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Austroassiminea letha

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Taxonomy [top]

Kingdom Phylum Class Order Family
ANIMALIA MOLLUSCA GASTROPODA LITTORINIMORPHA ASSIMINEIDAE

Scientific Name: Austroassiminea letha
Species Authority: Solem, Girardi, Slack-Smith & Kendrick, 1982
Common Name(s):
English Cape Leeuwin Snail, Leeuwin-naturaliste Amphibious Snail

Assessment Information [top]

Red List Category & Criteria: Endangered B1ab(iii,v) ver 3.1
Year Published: 2011
Date Assessed: 2011-07-22
Assessor(s): Clark, S.
Reviewer(s): Böhm, M. & Collen, B.
Contributor(s): Dyer, E., Soulsby, A.-M., Whitton, F., McGuinness, S., De Silva, R., Milligan, H.T., Kasthala, G., Herdson, R., Thorley, J., McMillan, K. & Collins, A.
Justification:
Austroassiminea letha has been listed as Endangered under criterion B1ab(iii,v). It has a restricted extent of occurrence of approximately 2,000 km²; its distribution is severely fragmented (with little possibility of migration between subpopulations); and there has been a continuing decline in habitat quality (from agriculture and reduced rainfall). Furthermore, although quantitative estimates are lacking, population size appears to be declining due to the effects of introduced species and habitat loss/degradation. Finally, A. letha has very specialized habitat requirements which further reduces its likelihood of recovery from declines.
History:
1996 Vulnerable
1994 Endangered (Groombridge 1994)

Geographic Range [top]

Range Description: This species is endemic to Western Australia, restricted to a few isolated localities between Cape Leeuwin and Cape Naturaliste. As a result, the species is found along a very narrow strip of coastline south of Perth (S. Clark pers. comm. 2011). Populations are known from five locations: the Cape Leeuwin swamp, Ellen Brook (two isolated sub-populations), Turner Brook, Deepdene Cliffs and Cosy Corner (Solem et al. 1982, Ponder et al. 1999). The extent of occurrence is approximately 2,000 km².
Countries:
Native:
Australia (Western Australia)
Range Map: Click here to open the map viewer and explore range.

Population [top]

Population: Solem et al. (1982) considered this species "both rare and endangered". The largest known population occurs at the Cape Leeuwin Wetland System, adjacent to the waterwheel on Cape Leeuwin (Limbourn and Westera 2006).

The fossil record indicates that this species was once much more widespread (three of six known occurrences are fossils only). It is now restricted to just a few localities, each several square metres in area, where it is "relatively abundant" (Solem et al. 1982).

Since the 1980s almost all populations have shown declines, particularly at Cape Leeuwin Swamp where suitable habitat has decreased, and at Ellen Brook where one population has almost disappeared with the introduction of several invasive snail species (S. Slack-Smith 2009 pers. comm.). A former locality at Gnoocardup was washed into the sea by strong wave action (S. Slack-Smith 2009 pers. comm.).

A second isolated population at Ellen Brook appears to be stable, and a previously unknown population - although small in size - was recently discovered along the Leeuwin-Naturaliste coast (S. Slack-Smith 2009 pers. comm.)
Population Trend: Decreasing

Habitat and Ecology [top]

Habitat and Ecology: This species is a habitat specialist, relying on natural seepages from limestone or lime sands (Solem et al. 1982, Ponder et al. 1999).
Systems: Freshwater

Threats [top]

Major Threat(s): According to Solem et al. (1982) "each of the three known living populations is small and in danger of destruction from agricultural or other human activity". These threats include weed invasion, sedimentation, erosion, salinity and loss of riparian vegetation (WRC 2001).

The Turner Brook population was previously impacted by chemical spraying and fertiliser application in surrounding agricultural areas. Although the effect of such activities on this species was unknown, Solem et al. (1982) state that "they are highly unlikely to be beneficial, and probably are quite harmful".

Due to their reliance on rain-fed springs and limited aestivation ability, these snails are also severely impacted by recent and predicted reduced rainfall in southwestern Australia. This is already evident in the population at Cape Leeuwin Swamp, where suitable habitat has reduced in size due to lower rainfall and prior use of the spring as a water supply to the nearby town of Augusta (S. Slack-Smith 2009 pers. comm.).

A former locality at Gnoocardup, halfway between Capes Leeuwin and Naturaliste, was located just two metres above intertidal boulders and was washed into the sea by strong wave action (S. Slack-Smith 2009 pers. comm.).

Another population - at Ellen Brook - has declined with the introduction of several species of snails since redevelopment in the area. These include competitive herbivores and a predator species of Oxychilus (S. Slack-Smith 2009 pers. comm.).

The above threats are compounded by the isolated nature of the known populations, as well as the snails' small size, both of which make it unlikely that a dwindling population in one locality will be 'rescued' by immigration and recruitment from neighbouring populations.

Conservation Actions [top]

Conservation Actions: This species' only habitat falls within the Fitzgerald Biosphere Reserve (FBR), the largest of six sub-regions on the South Coast of Western Australia, and within that the Fitzgerald River National Park (WRC 2001).

It is listed as a Schedule 1 species under the Australian Wildlife Conservation Act 1950, which formally recognises its restricted distribution (last updated 13th March 2008) (EPA 2009). It is also listed as Endangered in Western Australia under State legislation (Fukuda and Ponder 2003).

It is recommended that each of the known populations are protected to ensure no further declines from external anthropogenic activities (e.g., development, agrictulture), and these sites are managed in such a way that where suitable habitat has declined over the past ca 25 years it is restored. It may be necessary to control introduced competitors and predators at Ellenbrook, which appear to be causing local declines. Ex-situ propagation and benign reintroductions are also recommended to counteract population declines.

Further research is needed to determine whether these are the only known populations of this species, or whether its range extends into other areas of suitable habitat not yet surveyed. Population trends at existing sites must continue to be monitored to assess the beneficial impact of site/habitat protection.

Citation: Clark, S. 2011. Austroassiminea letha. The IUCN Red List of Threatened Species. Version 2014.2. <www.iucnredlist.org>. Downloaded on 22 September 2014.
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