|Scientific Name:||Partula thetis|
This species was described by Semper (1865) and is one of three congeners endemic to the Belau (Palau) Archipelago. It is distinguished by its distinctive mamillate apex.Pilsbry (1909-1910) re-described the species conchologically. Kondo described it's genitalia using specimens collected on Adelulu Hill, Airai, Babelthuap (Babeldaob) in 1936.
|Red List Category & Criteria:||Endangered B1ab(iii)+2ab(iii); D ver 3.1|
|Assessor/s:||O'Foighil, D. & Rundell, R.J.|
|Reviewer/s:||Cowie, R., Barker, G., Triantis, K., García, N. & Pippard, H.|
Although we lack data concerning contemporary (or within the past decade) population trends, this species has clearly experienced a major decline since the 1930s, when, according to Kondo (1955), Partula thetis was "distributed universally over the numerous Palau islands". Extensive surveys by Rundell in 2003, 2005 and 2007 records the species from six distinct islands/islets, the actual number of mature individuals observed in most of these sites did not exceed 1–2 snails within very restricted forest patches. The total area of occupancy at present is very small, and the species' habitat is experiencing severe developmental pressure around the Babeldaob and Koror locations, along with evidence of rat-induced predation on the uninhabited (by humans) locations such as the Ulong Islands. The species is therefore listed as Endangered.
This species is endemic to Palau. According to Kondo (1955), Partula thetis was "distributed universally over the numerous Palau islands" and was by far the most common of the three endemic partulids on this archipelago. He specifically mentioned one location: specimens collected on Adelulu Hill, Airai, Babeldaob in 1936 (Bishop Museum # 160529). Specimens deposited in the Florida Museum of Natural History (Gainsville) were sampled from the Ulong (West Island) Islands in 1995 (FLMNH252892), Wong Island in 1998 (FLMNH271838) and Ngeruktabel Island in 1998 (FLMNH271887). More recently, Rundell performed extensive land snail surveys on Palau and recovered this species from Babeldaob, Koror, Ngeruktabel and Ulong.
|Range Map:||Click here to open the map viewer and explore range.|
No rigorous quantitative data are available but Kondo (1955) states that it was the most abundant Palauan partulid, pointing out that the Bishop Museum had ~170 specimens of Partula thetis but only four of P. calypso. More recently, Rundell (2010) observed the following:
In 2003: six dead specimens collected and two live adults found in trees from limestone hills of Oikull (southeastern corner of Babeldaob), probably adjacent to “Adeluu Hill” mentioned by Kondo (1955); and one live specimen on leaves and limestone on the Ngerbechedesau limestone ridge, at the gate to Etpison house on Koror Island. In 2005: five live adults and one subadult on palms near limestone cliffs at Ngeruktabel Island and three live specimens, comprising one subadult and two adults in Ngermalk Island (Koror State). In 2007: several live individuals, mostly adults with a few juveniles, and eight dead shells in a protected limestone cliff area at Ulong Island; and five live adults on an island across from Ulong.
|Habitat and Ecology:||
This species currently lives on the leaves of palm shrubs and/or mixed vegetation in limestone forest (based on R. Rundell's surveys in 2003, 2005 and 2007).
The localities where this species occurs are low elevation areas near permanent or temporary human habitation. These localities are visited for collecting of firewood or crabs for fishing/human consumption, and thus portions of these localities are susceptible to occasional clearing and degradation. The Babeldaob locality in particular is very close to the centres of human population as well as the recently paved Compact Road; this area is also composed of limestone karst and is not formally protected--thus, it is under current threat of quarrying.
Rats such as Rattus norvegicus (Norway Rat), Rattus rattus (Ship Rat), and Rattus exulans (Polynesian Rat) represent a threat to the species. These species can be introduced and repeatedly re-introduced (by boat) by humans, but can also cross water independently. There are no known indigenous predatory land snails in Palau, and indigenous Palau species have evolved in the absence of such predation pressures. Euglandina rosea (Spiraxidae), Gonaxis kibweziensis and Gulella bicolor (Streptaxidae) have been introduced to Palau within the past 100 years (Cowie et al. 1996), and throughout the Pacific. Euglandina rosea and Gonaxis spp. in particular have been introduced in ill-conceived attempts at biocontrol for Achatina fulica (Meyer et al. 2008). Of these non-indigenous predatory species, Gulella bicolor was found by Cowie (Cowie et al. 1996) and Rundell (R.J. Rundell unpublished data, 2003, 2005, 2007). Although Euglandina rosea was not found, the presence of Achatina fulica (an agricultural pest) in Palau, particularly on the islands of Koror and Babeldaob (R.J. Rundell unpublished data, 2003, 2005, 2007) means that agricultural areas on these islands (e.g. Airai State) may be subject to renewed biocontrol attempts in the future, and vigilance is necessary regarding the potentially devastating consequences of E. rosea re-introductions. Note that accidental introductions (and distribution to different localities) are probably as a result of transportation of soil and organic debris (where snail eggs may be present), plants and produce.
This species would benefit from protection of the forest patches in which it lives, as well as periodic monitoring to assess the relative health of subpopulations (e.g. mark-recapture study). Introduction of predatory “biocontrol” snails (e.g. Euglandina rosea) or flatworms (e.g. Platydemus manokwari) would probably be catastrophic to these snail subpopulations, as has been demonstrated in similar partulid and achatinelline tree snails elsewhere. Subpopulations of this species co-occur with rat and mouse species (as of the 2003, 2005 and 2007 surveys), and the impacts of these invasive species on this snail are unknown. However, some rat species have been shown to have substantial impacts on tree snail populations, and therefore this snail species might benefit from rat-baiting efforts, with the caveat that potential negative impacts of such efforts on Palau’s endemic herpetofauna should be investigated and mitigated. The Palau Conservation Society has been supportive of recent land snail survey work in Palau.
|Citation:||O'Foighil, D. & Rundell, R.J. 2012. Partula thetis. In: IUCN 2013. IUCN Red List of Threatened Species. Version 2013.2. <www.iucnredlist.org>. Downloaded on 20 April 2014.|
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