B.1.1 Christmas Island - overviewB.1.1 Christmas Island - overview

B.1.1 Christmas Island - overview

Asplenium listeri is known only from Christmas Island, a territory of the Commonwealth of Australia, located in the Indian Ocean at about 10°25’S and 105°40’E. The island lies approximately 2,600 kilometres west of Darwin and 360 kilometres south of Jakarta and the western end of Java. The island covers approximately 135 square kilometres (13,500 hectares), of which about
85 square kilometres (8,500 hectares) (63%) is gazetted as Christmas Island National Park, a Commonwealth reserve under the EPBC Act.

The island is formed on the peak of a volcanic mountain which rises steeply about 5,000 metres from the ocean floor. Successive layers of coral reefs were formed over the igneous core at each stage of its emergence from the ocean, leading to development of a near‑continuous limestone cap. As the island became raised above sea level the ocean excavated new cliffs at each stage, leading to a topography of stepped terraces and inland cliffs.

Most of the coast consists of sheer rocky cliffs 10 to 20 metres high, often undercut, with a few beaches of sand and coral rubble. The interior comprises an irregular plateau from 160 to 360 metres elevation, separated from the coast by a series of steep slopes or cliffs with narrow terraces between. Across the island, substrates are almost entirely derived from limestone, with deep soils on the plateau and upper terraces, becoming progressively thinner and drier towards the more rocky lower terraces.

The island experiences a tropical equatorial climate, with a mean annual rainfall of 2,110 millimetres, reduced during years when the El Niño effect is in operation. The tropical climate leads to a marked seasonality in rainfall, with the dry season from May to November (August to October being the driest months, although some rainfall is received during this period) and the wet season from December to April when the north-west monsoon prevails (January to March being the wettest months). This seasonality is not expressed in other factors, with relative humidity stable year round at 80-90%, and similarly with temperature at 23 to 29°C.

The native vegetation of the island is predominantly evergreen, with a dense canopy, epiphytes, emergent trees and a sparse forest floor harvested by numerous land crabs. In addition to the primary rainforest (30 to 40 metres with emergents to 50 metres), other vegetation zones have been delineated as: marginal (terrace) rainforest (20 to 30 metres); open, scrubby and vine forests; coastal fringe; shorecliffs/spray zone; and mined areas (DuPuy 1993a).