Christmas Island rises steeply from the sea floor and can be viewed as a series of terraces around an irregular plateau with the lower terrace cliffs steeper and higher than the upper terraces. The deepest soils occur on the central plateau and the upper terraces. The distribution of plants can be correlated to soil depth and moisture, exposure and distance from the sea. The coastal zone is generally saline with thin soils. The zone immediately behind this is also a harsh habitat for most plants, especially where it faces the prevailing south-easterly winds. Pandanus and salt bush (Scaevola taccada) are typical of places exposed to salt spray. Further inland, the environment becomes more sheltered and rainforest develops with structure and floristics determined by the depth and type of soil.
Rich marine ecosystems
Northwest Point seacliff
The ocean waters, sand flats, caves, coral reefs and walls, and coral heads or 'bommies'.
- Shoreline rock platforms
These occur at many locations around the Island, particularly at North West Point and Egeria Point. At low tide there are many colourful pools with a variety of marine invertebrates and fish.
Formed of coarse coral and shell rubble, often with limestone outcrops; there are some small sand beaches on the north and east coasts. Dolly and Greta Beaches provide habitat for hermit and ghost crabs and are the only beaches with sufficient and stable deposits of sand to support turtle nesting activity.
These average 10-20 m high, rising to 60 m at Steep Point. A harsh environment, exposed to salt spray and salt laden wind. Plants such as Pemphis acidula, Pandanus christmatensis, Argusia argentea and Scaevola taccada are typical. A silver bosun and brown booby nesting habitat.
- Terrace forest
An area of generally shallow soils prone to dehydration in the dry season. Open, deciduous forest is typical on the coastal terraces, with scrambling and spiny shrubs and vines. Both vine and canopy forests also occur. Typical species are Acronychia trifoliolata, Berrya cordifolia, Calophyllum inophyllum (mainly east coast), Erythrina variegata, Hibiscus tiliaceus, Kleinhovia hospita, Ochrosia ackeringae, Pandanus elatus, Pisonia grandis and the endemic Grewia insularis, Gyrocarpus americanus, Terminalia catappa.
- Shallow soil rainforest on the higher terraces
Generally thin soils and exposed limestone pinnacles. Typical canopy species include Celtis timorensis, Dysoxylum gaudichaudianum, Ficus microcarpa, Inocarpus fagifer, Tristiropsis acutangula and Planchonella nitida. Vegetation is structurally lower and floristically richer than the climax rainforest of the plateau. Some pockets of deeper soil occur, supporting taller trees. In sheltered gullies especially on the south east coast below and to the south of Ross Hill, tall closed forest has developed.
- Limestone scree slopes and pinnacles
The inland cliffs rise out of the terrace forest and support a sparse community of plants. Banyans Ficus microcarpa, are common and the endemic stinging tree Dendrocnide peltata var.murrayana (one of three species of Dendrocnide) can be found on scree slopes.
- Deeper plateau and terrace soils rainforest
Typically a tall closed canopy rainforest with emergent trees to 45 m and a habitat for Abbott's booby. Typical emergent species are Syzygium nervosum, Ficus microcarpa, Planchonella nitida and Hernandia ovigera. The canopy comprises Barringtonia racemosa, Inocarpus fagifer, Cryptocarya nitens, Dysoxylum gaudichaudianum and Tristiropsis acutangula with an understorey of Arenga listeri, Pandanus elatus, Leea angulata, Ochrosia ackeringae, Pisonia umbellifera and various shrubs and herbs.
- Mangrove forest
There are no coastal mangroves, but a stand of normally estuarine Bruguiera gymnorhiza and B. sexangula occurs at Hosnie's Spring (registered as a Ramsar Wetlands site of international importance) about 50 metres above sea level. Two other mangrove species occur on the east coast. Heritiera littoralis occurs on the inland terrace above Greta Beach (outside the park) and further south towards Dolly Beach, as well as a discrete stand on the terrace above Dean's Point. Cynometra ramiflora occurs in two small stands south of Ross Hill.
- Perennially wet areas
Typically support Tahitian chestnuts Inocarpus fagifer that create a habitat used by blue crabs.
- Karst, comprising caves, overhangs, rock crevices and sinkholes
Glossy swiftlets nest in the caves and overhangs (refer to the cave management plan for details of other cave fauna).
- Mining fields
Typically limestone pinnacles resulting from the removal of soil. Thin soils support stunted Japanese cherries, the ferns Nephrolepis multiflora and Psilotum nudum and the exotics Mimosa invisa and M. pudica. Stockpiles of topsoil are colonised by Claoxylon indicum, Macaranga tanarius, Melochia umbellata and the exotic Leucaena leucocephala, among others.
The park includes a marine area extending 50 m seaward of the low water mark where terrestrial areas of the park include the coastline. This marine area incorporates approximately 46 km (63 per cent) of the Island's 73 km of coastline. Shoreline platforms slightly above sea level occur round the Island, predominantly on the coast between North West Point and Egeria Point. At low tide on these platforms there are many small pools maintained by wave splashes, with a variety of invertebrates and fish.
There is little algal growth due to the prevailing heavy seas. The shoreline platforms descend directly to a narrow band of shallow coral reefs with no intervening sandy, shallow reef flats. A few small beaches of coarse coral and shell rubble occur along the coastline. The shallow reefs drop off steeply so that there is little deep reef habitat before abyssal depths are reached.
The subterranean environment of Christmas Island is diverse and includes freshwater, marine, anchialine and terrestrial habitats. Although poorly known, the cave fauna is a significant component of the island's biodiversity. Subterranean fauna are found in air-filled (troglofauna) and water-filled (stygofauna) voids. With at least 12 endemic species, Christmas Island is a significant cave fauna province in an international context. The cave fauna comprises swiftlets, and a diverse assemblage of invertebrates, both terrestrial and aquatic, including a number of rare and endemic species of high conservation significance. Notable amongst cave collected fauna are terrestrial isopods, a new species of blind scorpion and new genus of cockroach.
These are ecosystems near coastal groundwater in caves with no surface connection to the sea but affected by marine tide. They contain an endemic aquatic fauna characteristically containing relictual of high taxonomic level. This is a distinctive ecosystem and within the last decade at least ten new families of Crustacea have been described from these newly environments and they are of high conservation significance. The only other anchialine system in Australia is the Cape Range/Barrow Island system. They are very vulnerable ecosystems and globally are the subject of widespread conservation assessment.
Within Christmas Island National Park are two Ramsar wetlands - The Dales and Hosnies Spring.
The Dales includes permanent and perennial streams, permanent springs, and include the majority of surface water on the Island. Most rainfall on Christmas Island filters down through the soil and limestone, and surface runoff only occurs after heavy rain. The Dales contain numerous wetland types including surface and karst features, and inland and coastal wetlands.
The Dales support a number of unique ecological and geomorphic features including anchialine cave communities, surface karst including the unique stepped tufa deposits at Hugh's waterfall, a stand of Tahitian chestnuts, a large number of endemic terrestrial species and a significant number of seabirds including Abbott's booby, red-footed booby and the brown booby, all of which breed at the site.
Vegetation in The Dales ranges from tall plateau rainforest to lower coastal vegetation. Migratory or vagrant bird species use The Dales as a staging site during migration, and a landfall for vagrant bird species outside their range.
Hosnies Spring is a small area of shallow freshwater streams and seepages, 20-45 metres above sea-level on the shore terrace of the east coast of the island. The Ramsar site consists of a stand of two species of mangroves of the usually tidal genus Bruguiera. The Ramsar site includes surrounding terrestrial areas with rainforest grading to coastal scrub, and includes an area of shoreline and coral reef.
Whilst mangroves of this group are distributed widely across the region, on Christmas Island the species are rare. The stand represented at Hosnies Spring is most unusual in that it occurs high above sea level (24-37 metres) on an inclined surface, the mangroves are unusually tall (up to 30-40 metres high) and because it appears that the stand has persisted at the site for approximately 120 000 years. The stand is maintained by the permanent freshwater spring.
The structure of the stand is also unusual in that it consists of a few very large individuals and abundant saplings and seedlings, typical of a population of long-lived trees where few individuals reach full height. The site is an example of a specific type of wetland unique to Christmas Island and perhaps unique worldwide.