Thersites mitchellae (Mitchell's Rainforest Snail)

Advice to the Minister for the Environment and Heritage from the Threatened Species Scientific Committee (TSSC) on Amendments to the list of Threatened Species under the Environment Protection and Biodiversity Conservation Act 1999 (EPBC Act)

1. Scientific name, common name (where appropriate), major taxon group

Thersites mitchellae (Mitchell's Rainforest Snail)

2. National Context

Mitchell's Rainforest Snail is a large land snail that is restricted to the coastal plains of northern NSW, between the Richmond and Tweed Rivers. Within this range, however, land clearing has been extensive and Mitchell's Rainforest Snail is now restricted to remaining areas of suitable habitat. The largest known population and largest remaining single area of suitable habitat is in Stotts Island Nature Reserve on the Tweed River, in northern NSW.

Mitchell's Rainforest Snail is found only in undisturbed remnant lowland subtropical rainforest and swamp sclerophyll forest with a rainforest understorey. 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).

Under the Threatened Species Conservation Act 1995 (NSW), Mitchell's Rainforest Snail is listed as endangered, and Stotts Island Nature Reserve has been declared critical habitat for the species under the NSW Act.

3. How judged by TSSC in relation to the EPBC Act criteria.

TSSC judges the species to be eligible for listing as critically endangered under the EPBC Act. The justification against the criteria is as follows:

Criterion 1 - Decline in numbers

Museum collections from last century show the Mitchell's Rainforest Snail was previously common within its range but has since declined in abundance. However, there is no quantitative data available against this criterion. Therefore, the species is not eligible for listing under this criterion.

Criterion 2 - Geographic distribution

Mitchell's Rainforest Snail is a large land snail that is restricted to the coastal plains of northern NSW, between the Richmond and Tweed Rivers The number of specimens from last century in museum collections indicate the Mitchell's Rainforest Snail was previously common within its range.

Within this range and habitat, however, land clearing for agriculture and urban development has been extensive. Over 90% of suitable habitat for the Mitchell's Rainforest Snail has been cleared. Suitable habitat for the species now occurs only in small, fragmented areas or remnants, which form part of the endangered ecological community - Lowland Rainforest on Floodplain in the NSW North Coast Bioregion, which was listed under the Threatened Species Conservation Act 1995 (NSW) in 1999.

Between 1955 and 1995, Mitchell's Rainforest Snail was recorded at only 2-3 sites, despite the Queensland Museum extensively collecting for species of snails in the littoral rainforests of the area. Since 1995, NSW National Parks and Wildlife has conducted surveys for Mitchell's Rainforest Snail, that targeted suitable habitat, and undertaken public awareness campaigns for the species. This research has found the species at a number of sites at five locations- Stotts Island, Banora Point, Byron Bay, Suffolk Park and Lennox Head. It is considered that specimens that were collected at a site at Wilsons Creek, west of Mullumbimby, in 1979-1980, outside the normal range for the species, may have been translocated specimens. This site is more elevated than other sites, and a targeted search of the area in 1999 did not find the species. Other snail species present at the site were indicative of an upland snail fauna assemblage rather than the lowland snail assemblage usually associated with Mitchell's Rainforest Snail.

The largest known subpopulation and largest remaining single area of suitable habitat (120 ha) is in Stotts Island Nature Reserve in the Tweed River near Murwillumbah. The other known subpopulations are much smaller in number and occur in smaller areas of remnant habitat, most of which are found outside conservation reserves. Theextent of occurrence of Mitchell's Rainforest Snail is estimated to be less than 400 km2, which appears the same as it was in the past. However, its current area of occupancy is estimated to be less than 5 km2 , which indicates a severely restricted distribution.

The greatest threat to Mitchell's Rainforest Snail is loss of habitat, through continuing agriculture and urban development. Within the species' range, the main local government areas, Tweed, Byron and Ballina are experiencing sustained and rapid development, particularly on the coastal plain, and many of the small pockets of surviving habitat for this species remain at risk from clearing and development. Recent information indicates that a number of sites providing known or high potential habitat for the species have been cleared for development or are currently subject to development proposals. If agricultural and urban development continues in areas of suitable habitat for the snail, it will result in further habitat loss for the species and an ongoing decline in numbers.

Additional threats to Mitchell's Rainforest Snail are predation and fire. The species appears to be vulnerable to predation from introduced mammals, such as rats. It is also known that the Noisy Pitta Pitta versicolor preys on these snails, and although the bird is a natural predator, it may have a considerable impact on the low numbers of Mitchell's Rainforest Snail that now remain. Fire will also impact adversely on the key habitat requirements for the Mitchell's Rainforest Snail, which are a well-developed leaf litter layer and an intact forest canopy.

The majority of known remaining populations of Mitchell's Rainforest Snail occur in small areas of remnant rainforest, including narrow strips of rainforest bordering coastal wetlands. Though breeding populations of the snail can persist in narrow strips of remnant rainforest, the long-term viability of populations at such sites is uncertain. These small remnant areas have a high perimeter to area ratio, and, thus, are at risk from edge-effects. Edge-effects on a particular habitat is the way adjoining habitats impact on that particular habitat along their adjoining edges. Edge-effects can result in increased loss of moisture from rainforest remnants, habitat disturbance, frequent fires, and invasion by exotic weeds and feral animals. There is little data available on the impact of these factors associated with remnants on land snails, but they are likely to be adverse.

In summary, Mitchell's Rainforest Snail, a species that was common last century, is now rare and has a very restricted geographic distribution, with its current area of occupancy estimated to be less than 5 km2. The species' distribution and habitat is now extremely limited and severely fragmented. On-going agriculture and urban development and other threatening processes could lead to a continuing decline in the area of occupancy, extent and quality of habitat, number of locations where the species is found and number of mature individuals. Therefore, the species is eligible for listing as critically endangered under this criterion.

Criterion 3 - Population size and decline in numbers or distribution

The number of specimens from last century in museum collections indicate the Mitchell's Rainforest Snail was previously common within its range in suitable habitat. Within this range and habitat, land clearing for agriculture and urban development has been extensive. Over 90% of its original habitat has been cleared.Recent surveys for the species, that targeted suitable habitat, and public awareness campaigns conducted by NSW National Parks and Wildlife Service have shown Mitchell's Rainforest Snail is now extremely rare. One estimate is that total population now numbers less than 500 mature individuals.

Recent research has found Mitchell's Rainforest Snail occurs at a number of sites at five locations(Stotts Island, Banora Point, Byron Bay, Suffolk Park and Lennox Head) distributed along the coastal plain of northern NSW. The largest known population, and largest remaining area of habitat, is in Stotts Island Nature Reserve. The status and occurrence of Mitchell's Rainforest Snail and its habitat on Stotts Island was investigated by the NSW NPWS and Queensland Museum in 1999. The total subpopulation on Stotts Island is estimated to be several hundred, and its long-term viability is considered good. It is claimed that the number of mature individuals within this population is less than 250. Other subpopulations are considerably smaller - most occur in very small habitat remnants and are known from between one and three specimens.

Mitchell's Rainforest Snail's greatest threat is loss of habitat, through continuing agriculture and urban development. Additional potential threats include predation by birds and introduced mammals, fire, and those threats associated with subpopulations that occur in small remnants (e.g. invasion of weeds and predators). For details of these threats, see Criterion 2.

Mitchell's Rainforest Snail has a very restricted geographic distribution, with its current area of occupancy estimated to be less than 5 km2. The species' distribution and habitat is now extremely limited and severely fragmented. On-going threats could lead to a continuing decline in the area of occupancy, extent and quality of habitat, and the number of locations where the species is found.

Mitchell's Rainforest Snail is eligible to be listed under this criterion because: its estimated population size of less than 500 mature individuals is low, with no subpopulation considered to contain more than 250 mature individuals; a continuing decline is projected because of ongoing threats; and its geographic distribution is precarious for its survival.

Therefore, the species is eligible for listing as endangered under this criterion.

Criterion 4 - Population size

Recent surveys for the Mitchell's Rainforest Snail, that targeted suitable habitat, and public awareness campaigns conducted by NSW National Parks and Wildlife Service have shown the species to be extremely rare. The estimated number of mature individuals is low, with an estimated total population of less than 500 mature individuals.

Therefore, the species is eligible for listing as vulnerable under this criterion.

Criterion 5 - Probability of extinction in the wild

There is no quantitative data available against this criterion.

4. Conclusion

Museum collections from last century show the Mitchell's Rainforest Snail was previously common within its range, but has since declined in abundance. Since this time, much of the habitat that the species occupied, remnant lowland subtropical rainforest and swamp sclerophyll forest, has been cleared. Recent research shows that Mitchell's Rainforest Snail now has a restricted and fragmented geographic distribution, with an area of occupancy estimated to be less than 5km2, and a low number of individuals, with a population of less than 500 mature individuals. Ongoing decline is projected due to continuing degradation of habitat.

The species is eligible for listing as critically endangered under criterion 2.

5. Recommendation

TSSC recommends that the list referred to in section 178 of the EPBC Act be amended by
including in the list in the critically endangered category:

Thersites mitchellae (Mitchell's Rainforest Snail)