds from Japan and China, and are covered by the migratory provisions of the Environment Protection and Biodiversity Conservation (EPBC) Act. These migratory species include the Abbott's booby, Christmas Island frigate bird, white-tailed tropicbird, greater frigate bird (FREGETA MINOR), red-footed booby (SULA SULA) and the endangered Christmas Island goshawk (ACCIPTER FASCIATUS NATALIS). Vagrant species from Asia and Australia also visit the island.
The flora comprises about 385 documented species of flowering plants and ferns of which seventeen species are endemic. Plateau and terrace rainforest formations determine the two broad subdivisions of vegetation types on the Island.
The plateau rainforest is floristically simple in structure and carries a well-preserved and tall primary rainforest, between 30-40 metres in height, with a sparse understorey and the predominantly litter bare floor, a result of crabs scavenging leaf litter and other vegetative material. Dominant tree species include PLANCHONELLA NITIDA, SYZIGIUM NERVOSUM, and INOCARPUS FAGIFER, which have tall boles and are elaborately buttressed. The island's plateau rainforest is also notable as it lacks the dense understorey normally associated with their related Indo-Malaysian rainforests and instead have a aesthetically pleasing clean open forest floor which is in part due to the dominant grazing pressure of the island's abundant land crabs.
The island's terrace vegetation is more diverse, due to the variability of the habitats from sea cliffs to shore terraces to the margins of the deeper soiled plateau. The marginal rainforests are more diverse species-wise, more open and with lower canopies of 20-30 metres. The sea cliffs have salt tolerant species (PANDANUS thickets and heaths), and grade through to vine thickets and open forests on the shore terrace to marginal rainforests. The marginal rainforest includes both evergreen and deciduous species including TERMINALIA CATAPPA, MACARANGA TENARIUS and CELTIS TIMORENSIS. In some areas tree species become dominant. Examples include an ARENGA LISTERI palm stand on basaltic soil near Great Beach and shore terrace stands of PISONIA GRANDIS and BARRINGTONIA RACEMOSA. Areas with surface water are often the habitat of a cycad CYCAS RUMPII and also back- mangrove species, such as the Ramsar convention listed Wetland of International Importance sited at Hosnie's Spring. This internationally significant locality comprises a freshwater spring with an unusually tall BRUGIERA species mangrove stand and is located 37 metres above sea level. This stand is estimated to be have been stranded by an uplift of the island around 120,000 years ago. Another two remnant stands of back-mangroves occur and are comprised of the two species HERITIERA LITTORALIS and CYNOMETRA RAMIFLORA.
In 1991 an Australian botanist Tracy recognised two structural groups in the plateau rainforest; one as semi-deciduous mesophyll vine forest (SDMVF) and the other as a complex mesophyll vine forest (CMVF) of a floristically simpler form when compared to similar rainforests elsewhere. Tracey also classified the terrace rainforest as SDMVF or as deciduous vine forest and observed that its tree species were shared with Australia's tropical coast but that they were unusually tall on Christmas Island.
The islands diverse land crab assemblage is ecologically important, and includes members of the widespread tropical genus's GECARCINUS, BIRGUS and COENOBITA. These land crabs dominate the scavenging role usually occupied by small mammals and ground birds and exert an ecologically significant selective pressure on the recruitment and distribution of the island's rainforest plant species and so are partially responsible for the simple structure of the islands plateau rainforest. Fourteen species of land crab are present and include the notably abundant and endemic red crab (GECARCOIDEA NATALIS) whose spectacular migration in large numbers from the plateau rainforest to the sea during the wet season to breed is a well known feature of the Island. Another land crab, Jackson's crab (SESARMA JACKSONI) is yet to be confirmed as an additional endemic species but information sources indicate that it is yet to be recorded anywhere else. The blue crab (CARDISOMA HIRTIPES) only occurs in its blue form on the Island, however further genetic investigations may show it to be endemic sub-species. Notably the whale shark (RHINIODIN TYPUS), which is a plankton feeder, is thought to time its visits to the island to coincide with the summer red crab larvae bloom. Another plankton feeder observed in the island waters is the manta ray (MANTA BIROSTRIS).
There are few inhabited oceanic islands at similar latitudes that have a similar ecological integrity as exhibited by Christmas Island and this is in part due to the understorey grazing and predation pressure exerted by the island's land crabs.
Eight species of terrestrial reptile have been reported from the Island, and it provides important habitat for two of these, the blue tailed skink (CRYPTOBLEPHARUS EGERIAE) and the tree gecko (LEPIDODACTYLUS LISTERI) as both have respectively had a reduction in range or numbers in recent times.
In addition to reptiles the island provides important habitat for a depauperate mammal fauna. The endemic fruit bat (PTEROPUS NATALIS) is plentiful, and another endemic the insectivorous bat (PIPISTRELLUS MURRAYI) is now nationally endangered. The nationally endangered endemic shrew was thought to be extinct until specimens were found in the mid-80's and the island also historically had two endemic rats which are now extinct due to the introduction of exotic rats. These two endemics may have allowed the island's endemic birds to adapt to coexisting with rats and so may have minimised the impacts of the introduction of exotic rat species, which have decimated other oceanic island's bird assemblages elsewhere in the Indian Ocean.
The islands recorded marine species diversity include five hundred and seventy five fish species and eighty eight coral species and over three hundred molluscs, ninety echinoderms and two hundred decapod crustacean species. The fringing reef platform abounds with living corals, particularly ACROPORA and provides an important source of food for terns and tropic birds. Reef waters offshore are clear with good coral formation on the narrow coastal shelf which ends abruptly in a steep seaward slope, often with a spectacular vertical edged drop-off.
Christmas Island has been referred to in a variety of ecological and geological papers. Ridley (1930) used Christmas Island's distinct plant ecology in a treatise on 'The Dispersal of Plants around the World' and Carlquist (1965) highlighted the importance of islands such as Christmas Island in highlighting evolutionary processes.
The Island has also historic connections with the birth of oceanography. Sir John Murray, of "Challenger" fame, was the first to recognise the importance of the Island's phosphate deposits and lobbied for their exploitation. Murray was both the founder of the settlement and also the sponsor of a classic comprehensive natural history survey of the island, which was conducted prior to phosphate mining being started on the island