Christmas and Cocos Islands tropical forests
Christmas Island and the Cocos (Keeling) Islands are two distinct Australian territories located in the Indian Ocean, known for their unique tropical forests. Christmas Island is characterized by its volcanic terrain, steep coastal cliffs, and extensive primary monsoonal rainforest, which is home to a diverse range of endemic species, including various bird and crab species. The island's tropical broadleaf forests cover approximately 75% of its land area and provide crucial habitats for breeding seabirds and other wildlife, supported by a rich epiphyte community.
In contrast, the Cocos Islands consist of flat coral atolls with limited freshwater sources and soils primarily made up of guano and coral. Their vegetation is dominated by coconut palms, and while the southern atoll has seen significant habitat loss due to plantations, the northern atoll retains much of its native forest. The islands host fewer endemic species compared to Christmas Island, although they support important seabird colonies and unique fauna, including the Cocos buff-banded rail.
Human activities, such as the introduction of invasive species and habitat alteration for agriculture, have posed threats to these fragile ecosystems. Additionally, both islands face challenges related to climate change, particularly concerning rising sea levels and extreme weather events. Conservation efforts are ongoing, including the establishment of national parks to protect these valuable environments and their biodiversity.
Christmas and Cocos Islands tropical forests
- Category: Forest Biomes.
- Geographic Location: Indian Ocean.
- Summary: These isolated oceanic islands possess large tracts of tropical forests that provide habitat for endemic terrestrial species; they also serve as important seabird rookeries and stopover points for migratory birds.
Christmas Island and Cocos (Keeling) Islands are two Australian territories located southwest of the island of Java in Indonesia and northwest of Australia in the Indian Ocean. Christmas Island is about 560 miles (900 kilometers) northeast of the Cocos Islands. Both territories have equatorial oceanic climates, with distinct tropical wet and dry seasons; average temperatures range from about 73–84 degrees F (23–29 degrees C), and average annual precipitation may exceed 79 inches (200 centimeters).
Christmas Island and the Cocos Islands are both volcanic islands, the latter being more typical of atoll ecosystems in the Pacific. The islands possess large tracts of broadleaf forests that are comprised of a combination of Indo-Pacific and Melanesian tree species, and serve as breeding grounds for colonizing birds. They also function as habitat for endemic (exclusively found in this biome) plant, bird, bat, reptile, and crab taxa.
Christmas Island
Christmas Island is situated at the peak of an extinct volcano rising from the ocean floor. Its landscape is comprised of steep coastal cliffs surrounded by a series of terraces and an interior plateau with large tracts of primary monsoonal rainforest. The highest points on the island are approximately 1,150 feet (350 meters) above sea level. Rainwater falling here filters through the soil and limestone, supplying several freshwater streams on the island. The role of Christmas Island as an important seabird rookery is underscored by the buildup of phosphate from the deposition of large amounts of bird guano over the millennia. Although phosphate mining has threatened the island's unique forest community in the past, approximately two-thirds of Christmas Island's land area is now protected within Christmas Island National Park, managed by Parks Australia.
The island's plant communities are dependent on soil type, depth, moisture, and distance from the sea, with tropical broadleaf forest making up 75 percent of the island's total area. The inland plateau is somewhat sheltered from the ocean, allowing for the development of relatively deeper soils compared with the terraced coastal regions of the island. These deeper soils permit the establishment of rainforest vegetation that creates a thick evergreen canopy and an open understory that supports a rich epiphyte community. Common canopy species include Malayan banyan (Ficus microcarpa), Tahitian chestnut (Inocarpus fagifer), satinash (Syzygium nervosum), soapberry tree (Tristiropsis acutangula), and Jack-in-the-box tree (Hernandia ovigera). Two endemic species—the Christmas Island palm (Arenga listeri) and a species of screwpine (Pandanus elatus)—dominate the understory.
Christmas Island is home to 16 land-crab species, representing very high diversity among island populations of such crabs. In addition to the Christmas Island red crab (Gecarcoideanatalis), another species of note is the robber crab (Birgus latro), which is the largest extant land-crab species.
Endemism within the forests of the island is especially pronounced. The Christmas Island fruit bat (Pteropus natalis) as well as a variety of endemic bird species are found here. The forest serves as important breeding habitat for Abbott's booby (Papasula abbotti); the Christmas Island frigatebird (Fregata andrewsi); the Christmas Island white-eye (Zosterops natalis); the Christmas Island imperial pigeon (Ducula whartoni); and the Christmas Island hawk owl (Ninox natalis), which is one of the rarest owls on Earth.
Only about 1,400 people inhabit the island. A concern to residents is the effect global warming could have on this small ecoregion. Although Christmas Island is less vulnerable to the potential impacts of climate change and sea-level rise than the Cocos Islands, being almost completely surrounded by a sea cliff of about 50 feet (15 meters) above sea level, there are a number of low-lying areas (such as the Kampong and Settlement) that are potentially exposed to sea-level rise and the effects of storm surge associated with climate change.
Cocos Islands
Also known as the Keeling Islands, the Cocos Islands are among the most isolated islands in the Indian Ocean. They consist of two separate groups of coral islands and reefs that make up two atolls, each sitting atop a volcanic seamount. The atolls are relatively flat, with soils composed of guano, pumice, and coral, and with thick stands of pisonia (Pisoniagrandis) and coconut palm (Cocosnucifera). Other species present include Laportea aestuans, Canavalia cathartica, and Erythrina variegata. Freshwater is available in lenses found only on the larger islets here. Vegetation of the southern atoll has been largely replaced by coconut plantations or other introduced plant species. However, the uninhabited North Keeling Island possesses large areas where the native forest community is thought to be largely intact.
Pulu Keeling National Park on North Keeling Island serves as a seabird rookery, supporting large breeding colonies of red-footed booby (Sula sula), great frigatebird (Fregata minor), lesser frigatebird (Fregata ariel), common noddy (Anous stolidus), and white tern (Gygis alba). It is also home to the one existing population of the endemic Cocos buff-banded rail (Rallus phillippensis andrewsi).
Because of isolation, low total land area, frequent inundation by oceanic cyclones, and likely recent (geologically speaking—about 4,000 years ago) submergence, the Cocos Islands terrestrial fauna has low species richness, and endemism is not as evident as on Christmas Island. No land mammals are native to the Cocos Islands, but there are a number of land crabs, insects, birds, and four reptiles, including three gecko and one blind snake species, that are native here.
Land crabs are common in the forest, including the little nipper crab (Geograpsusgrayi), the red land crab, the robber crab, and three species of hermit crabs. As on Christmas Island, these crabs quickly consume tree seeds, seedlings, and leaf litter on the forest floor and play a major role in regulating the plant community.
Human Impact
Human presence here has only been since the early 1800s. About 600 people inhabit this region—and only on the Southern Keeling Islands. Wildlife in the South Islands continues to be threatened by loss of forest due to coconut plantations. Although not inhabited by humans, there has been a great loss of bird habitat on the North Island, due to hunting and poaching parties that visit from the south.
One notable, human-induced habitat threat was the accidental introduction in the early twentieth century of the yellow crazy ant (Anoplolepisgracilipes) to Christmas Island. With few predators, an ideal climate, and very abundant food sources, these ants became widely established within a few decades. By the latter twentieth century, the thriving yellow crazy ant had built supercolonies on Christmas Island, and had devastated many native crab species, among other fauna; they also negatively altered the balance of rainforest species and canopy structure. Mitigation since the 1990s has come in the form of baiting with contagious poisons, sometimes by aerial spraying; these measures seem to have depleted the population of this tramp ant to manageable levels.
A larger concern may be that of global warming trends. The Cocos Islands are extremely vulnerable to the potential impacts of climate change and sea-level rise. These atolls have elevations between 3 and 12 feet (1 to 4 meters) above sea level, and any change in the mean sea level, combined with the effects of storm surge associated with large storms or cyclones, are likely to have dramatic consequences, especially for settlements on South Keeling.
To better protect the area, Cocos Island National Park, which was a protected marine reserve since 1982, became part of a larger reserve in 2021 called Bicentennial Marine Managed Area. The region is home to fourteen shark species, three of which are endangered.
Bibliography
Australian Government Publications. “Climate Change Risk Assessment for the Australian Indian Ocean Territories: Cocos (Keeling) Islands and Christmas Island.” January 2009. www.infrastructure.gov.au/sites/default/files/migrated/territories/publications/files/Final‗Report‗CKI‗and‗CC.pdf. Accessed 23 Dec. 2024.
Du Puy, D. J. “Christmas Island.” In A. S. George, A. E. Orchard, and H. J. Hewson, eds. Flora of Australia, Volume 50, Oceanic Islands 2. Canberra: Australian Government Publishing Service, 1993.
Gibbens, Sarah. "Costa Rico's Pristine Shark Island Now a Massive Marine Reserve." National Geographic, 17 Dec. 2021, www.nationalgeographic.com/environment/article/costa-rica-shark-island-now-massive-marine-reserve. Accessed 23 Dec. 2024.
Green, Peter T., Dennis J. O'Dowd, and P. S. Lake. “Control of Seedling Recruitment by Land Crabs in Rainforest on a Remote Oceanic Island.” Ecology 78, no. 8 (1997).
O'Dowd, Dennis J., Peter T. Green, and P. S. Lake. “Invasional ‘Meltdown' on an Oceanic Island.” Ecology Letters 6, no. 9 (2003).
Seo, Hannah. "Extinction Obituary: The Sudden, Sad Disappearance of the Christmas Island Forest Skink." The Guardian, 18 May 2022, www.theguardian.com/environment/2022/may/18/christmas-island-forest-skinks-lizard-extinct-aoe. Accessed 23 Dec. 2024.