SummarySummary

Summary

Lister’s Gecko, Lepidodactylus listeri, and the Christmas Island Blind Snake, Typhlops exocoeti, are reptile species endemic to Christmas Island. Both are currently listed as Vulnerable (T. exocoeti as Ramphotyphlops exocoeti) under the Environmental Protection and Biodiversity Act 1999 (EPBC Act). They are also listed as Vulnerable by IUCN (IUCN 2004). However the current conservation status of both is effectively unknown; Lister’s Gecko was last seen in 1987, and the last blind snake record was made in 1986.

James (2004) has recorded significant declines in other, previously common endemic Christmas Island reptiles since the last targeted reptile survey in 1998 (Cogger & Sadlier, 2000). These declines, in relation to the range and relative abundance recorded in the first targeted reptile survey conducted in 1979, can only be termed “catastrophic”. Consequently this Recovery Plan has been constructed within a context of no recent sightings of either threatened species but a clear record of substantial recent declines of other endemic reptiles on Christmas Island. There is a possibility that one or both of these species is now extinct.

A number of actions are identified that are needed to secure any remaining populations, including investigation of the potential threats to the species and to the integrity of the habitats in which they have been found in the past. The primary action required is to determine, through survey and monitoring, the presence, location and status of any remaining populations.

Distribution and Ecology

Lister’s Gecko is a small (average snout-vent length ca. 50 mm) arboreal species found active on tree trunks at night or sheltering in the day under bark of living or dead trees (Gibson-Hill, 1947).

The Christmas Island Blind Snake is a fossorial snake found in the sub-surface soil and surface litter of the forest floor and almost certainly, like most of its congeners, preying on the eggs, larvae and pupae of ants and termites that it devours in large numbers. It is usually active on the surface only at night.

The two species occur only on Christmas Island. All provenanced records are shown in Figures 1 and 2 below.

Habitat Critical to Survival

Lister’s Gecko: The only records for which habitat information is available (rather than inferred) were made in 1938-1940 (Gibson-Hill, 1947) and 1979 (Cogger and Sadlier, 1981; Cogger et al., 1983). These observations indicated that at the time these papers were written the gecko was:

• most abundant in primary forest on the plateau, but that within this area it will readily adapt to disturbed habitats and secondary growth that includes at least some mature trees;
• least abundant on the terraces sampled on the north-eastern, eastern and southern sides of the island;
• apparently absent from all mined areas even when covered by dense shrubby vegetation; and
• an arboreal species found active on tree trunks at night (from near ground level to the canopy of even large rainforest trees) or sheltering during the day under the bark of living or dead trees (Gibson-Hill, 1947, records them only from under the bark of dead trees, both standing and fallen). All recorded eggs were located under bark on standing tree trunks, within human reach.

At the time of the 1979 study, there is little doubt that the primary forest on the plateau would have been regarded as the island habitat most critical to survival of Lister’s Gecko. Until fuller knowledge of the current population size and ecological requirements is available, this habitat should be regarded as potential habitat critical to survival. The general characteristics of this habitat, including a description of its floristics and structure are provided by Gillison (1976) and are illustrated in Cogger et al. (1983) and Clarke (1995).

Christmas Island Blind Snake: The few records with information on locality or habitat suggest that this species occurs primarily where the deeper soils and primary rainforests occur on the island’s central plateau. Most of this area is now located within the boundaries of Christmas Island National Park, managed for conservation by Parks Australia North (PAN), and is therefore secure as far as land tenure. However, this original primary habitat is currently being subject to immense environmental stress due to recent droughts and the explosive increases in populations of the Yellow Crazy Ant (Anoplolepis gracilipes) and its mutualist scale insects - Tachardina aurantiaca and introduced Coccus celatus (vide O’Dowd et al. 2003). Until knowledge of the population size and ecological requirements of the blind snake is available and understood, all forested parts of the island should be regarded as potential habitat critical for this species.

Given the absence of any records of either species for nearly 20 years, despite periodic intensive searches, and given the profound habitat changes that have occurred directly or indirectly during this period, it is not possible at this time to map habitat critical to the survival of these species. Rather, habitats in which the species were last recorded should be treated as surrogates for habitat critical to survival. Given the absence of recent sightings of either species, any habitat in which one or other species is found in future should be regarded as habitat critical to the species’ survival and the discovered population treated as an important population.

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Figure 1: All records with specified provenance of Lister’s Gecko from Christmas Island (from Cogger & Sadlier, 2000); grid at 30” intervals.

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Figure 2: The only provenanced record of the Christmas Island Blind Snake is from near Stewart Hill, in primary rainforest near the centre of the Christmas Island central plateau.

Threats

Various threatening processes that are likely to impact on Lister’s Gecko and the Christmas Island Blind Snake are listed below and may vary in their impacts according to the different ecologies of these two reptiles. However, because no ecological studies of the reptile species have been conducted, all are potential threats and the magnitude of impact, if any, is unquantifiable.

Predators

Other than native species which were sympatric with the Lister’s Gecko before their declines, known or probable predators on Lister’s Gecko and/or its eggs include the Black Rat (Rattus rattus), domestic and feral cats (Felis cattus), the Asian Wolf Snake (Lycodon capucinus), the Yellow Crazy Ant, and the giant centipede. There is a high probability that the Wolf Snake is a major threat to all endemic reptiles of Christmas Island (Fritts, 1993). There are many examples of both cats and black rats having dramatic impacts on the native fauna, including lizards, of oceanic islands. While their impacts on the native reptiles of Christmas Island are unresearched and unknown, available data would caution against underestimating their potential impacts. There are also few data available on known or potential native predators and their current impacts on either of the two threatened reptiles.

Habitat loss, fragmentation and degradation

Before 1989 Christmas Island was subjected to more or less continuous loss and degradation of its native vegetation through clearing (for buildings, phosphate mining, airport construction, road infrastructure, etc.) and the impacts of exotic plants and animals. Christmas Island National Park was gazetted in 1980 (and extensions in 1986 and 1989), setting aside of more than 60% of the island, together with the resources and personnel to manage it. Outside the National Park the EPBC Act provisions provide for regulation of actions which may have significant impacts on threatened species.

However, past and ongoing fragmentation of native habitat, either through excision of forests for other purposes or the construction of roads and tracks, may also have had impacts on Lister’s Gecko and the blind snake. The impacts of such intrusions into forests are usually greater than the actual areas excised, due to degraded edges being used by introduced species that are competitors or predators, and by changing forest micro-climate significantly.

Probably the greatest impact on the ecology of the island, including the National Park, has been the development of supercolonies of the crazy ant, whose activities have seriously degraded about half of the forested habitats, and resulted in massive mortalities of terrestrial crabs and other organisms, that have led in turn to gross deleterious structural and floristic changes in the island’s forests (Green et al., 2001). On-going control of the crazy ant and rehabilitation of the affected forests is probably critical to the survival of the endemic reptiles, including Lister’s Gecko and the blind snake.

Information gaps

Knowledge of the biology, ecology, habitat preferences and spatial distribution of Lister’s Gecko and the Christmas Island Blind Snake is insufficient to inform specific recovery actions. Consequently priority must be given to locating and determining relative abundance of any remaining populations through survey and monitoring, then targeted research on the species’ ecologies. Interim actions are aimed at maintaining the ecological integrity of known or suspected habitats and determining the likelihood of impacts of potential threats.

New invasive species

A number of exotic species have been previously introduced to Christmas Island, including the crazy ant, black rats and cats, and more recently the scincid lizard, Lygosoma bowringii (sometime before 1979) and the Wolf Snake (ca. 1983). The probability of introduction of the invasive gecko Lepidodactylus lugubris (a congener of Lister’s Gecko) is high and could possibly lead to eradication of any remaining populations of the latter should it become established on Christmas Island. The colonisation of Christmas Island by the latterly introduced species represents a failure, at the time of entry, of quarantine mechanisms then in place, and are indicative of the special vulnerability of isolated oceanic islands to the impacts of exotic species. The exclusion of further exotic species may be a critical factor in maintaining remaining populations of Christmas Island’s native and endemic biota.

Disease

There is currently no evidence to suggest that exotic pathogens may be all or partly responsible for the recorded declines (Cogger et al., 1983; James, 2004) in the endemic reptiles of Christmas Island, including Lister’s Gecko. However, this is a possibility that cannot be currently tested or dismissed. Without locating and regularly monitoring any remaining populations, as proposed in this Recovery Plan, pathogenic mortalities have little chance of being identified.

Competition

The introduced geckos Hemidactylus frenatus and Gehyra mutilata are both arboreal and feed on small arthropods, and so are potential competitors with Lister’s Gecko. These species tend to occupy more disturbed habitats, but the occurrence of both at sites in 1998 that were occupied solely by Lister’s Gecko in 1979 (Cogger & Sadlier, 2000) suggests either that they have succeeded in displacing it or have moved into such areas following the latter’s decline from other causes.

Currently there is no information available on potential competition between the blind snake and the introduced Flowerpot Snake (Ramphotyphlops braminus). Both are primarily fossorial species feeding mostly or solely on the eggs, larvae and pupae of ants and termites, indicating that there could be significant competition for food and preferred space.

Both Lister’s Gecko and the blind snake are likely to compete with the crazy ant. The latter preys on a wide range of invertebrate taxa, including all taxa likely to be eaten by both the gecko and the blind snake.

Crazy Ant control measures

The insecticide fipronil is currently used to control populations of the Yellow Crazy Ant on Christmas Island. Although fipronil is relatively toxic to a range of reptilian species (Peveling, 2000; Tingle and McWilliam, 1999), it has been used in what are considered to be safe dilutions for non-target species, and there is no evidence that fipronil at the concentrations used has an effect on the reptiles of Christmas Island. Trials of manual and aerial baiting with fipronil have included pre-and post-baiting surveys of the impact of baiting on non-target species, including reptiles (Stork et al., 2003), however no individuals of Lister’s Gecko or the blind snake have been seen in any of these surveys and so the effects, if any, of baiting programs on these species are unknown. While it may well be that the overall impacts of the crazy ant on the Christmas Island biota are so severe that their control would be given priority over the conservation of any individual species, there is some potential for the control program to impact on both Lister’s Gecko and the blind snake. Alternatives methods are currently being explored, including the use of insect growth regulators instead of insecticides.

Displacement from preferred habitat by the crazy ant

It is likely that both Lister’s Gecko and the Christmas Island Blind Snake, like other endemic reptiles, are displaced from their habitats within areas grossly affected by ant supercolonies through physical distress when traversing areas with swarming ants, through olfactory responses to ant secretions in gecko-ant and snake-ant interactions, or through changes in forest microclimates following crazy ant infestations.