Christmas Island rises steeply from the sea floor. A series of terraces surrounds 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 national park also includes two Ramsar Wetlands of International Importance.
The island environment is classified into 12 broad habitats:
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.
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 catapa.
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.
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.
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.
Christmas Island has a diverse subterranean environment with freshwater, marine, anchialine and terrestrial habitats. Although poorly known, the cave fauna is a significant component of the island's biodiversity.
Subterranean animals are found in air-filled (troglofauna) and water-filled (stygofauna) voids. With at least 12 endemic species, Christmas Island’s cave fauna are internationally significant.
In addition to swiftlets, a diverse range of terrestrial and aquatic invertebrates live here, including a number of rare and endemic species of high conservation significance. Terrestrial isopods, a new species of blind scorpion and a new genus of cockroach are among the latest identified fauna.
These are ecosystems near coastal groundwater, in caves with no surface connection to the sea but affected by the marine tide. One of only two anchialine systems known in Australia, they contain endemic aquatic fauna, including 10 new families of crustacea identified in the last decade. Anchialine systems are very vulnerable ecosystems and globally are the subject of widespread conservation assessment.
Christmas Island National Park has two listed Wetlands of International Importance under the Ramsar Convention: the Dales and Hosnies Springs.
The Dales is a picturesque series of seven watercourses, three with permanent springs and permanent flowing water and four with intermittent streams.
The Dales is surrounded predominantly by semi-deciduous forest. On the seaward side a line of coastal shrubland merges with sea cliffs and rocky marine shores, extending to part of a narrow, shallow, sloping reef. Mixed amongst the terrestrial and marine environments are a range of karst features. The variety of habitats and the permanent surface water support a wide diversity of endemic and threatened species.
Plants and animals
At Hugh’s Dale, and in parts of Anderson Dale, lie tall stands of splendid Tahitian chestnuts (Inocarpus fagifer). The rare epiphytic ribbon fern (Ophioglossum pendulum) grows in trees and endemic plants such as the arenga palm (Arenga listeri) and Ridley’s orchid (Brachypeza archytus) are common.
The Dales is significant habitat for endemic blue crabs which are only found in wet areas of Christmas Island, as they are dependent on freshwater streams for their reproductive cycle. Migrating red crabs pass through the area on their annual breeding migration and the area supports a resident population of red crabs and land crabs such as the robber crab.
Endemic birds include the Abbott’s booby red-footed booby and the brown booby, all of which breed at the site. Migratory bird species use The Dales as a staging site during migration, and it is a landfall for vagrant bird species outside their range.
Christmas Island’s only native freshwater fish, the brown gudgeon (Eleotris fusca) is also found in the streams at The Dales.
Hosnies Spring is an entire mangrove ecosystem growing in a freshwater spring 37 metres above sea level. The mangroves are the largest of their species ever seen with canopy heights 30-40 metres tall, and the stand is estimated to have persisted at the site for around 120,000 years.
Hosnies Spring is an area of permanent, shallow freshwater wetland, one of the few permanent freshwater areas on Christmas Island, fed by a natural spring system located approximately 30 metres above sea level.
Plants and animals
The area that surrounds the wetland site is predominantly rainforest, characterised by a 20 to 30 metre tall canopy of evergreen and deciduous trees such as Pisonia grandis and Barringtonia racemosa and a conspicuous lack of herb and shrub layers. A narrow band of coastal scrub with hardy species such as Scaevola taccada lies at the seaward edge of the shore terrace, with an unvegetated area of limestone pinnacles at the top of the sea cliff. The cliff descends some 17 metres almost vertically to the rocky marine shore below. The Ramsar site extends 50 metres seaward of the low water mark and includes areas of shallow coral reef.
Hosnies Spring is remarkable not only for the extraordinary age of the mangroves and the very large size of individual trees, but their occurrence at an elevation not recorded anywhere else in the world.
The site supports endemic and significant fauna species including land crabs (in particular red, robber and blue crabs), sea and forest birds and the Christmas Island flying fox.