Prepared by Mark Butz
Futures by Design
Department of the Environment and Heritage, 2004
ISBN 0 642 55049 2
Part C: Threats
Asplenium listeri is a small terrestrial fern with shortly creeping rhizomes and fronds up to 9cm long held in a crown. The species is described (DuPuy 1993b) as an occasional (rare) component of the sparse vegetation community characteristic of inland cliffs which rise above marginal rainforest on the terraces. It is not recorded from the other vegetation zones (being primary rainforest; marginal terrace rainforest; open, scrubby and vine forests; coastal fringe; shore cliffs/spray zone; and mined areas).
Asplenium listeri appears to be quite specialised in its habitat of limestone rock crevices in otherwise dry and exposed sites, where few other species are found. These sites tend to be rocky and narrow cliff-top strips up to 15 metres wide, between a very open aspect on the seaward side and a forest structure increasing up to 40 metres high on the inland side (Reddell pers. comm.). Such situations are well placed to interrupt and capture moist flow from south-easterly trade winds (Tranter pers. comm.). Some sites may be partly shaded, beneath or near Ficus microcarpa (Holmes & Holmes 2002).
It is highly likely that distribution and propagation of the species are related to moisture retention. While the island experiences generally high rainfall, the limestone outcrops are highly porous due to joints and solution. As a result, surface water is rare, being restricted to perennial springs. These are effluxes associated with the junction of limestone with underlying basalt, and surface flow continues where they flow over volcanic rock. Most effluxes lie at elevations below the occurrences of Asplenium listeri (which are >100 metres elevation).
It is interesting to note that the first collection of the species by Lister took place at the peak of the dry season and probably on the northern side of the island (DuPuy 1993b). The specimen that Lister collected may have been in an ideal microhabitat, perhaps associated with a seep, or it may have been collected after unseasonal rain. In 2001 specimens kept at a nursery on Christmas Island would apparently die off when deprived of water for a short period and quickly recover when watered again. This suggests that the vegetative parts would more normally die off during the dry season, and re sprout during the next wet (Sewell pers. comm.). It was observed that the specimens did not appear to benefit from regular abundant irrigation, as befits the habitat preferences of the species (Claussen pers. comm.).
The habit of growth in rock crevices is likely to afford the rhizome a degree of protection from drying out in the dry season, a protective effect heightened by accumulation of organic debris as mulch in the crevices (Hart pers. comm.). In other situations, this habit may also protect the rhizome from occasional fire, which is not currently seen as an issue on Christmas Island (Claussen pers. comm; Reddell pers. comm.)
The species appears to be slow-growing and not vigorous (Reddell pers. comm.). Its restricted distribution and its apparent absence from potential habitat suggest that the species is by no means aggressive in colonising or propagating.
Despite the high degree of specialisation implied by the habit of growing in exposed rock crevices, Asplenium listeri was recorded in one location growing in the same crevice as the closely related but larger Asplenium polyodon G. Forst. (Sickle Spleenwort, Mare's Tail Fern). This is an unusual occurrence for the latter, which is more normally (and frequently) found in epiphytic situations utilising the root ball of Asplenium nidus L. (Bird's Nest Fern) (DuPuy 1993b).
One can only speculate on the implications of the two cohabiting in a rock crevice. The rarity of the situation might suggest A. polyodon is not a significant competitor for A. listeri. Alternatively, the already more widespread A. polyodon might be capable of broadening its habitat choices at the expense of A. listeri.
It has been speculated that A. listeri is actually a variant of A. polyodon that has become a rock crevice specialist (Reddell pers. comm.). If this is so, the occurrence of two distinct forms in the one crevice is of considerable interest. Greater certainty would require professional taxonomic review.
Potential threatening processes for Asplenium listeri include:
- removal or modification of actual or potential habitat by phosphate mining or by construction of roads or other developments
- weed invasion arising from rehabilitation of mined areas or from construction
- predation by exotic species or changes in native species composition arising from activity of exotic species
- human traffic
- unauthorised collecting of specimens
- stochastic disturbance events e.g. cyclones, severe dry seasons
The factors that have led to the limited occurrence and loss of locations for the species are not clear. At the time of initial listing, stochastic events were presumed to be significant (TSSC 2002).
Mining of phosphate-rich soils from between limestone pinnacles began near Phosphate Hill in 1899 and has continued in most years to the present day. The original mining took place on the eastern side of the island and spread westwards with associated construction of access roads. In the late 1960's drill line surveying to map accurately the phosphate reserves resulted in a parallel grid of lines being cleared every 120 metres over most of the plateau (CofA 2002).
Rehabilitation of mining fields has been carried out in several phases. The earliest rehabilitation involved leveling of pinnacles, reintroduction of stockpiled low phosphate overburden, and planting with exotic tree and shrub species. In later phases, replanting utilised only native species. Mining fields in the geographic area of the National Park were excluded from the Park to allow them to be worked out, subject to environmental controls. In February 1998 a lease was signed to allow mining activities to continue for a further 21 years (CofA 2002).
The current distribution of Asplenium listeri on exposed cliff-tops suggests that it is unlikely that past mining or associated roading and surveying have removed actual or potential habitat. It is not known whether deposition of dust from mining and associated roading and transport (which can smother vegetation) has affected any population of Asplenium listeri.
It is possible that Population 5 is adjacent to an area that may be proposed for mining in the future. The draft EIS for the proposed expansion of mining submitted in 2003 states that all populations of Asplenium listeri will be preserved and sufficient buffer zones established (Bennett in litt.).
Location of a population in an area likely to be affected by heavy dust deposition should result in specific requirements to mitigate this potential impact.
There are several recent, current or potential developments involving construction activity on the island. These include:
- proposed expansion of nine sites for phosphate mining (EPBC referral 2001/487) - the environmental assessment for this proposed expansion is due to be released in 2004.
- construction of the Asia Pacific Space Centre near South Point - the Centre is primarily located in former mined areas; survey has not located any specimens (APSC 2003), and it is not likely to affect a population. The construction environmental management plan states that any specimens of threatened species identified on the site would be retained (APSC 2003). The Centre will be located above one of the areas offering potential habitat for Asplenium listeri on cliff-tops above the eastern terraces towards South Point.
- associated construction of common use infrastructure including a new port facility between Waterfall and Norris Point on the east coast (referral 2001/435), upgrade of the Linkwater Road between the new port and Lily Beach Road (referral 2001/436) and an expansion of the airport (referral 2001/434) - the environmental assessment reports comment on the presence of a number of endemic and listed plant species but Asplenium listeri was not recorded or expected (GHD 2002a; 2002b; 2003).
- construction of an Immigration Reception and Processing Centre (IRPC) on the central plateau area towards North West Point and associated infrastructure (exempt from the EPBC Act by Ministerial decision 3 April 2002) - this construction was not in a location likely to contain Asplenium listeri. Most of the activity was within a former mined area (ML 138 and ML 139) (Exemption notice April 2002).
- a radio system upgrade for the Australian Federal Police at Murray Hill (referral 2002/718) - no additional information located
- construction of a mobile phone tower/base station at Limestone Hill near South Point (referral 2002/694) - the environmental assessment report did not record Asplenium listeri (Holmes 2002).
- possible extension or modification of the Christmas Island Tourism Resort and Casino on the eastern coast north of Ethel Beach (closed in 1998 but may be redeveloped) - no additional information located.
None of these developments are likely to affect a known population of Asplenium listeri.
Apart from new developments, there is potential for construction as part of maintenance to be a potential threat, specifically:
- any widening or modification of the Greta Beach Road in the vicinity of Population 2; and
- maintenance or construction for water supply infrastructure that may be potential habitat [see B.2.2 above].
No Weeds of National Significance are known to affect any populations of Asplenium listeri.
Numerous species of weeds have invaded forest margins along roads and tracks and around mining fields, and sparse vegetation communities, such as those on the terraces, are also vulnerable to such invasion.
The very hardy native ferns Nephrolepis multiflora (scurfy sword fern) and Nephrolepis biserrata (broad sword fern) can spread aggressively to form dense and tangled colonies which confound access, and compete strongly with other plants (Swarbrick 1997). N. multiflora is particularly noted as an invader of mining fields (CofA 2002) where it is often dominant in areas of poor thin soil and limestone pinnacles, while also forming dense thickets more than 2 metres tall in damper, more shaded gullies towards the margins of old mines (DuPuy 1993c). However, there is no evidence that either species is competing with Asplenium listeri in its exposed cliff-top habitat (Reddell pers. comm.).
With the possible (and uncertain) exception of former habitat that may have been affected by weed invasion following mining, the specialised habitat now occupied by Asplenium listeri makes it unlikely that weed invasion has contributed to the restricted distribution of the species through competition, and there is no reference to weed species in records of known localities. The most recent survey notes that the species is not subject to tangible threats from invasive weeds (Holmes & Holmes 2002).
There are no records indicating any specific threats from the activity of exotic fauna.
The species occasioning most concern on the island in recent times is yellow crazy ant (Anoplolepsis gracilipes), which was introduced accidentally between 1915 and 1934. It has recently undergone a population explosion and the ants have formed multi-queened 'supercolonies'. This has had a marked impact on other terrestrial fauna such as land crabs and in turn has changed the vegetation profile in some forest types. Although Asplenium listeri may be vulnerable to episodic changes in canopy density or encroachment by other vegetation (Holmes & Holmes 2002), the effects brought about by the yellow crazy ant are not considered likely to affect the areas associated with Asplenium listeri (Claussen pers. comm.).
Of greater potential impact may be the giant African snail (Achatina fulica) which feeds on a wide variety of plants but may be expected to take refuge in moist locations, including those favoured by ferns. It is not known whether this species has had, or is likely to have, any impact on Asplenium listeri. Although the presence of land crabs appears to restrict the distribution of the snail, this controlling effect may be compromised by reduced crab populations brought about by the yellow crazy ant infestation (CofA 2002).
Crazy ant populations are responding to a program of aerial baiting, however the potential for increased spread of the snail may be inferred, because there may be a gap of many years before land crabs recover to a level sufficient to control snail populations. Parks Australia monitor presence/absence of the snail during surveys for crazy ant populations, and these data could be related to the distribution of Asplenium listeri to assess any interactions (Jeffery pers. comm.).
Also of potential concern is an exotic millipede which feeds on vegetable matter and occurs in very high densities in certain areas (Sewell pers. comm.). No additional information was located on this.
The potential habitat for Asplenium listeri is for the most part highly inaccessible due to the steep and rocky cliffs, pinnacles and screes, and inhospitable due to the dry and exposed nature of the sites and the types of vegetation in the vicinity (often dense and dominated by stinging trees and prickly plants). As a result, pressure of human traffic is not likely to pose a threat to populations of the species (Reddell pers. comm.).
There is no evidence of unauthorised collecting of Asplenium listeri. However, members of this extensive genus are particularly attractive for cultivation, with some highly variable in habit and suitable for selection for cultivation, while others form excellent specimens for baskets (Jones & Clemesha 1981; DuPuy 1993b).
There is potential for collecting of specimens or propagules of rare species by or on behalf of fern enthusiasts. The likelihood or extent of such a demand cannot be predicted. However, in view of the small population numbers known and the uncertainty of factors restricting distribution, any such predation could pose a significant threat to the survival of the species. Confidentiality of precise locations would appear to offer the best safeguard against this practice.
Because of the tropical location of the island, severe rainfall events associated with the monsoon are common in the wet season, although the extent and impacts of these cannot be predicted. Severe dry seasons tend to coincide with the El Niño effect in the Pacific (CofA 2002), and these events may be increasing in frequency and severity.
Confident assessment of the impact of stochastic events (as for other potential impacts) requires further study of the life cycle, longevity and regeneration requirements of the species - information which is currently not available (Reddell pers. comm.).
However, applying the precautionary principle, it is reasonable to expect that Asplenium listeri could be affected by severe rainfall events through scouring and transport of soils and mulch from rock crevices to expose the rhizome, and/or by severe dry seasons through desiccation.
In view of the exposed nature of the species' cliff-top habitat, such events have the potential to be significant factors in limiting its occurrence and potentially in threatening its continued survival. These events were seen as the most concerning threat at the time of initial listing (TSSC 2002).
Fire is not currently seen as an issue on Christmas Island (Claussen pers. comm; Reddell pers. comm.), however it is noted that a fire did occur in the terrace rainforests during the long dry of 1994 and again in September 1997 (GHD 2002a). If dry seasons become more severe more frequently, then impact from fires may become an issue for many species that are not adapted to such events.
In view of uncertainty about the factors that have restricted distribution, the degree of threat to any particular area or population is difficult to estimate.
No populations are known to be threatened by current or proposed mining or construction, and there is no imminent concern about weed invasion, the impact of exotic fauna, or unauthorised collecting.
Some of the known populations may be less likely to face these types of threats because they are located within the Christmas Island National Park. These include:
- Population 1 - Gannet Hill
- Population 3 - Aldrich Hill; and
- Population 4 - Sydney's Dale.
It is also notable that The Dales area, being a Wetland of International Importance under the Ramsar Convention, is a matter of national environmental significance under the EPBC Act.
The Plan of Management for the National Park noted a number of proposed extensions to the park, including:
- an area on the east coast between Margaret Knoll and Ross Hill (excluding mining fields) and taking in Gannet Hill (excluding mining fields); and
- an area on the east coast south of the Ross Hill section around South Point (excluding mining fields) to join the existing Park boundary.
These extensions may afford a higher degree of protection from development to Population 2 and some of the areas likely to contain potential habitat for Asplenium listeri, but may not afford a higher degree of protection from development to Population 5.
However, listing of the species as a threatened species under the EPBC Act offers a degree of protection irrespective of land tenure.