Ascension scrub and grasslands
Ascension scrub and grasslands refer to the unique ecosystems found on Ascension Island, a volcanic island located in the South Atlantic Ocean. First discovered in 1501, the island has undergone significant transformation due to human settlement and attempts at terraforming over the last two centuries. Initially characterized by a harsh, desert landscape dominated by volcanic rock and minimal vegetation, the island's climate ranges from a hot desert at sea level to a more humid environment on the peak of Green Mountain.
The island's flora includes endemic species such as certain grasses and ferns, although many native plants have faced decline due to the introduction of non-native species and animals. Birdlife is notably diverse, with species like the endemic Ascension frigate bird and various terns thriving in the region, although several bird species have become extinct due to habitat disruption. Ascension Island also supports unique animal life, including green turtles that nest on its shores. Human activities, especially since British naval occupation, have led to the introduction of new plant species and significant ecological changes, creating novel habitats that challenge the native ecosystems. The ongoing effects of climate change further threaten these delicate habitats, highlighting the complex interplay between human influence and natural environments.
Ascension scrub and grasslands
- Category: Desert Biomes.
- Geographic Location: Atlantic Ocean.
- Summary: First discovered in 1501, Ascension was a true desert island. Nearly 200 years of settlement have produced a landscape in rapid transition.
On first stepping out of an airplane at Ascension Island's Wideawake Field, a visitor cannot avoid being thunderstruck by the desolation of the landscape, dominated by vast expanses of jagged, gray volcanic rock with occasional rust-red volcanic cones. A glimpse to the east, however, reveals something different: a tall greenish peak obscured by clouds. That peak, Green Mountain, is the highest point on Ascension. It is also the focus of a two-centuries-long effort to transform—terraform—the island's landscape.
Ascension Island, just south of the equator, is the tip of a large volcano about 60 miles (97 kilometers) west of the mid-Atlantic Ridge. This volcano rises from the ocean floor, about 10,000 feet (3,048 meters) deep, ascending to 2,817 feet (859 meters) above sea level on the summit of Green Mountain. The tip of the volcano forms a roughly arrowhead-shaped island, roughly as broad as it is long, with the tip pointing to the east. The island—only 34 square miles (88 square kilometers) in area—features more than 60 volcanic vents, ranging from small craters to the summit of Green Mountain.
More than half the surface consists of lava flows; most of the rest, with the exception of storm-deposited material along the island's beaches, consists of pyroclastic deposits. The oldest rocks on the surface range from 0.6 to 1.5 million years old. The most recent eruption may have taken place a few hundred years ago, but no eruptions have occurred there since the island's discovery by Portuguese admiral João da Nova in 1501.
The climate at sea level is a hot desert climate—high temperatures with little precipitation. In the island's main settlement of Georgetown, the mean annual temperature is about 84 degrees F (29 degrees C), and the mean annual precipitation is about 5 inches (127 millimeters). At higher elevations on Green Mountain, the desert climate grades into a hot steppe climate and ultimately into a mild, humid climate with no dry season. Mean annual temperature on Green Mountain is about 66 degrees F (19 degrees C); mean annual precipitation there is about 26 inches (661 millimeters).
Vegetation
Before its discovery, the island was sparsely vegetated, the only hint of visible greenery being at higher-elevation slopes. The island's known vegetation consisted of only one shrub, Oldenlandia adscenionis, scattered around on some slopes, along with Portulaca oleracea, grasses including Aristida adscensionis, and ferns and club mosses including Marrattia purpurascens and Lycopodium cernuum. On lava flows and other barren areas, Euphorbia origanoides and other grasses could be found, whereas A. adscensionis and Cyperus appendiculatus could be found in gullies. P. oleracea and Ipomoea pes-caprae were scattered along the coast. A variety of ferns dominated the summit of Green Mountain.
Besides all these plants, O. adscensionis was endemic to the island (found only here; evolved uniquely to fit this biome) but is now believed to be extinct. Extant endemic species include the grasses Sporobolus caespitosus and S. durus and the ferns Anogramma ascensionis, Asplenium ascensionis, Dryopteris ascensionis, Marratia purpurascens, Pteris adscensionis, and Xiphopteris ascensionse.
Bird Life
By contrast with the paucity of vegetative flora at the time of discovery, Ascension was rich in bird life, as it was one of the few large sites in this marine region suitable to oceanic birds for breeding and rearing young. The island's signature avian residents were arguably the endemic Ascension frigate birds (Fregata aquila). Another notable resident included the sooty tern, or Wideawake (Sterna fuscata).
Further common bird species were the masked, brown, and red-footed boobies (Sula dactylatra, S. leucogaster, and S. sula); the brown and black noddies (Anous stolidus and A. minutus); the red-billed and white-tailed tropicbirds (Phaethon aethereus and P. lepturus); the Madeiran storm petrel (Oceanodroma castro); the white tern (Gygis alba); Audubon's shearwater (Puffinus lherminieri); the Ascension night heron (Nycticorax sp.); and the Ascension rail (Atlantisia elpenor). The latter two species are now extinct. Although the Ascension rail was noted (and sketched) by the seventeeth-century explorer Peter Mundy, the Ascension night heron is known only from fossils.
Animal Life
Ascension had only one large land animal: a crab, Johngarthia lagostoma, that can still be found from sea level to the summit of Green Mountain. Its most prized land visitor, however, is its breeding population of green turtles (Chelonia mydas). For several months of the year, the females of the species climb out of the sea to lay eggs on the thin ribbons of sandy beach scattered along the coast. Sailors frequently stocked their holds with adult turtles, which they used for fresh meat during long sea voyages.
Effects of Human Activities
Ascension's terrestrial life changed radically after its discovery. Introduced animals, such as rats, goats, and cats, devastated the island's native plant life and drove many of its seabird colonies to rookeries on adjacent Boatswainbird Island. While conservation efforts have managed to eradicate the feral cats on the island, the rat population has only grown larger due to the lack of predators they face. The island's greatest transformation, however, began after Britain's Royal Navy occupied the island in 1815.
Determined to make it more hospitable to humans, the British sought the advice of Sir Joseph Hooker, who prepared a plan to terraform the island by planting species new to the area. Hooker's proposal included planting trees at higher elevations to increase rainfall, planting valley slopes with species that promote soil formation, planting dry areas with drought-tolerant species, and adding a mix of European and tropical species to gardens on the upper slopes of Green Mountain.
The results of this effort are novel terrestrial ecosystems. A lush cloud forest consisting of a mixture of tropical and subtropical species—including bamboo (Bambusa spp.), banana (Musa spp.), Casuarina equisetifolia, Eucalyptus camaldulensis, fig (Ficus carica), ginger (Alpinia speciosa), mulberry (Morus spp.), Norfolk Island pine (Araucaria excelsa), and screwpine (Pandanus spp.)—occupies the upper slopes of Green Mountain. Vast expanses of greasy grass (Melinis minutiflora), guava (Psidium guajava), and other introduced grasses and shrubs occupy the middle and lower slopes of Green Mountain. Introduced desert species, such as mesquite (Prosopis juliflora) and prickly pear cactus (Opuntia cochinillifera and O. vulgaris), are spreading across the volcanic flats. One of the introduced species, the Mexican thorn bush, presents an ongoing threat to marine turtles. Climate change as a result of human activity and rising sea levels also threaten the biome.
Bibliography
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Duffey, E. “The Terrestrial Ecology of Ascension Island.” The Journal of Applied Ecology 1, no. 2 (1964).
Grazier, Kevin R., ed. The Science of Dune: An Unauthorized Exploration Into the Real Science Behind Frank Herbert's Fictional Universe. Dallas, TX: BenBella Books, 2008.
Hart-Davis, Duff. Ascension: The Story of a South Atlantic Island. Garden City, NY: Doubleday, 1973.
Lawrence, David M. “The Shade of Uliet: Musings on the Ecology of Dune.” In Kevin R. Grazier, ed. The Science of Dune: An Unauthorized Exploration into the Real Science Behind Frank Herbert's Fictional Universe. Dallas, TX: BenBella Books, 2008.
Martin, Emma, and Neil Burgess. “Ascension Scrub and Grasslands.” One Earth, 23 Sept. 2020, www.oneearth.org/ecoregions/ascension-scrub-and-grasslands/. Accessed 16 Dec. 2024.
Selkirk, Diane. "The Island with a Key to Our Future." BBC, 15 June 2020, www.bbc.com/travel/article/20200614-ascension-island-the-island-with-a-key-to-our-future. Accessed 16 Dec. 2024.