Mitchell's Rainforest Snail Thersites mitchellae Recovery Plan

NSW National Parks and Wildlife Service, July 2001
ISBN 0 731 6298 5

Appendix 1 - Mitchell's Rainforest Snail Thersites mitchellae (Cox, 1864)

Status: Endangered species

These guidelines have been prepared to assist persons required to prepare or review assessments of likely impacts on Mitchell's Rainforest Snail. The guidelines will assist in the following:

  • preparation of environmental impact assessment reports, Statements of Environmental Effects and Species Impact Statements by proponents of developments or activities pursuant to the Environmental Planning and Assessment Act 1979;
  • review of environmental reports by consent and determining authorities; and
  • preparation of licence applications under the Threatened Species Conservation Act 1995 (TSC Act).

The guidelines should be read in conjunction with the NPWS Information Circular No. 2: Threatened Species Assessment under the EP&A Act: The '8 Part Test' of Significance (November 1996).

Survey

Field survey and habitat assessment for Mitchell's Rainforest Snail should be undertaken by persons with suitable demonstrated experience in land snail surveys and land snail identification.

Field surveys and collection of specimens for identification purposes may result in harm to Mitchell's Rainforest Snail or damage to its habitat, and accordingly should be licensed under the TSC Act. The Mitchell's Rainforest Snail recovery plan (July 2001) includes guidelines for threatened land snail surveys (Appendix 3). These guidelines have been designed to facilitate the timely licensing of land snail survey and research licence applications under the TSC Act, whilst ensuring that licensed activities will not further threaten the survival of threatened land snail species or populations of these species.

Current knowledge indicates that Mitchell's Rainforest Snail is restricted to fragments of remaining habitat on the coastal lowlands between the Tweed and Richmond Rivers in north-east New South Wales. Mitchell's Rainforest Snail occurs in subtropical rainforest remnants and swamp sclerophyll forest with rainforest elements, typically in areas of alluvial soils with a basaltic influence. Live snails shelter in leaf litter on the ground and sometimes under bark on tree trunks. Most of the sites where the species has been recorded are on slightly higher ground at the edge of coastal wetlands, which may reflect either patterns of land clearing or actual habitat preferences. Areas with palms and figs around the borders of coastal wetlands are particularly favoured habitat.

It is recommended that any bush remnant containing lowland subtropical rainforest in particular, but also other lowland moist forest communities, be considered as potentially harbouring Mitchell's Rainforest Snail no matter how small or degraded it might appear, especially if there is any sort of leaf litter layer present.

The shell of Mitchell's Rainforest Snail can remain in the leaf litter for extended periods following the death of the animal. Presence of the species at a site can most easily be determined by searching for empty shells or shell fragments. Accumulations of shell material can sometimes be found around feeding sites (anvils) used by the Noisy Pitta (Pitta versicolor).

Searches for shell material can be undertaken year round. However, surveys to locate active live animals are best undertaken at night during warm, wet weather. Live animals can also be searched for by day by identifying and investigating potential shelter sites such as fallen palm fronds, leaf litter and loose bark on tree trunks. The minimum time required to search for shell material or live animals will depend on the size and amount of potential habitat present within the site being assessed. However, it is suggested that for small sites (less than 0.5 ha) at least three hours be spent searching and then scaled up appropriately for larger areas.

It should be noted that the presence of one or more individuals would generally indicate the presence of further individuals within the site. In addition the presence of empty shells, particularly fresh shells with an intact periostracum (colouring), should be treated as indicating the presence of living individuals.

Once Mitchell's Rainforest Snail shells or live animals have been recorded on a site, or when habitat assessment suggests suitable potential habitat is present, further assessment of the distribution and extent of habitat on the site (and adjacent to the site if access is possible) should be undertaken.

Mitchell's Rainforest Snail can be easily confused with other land snail species which also occur on the coastal plain of north-east NSW, such as the common Fraser's Snail (Sphaerospira fraseri). Mitchell's Rainforest Snail differs from Fraser's Snail in the following characters: i) intact shell deep reddish brown to black in colour with two prominent yellow bands; ii) shell triangular in profile with a keel around the outer margin and a slightly flattened base; and iii) aperture (opening) of the shell parabolic rather than circular in outline.

Life cycle of the species

Part (a) of the eight part test asks whether the life cycle of the species is likely to be disrupted such that a viable local population of the species is likely to be placed at risk of extinction. Very little is currently known about the biology and life history of Mitchell's Rainforest Snail. It is hermaphroditic and lays clutches of up to 70 small round white eggs below the surface of leaf litter. The species is herbivorous and is thought to feed on leaf litter, fungus and lichen. It is generally active at night, particularly during or following rain. Nothing is currently known about rates of fecundity, length of life span, dispersal patterns and over what distances individuals can move.

The life cycle of Mitchell's Rainforest Snail is such that if the proposed development or activity causes the destruction of all the available habitat present within the site then the local population will most likely be lost as there is little potential to recolonise. However, if sections of the site are to be left as habitat, then the impacts of factors such as changes to microclimate, increased risk of weed invasion, fire and introduced predators will have to be addressed.

Viable local population of the species

It is currently not known what number of individuals constitutes a viable population for Mitchell's Rainforest Snail but it is not considered to be large. It is recommended that, in accordance with the precautionary principle, populations are considered viable unless proven otherwise.

Endangered populations

Part (b) of the eight part test asks whether the viability of an endangered population is likely to be significantly compromised. This question relates to populations listed under schedule 1 part 2 of the TSC Act, and does not apply to the Mitchell's Rainforest Snail.

A significant area of habitat

Part (c) of the eight part test asks whether a significant area of known habitat is to be modified or removed. Based on the current understanding of the distribution and abundance of Mitchell's Rainforest Snail, assessment of the significance of an area of habitat should include consideration of the following:

  1. any area of habitat known to support the species;
  2. the size and extent of an area of potential habitat;
  3. whether an area of potential habitat has vegetation communities largely unaffected by disturbances such as fire or weed invasion;
  4. whether the area is at the edge of the species' range; or
  5. the long term security of the area.

Where an area of potential habitat is assessed as being significant by one or more of the above criteria, either a targeted survey for Mitchell's Rainforest Snail at the site should be undertaken, or the area may be assumed to be known habitat for the purposes of the assessment.

Regional distribution of the habitat

The known distribution of Mitchell's Rainforest Snail is restricted to the coastal lowlands between the Tweed and Richmond Rivers in the NSW north coast bioregion. Populations have been recorded in the local government areas (LGAs) of Tweed, Byron and Ballina.

Isolation/fragmentation

Part (d) of the eight part test asks whether a development or activity might isolate populations such that they become no longer viable (such as through inbreeding depression). Thus the development need not directly impact on a population, but merely separate it from others by preventing movement of animals.

Mitchell's Rainforest Snail was formerly widespread but is now restricted to remnant areas of habitat in a highly fragmented urban and agricultural environment. This, and the likely limited dispersal ability, means that many of the populations of this species are already effectively isolated from others, except where they exist within the same bushland remnant or similar immediately adjacent locations. Consideration of this part of the eight part test therefore needs to be undertaken within the context that most populations of Mitchell's Rainforest Snail are already highly isolated.

As dispersal behaviour and ability are presently poorly known, it is difficult to determine what type of development may isolate populations that are currently interacting with others.

Critical habitat

Part (e) of the eight part test asks whether critical habitat will be affected. There is presently no critical habitat listed for Mitchell's Rainforest Snail. However, the Mitchell's Rainforest Snail recovery plan includes actions for the declaration of Stotts Island as critical habitat and assessment of additional areas for identification as critical habitat. The NPWS Register of critical habitat should be consulted to obtain information on the current status of critical habitat for the species.

Adequacy of representation in conservation reserves or other similar protected areas

Part (f) of the eight part test asks whether the species or its habitat is adequately conserved in the region. The majority of the known populations of Mitchell's Rainforest Snail occur outside of current conservation reserves. The largest known population and largest remaining single area of habitat is in Stotts Island Nature Reserve in the Tweed River near Murwillumbah. Given the distribution and likely genetic diversity within Mitchell's Rainforest Snail, the current reserve system does not represent an adequate coverage of the species, particularly in the southern part of its' range. The NPWS Atlas of NSW Wildlife and Museum records should be consulted for current information on records of the species in conservation reserves or other similar protected areas.

Threatening processes

Part (g) of the eight part test asks the development or activity is a threatening process. The TSC Act currently lists "high frequency fire resulting in the disruption of life cycle processes in plants and animals and loss of vegetation structure and composition" as a key threatening process. Key habitat components for Mitchell's Rainforest Snail are a well-developed leaf litter layer (providing food, shelter and breeding sites) and an intact forest canopy (maintaining a moist microclimate and providing a source of leaf litter). Fire will impact adversely on these key habitat components.

The NSW Scientific Committee has made a preliminary determination to list "loss of biodiversity as a result of loss and/or degradation of habitat following clearing and fragmentation of native vegetation" as a key threatening process. The Mitchell's Rainforest Snail recovery plan identifies loss of habitat as the major threat to the recovery of the species.

Schedule 3 of the TSC Act should be consulted to determine whether other relevant key threatening processes are listed.

Processes that are generally considered by the scientific community as being detrimental to a species or its habitat, and which are relevant to Mitchell's Rainforest Snail, include habitat clearance and/or modification, fire, weed invasion and introduction of feral predators such as rats.

Limit of known distribution

Part (h) of the eight part test asks whether the species is at the limit of its known distribution. Mitchell's Rainforest Snail occurs only on the coastal plain of the NSW far north coast, bounded by the lower Tweed River (to the north), the lower Richmond River (to the south) and the foothills of the ranges (to the west). The most northerly recent record of the species is at Banora Point, the most southerly at Lennox Head, and the most westerly at Stotts Island near Murwillumbah. The NPWS Atlas of NSW Wildlife and Museum records should be consulted for current information on the distribution of records.

For Further Information contact

National Parks and Wildlife Service Northern Directorate Threatened Species Unit, Locked Bag 914 Coffs Harbour NSW 2450. Phone (02) 66515 946.

References

National Parks and Wildlife Service (2001). Mitchell's Rainforest Snail Thersites mitchellae recovery plan. NPWS, Hurstville, NSW.

Australian Museum (2001). Threatened and Endangered Landsnail species - Thersites mitchellae. http://www.austmus.gov.au/science/division/invert/mal/endangered/thersites.htm
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