National recovery plan for the Christmas Island Spleenwort Asplenium listeri
Prepared by Mark Butz
Futures by Design
Department of the Environment and Heritage, 2004
ISBN 0 642 55049 2
Part B: Distribution and location
B.1 Distribution
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; shore cliffs/spray zone; and mined areas (DuPuy 1993a).
B.1.2 Distribution of Asplenium listeri
Asplenium listeri is endemic to Christmas Island (DuPuy 1993b).
Historical
The species is named for naturalist Joseph Jackson Lister, who visited in H M Survey Vessel Egeria in September-October 1887. Lister collected a specimen of the fern, probably near Flying Fish Cove towards the north-eastern corner of the island. A single specimen was reported by H N Ridley in 1906 from this same vicinity at Toms Ladder (DuPuy 1993b). There is no longer evidence for the species in this area, despite attempts to locate the type locality (Green pers. comm.).
Known
The species is now known from five localities.
Population 1 was the first locality recorded (and the only one known at the time the species was nominated for listing) at Gannet Hill, above the limestone terraces on the eastern side of the island. Because the original collections by Lister and Ridley no longer exist, the species was newly described from a single specimen collected at this locality in April 1987 (DuPuy 1993b).
Three additional localities were located during a survey of flora in March-April 2002 (Holmes & Holmes 2002):
- Population 2 - on the eastern terrace of the island on the Greta Beach Road a few kilometres south-west of the Gannet Hill locality.
- Population 3 - on the southern side of the island at Aldrich Hill approximately 9 kilometres to the west of the Greta Beach Road locality.
- Population 4 - near Sydney's Dale (Dale No.6) on the western side of the island; about 4 kilometres west of Aldrich Hill and 13 kilometres west of Greta Beach Road/Gannet Hill.
Note: this population was not relocated despite two attempts (Claussen pers. comm.).
These populations when combined total less than 300 individuals.
Population 5 was located in March 2003 by plant ecologists carrying out environment impact assessment for an expansion of phosphate mining. This was north of the Resort on the eastern terrace. The precise location will be provided when the draft environmental impact statement has been released, but will be kept confidential (Zimmermann; Reddell pers. comm.).
Unconfirmed
Although other occurrences have been reported, these appear to coincide with confirmed sites:
- A population of about 20 mature individuals with hundreds of sporelings, was located in about 1996 north of the Resort, above a spring and stream bed, and below a mining lease (Hart pers. comm.). This site is apparently not the same as Population 5 (Bennett in litt.)
- The Commonwealth database notes a site at Jones Spring (about 600 metres north of the Resort on the east coast) and said to have approximately 500 individuals (90% juveniles). This was not relocated despite attempts in 2001 and 2002 (Claussen pers. comm.). This site is almost certainly the site noted above which was reported in about 1996.
- A population of unknown size was reported in the vicinity of the Greta Beach Road on the east coast in approximately late 1996, but this was not readily relocated (Hart pers. comm.). This is highly likely to be Population 2 which was (re)located in March-April 2002.
B.1.3 Important populations
An important population is one that is necessary for the species' long-term survival and recovery. This may include populations that are:
- key sources for reproduction and dispersal
- necessary for maintaining genetic diversity
- near the limit of the range of the species.
All known occurrences of Asplenium listeri need to be regarded as important populations, based on:
- endemic status
- highly restricted occurrence; and
- uncertainty surrounding the reasons for its rarity.
Any additional populations that are located within the term of this plan need to be similarly regarded as important populations.
B.1.4 Potential habitat
Despite the limited occurrences, 'potential habitat' appears to be:
- limestone rock crevices at the uppermost part of inland cliffs above terraces (or cuttings of similar structure)
- between about 110 and 255 metres elevation; and
- often beneath or near Ficus microcarpa.
Populations can easily remain undetected in this habitat, since much of the terraces and associated cliffs (particularly on the northern and southern sides) are infrequently visited. Survey of cliff-tops for populations is an extremely difficult undertaking due to inhospitable terrain and the nature of the vegetation (Claussen pers. comm.). These same factors make it difficult to assess the suitability of such areas as potential habitat for colonisation or (re)introduction.
A number of informants (Tranter pers. comm.; Hart pers. comm.) suggest that the greatest potential for new populations (or for colonisation or reintroduction) is at the inland cliff-tops of the eastern terraces, broadly facing the prevailing south-easterly trade winds which provide moisture to an otherwise very dry and exposed situation.
This is the zone with the largest area lying outside the national park and is the focus of most of the island's recreational development (the eastern beaches) and some of the most significant infrastructure development (a new port and associated roading, and expansion of the airport), with possible expansion of phosphate mining on the inland side of cliff tops.
In view of this, all potential habitat (as described above) needs to be carefully surveyed for the presence of Asplenium listeri when proposed developments are assessed.
It has been suggested that potential habitat may also include artificially moist areas in the vicinity of water supply works, such as Jedda Cave and Jane-Up, within the altitude range for the species at 210 and 200 metres elevation respectively (Sewell pers. comm.). However, these sites have previously been surveyed with no Asplenium listeri being located (Claussen pers. comm.).
B.2 Habitat that is critical to the survival of the species
B.2.1 Defining and mapping habitat critical to the survival of the species
Definition of habitat that is critical to the survival of Asplenium listeri is based on the following matters from EPBC Regulation 7.09:
- habitat used in periods of stress
- habitat used to meet essential life cycles
- habitat used by important populations
- habitat necessary to maintain genetic diversity and long-term evolutionary development
- habitat necessary to ensure the long-term future of the species through reintroduction or re colonisation.
Based on current knowledge of the biology and habitat in known locations, habitat critical to the survival of Asplenium listeri needs to include:
- all limestone rock crevices in the vicinity of known occurrences - reflecting uncertainty regarding reasons for the extremely limited distribution of the species and potential threats to survival; and
- taller vegetation structures on the inland side of cliff-top sites and relatively open exposure to the coast (refer C.1 below)
The environs of any additional populations that are either newly confirmed or newly located or more precisely defined within the term of this plan need to be similarly regarded.
Due to the uncertainty of current location information and limited knowledge on the ecology and specific habitat requirements, the habitat critical to the survival of the species cannot be mapped accurately at this time.
To protect the species the precise locations of all populations should remain confidential in any listing or public record. As an aid to confidentiality, a buffer of at least 1 kilometre around all known populations should be applied in publicly available mapping for EPBC referral and enquiry purposes.
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