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French Polynesia. Source: Emulateme.com.
The story about the Partula tree snails on several Pacific islands is a sad chapters of man's taking influence in existing ecosystems, of which the effects are unforeseeable.
The Partila tree snails (Family Partulidae, not to be confused with the European tree snail Arianta arbustorum belonging to a completely other family) are pulmonate land snails (Stylommatophora). There they belong to the subgroup Sigmurethra, and, together with their by far larger relatives, the achataine snails (Achatinidae), to the tribus Achatinoinei. In contrary to their large relatives the Partula snails only become 2 cm in size, about as large as an achatine snail's egg.
In contrary to the achatine snails, Partula snails are ovoviviparous, instead of eggs being layed, the young hatch inside the parent and the parent gives birth to living young snails. Like other pulmonate land snails the Partula snails also are hermaphroditic animals.
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Picture right: Partula snail.
Source: Phillip Coffey, on Partula Snails.
It is already since the 19th century, that Partula snails are of interest to biologists. Like the Darwin finches from a common ancestor on the Pacific islands a large diversity of different species has evolved, some of which only occur on one single island on trees of one single species. The process basic to this, in which from a common ancestor by adaptation, that is by variation and development, to existing ecologic niches, new subgroups of existing groups arise, is called adaptive radiation.
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Picture left: Partula subangulata from Raiatea, society islands, Fr. Polynesia.
Source: Image plate Partula snails.
The Partulidae family, to which besides the genus Partula also belong the genera Samoana and Eua, comprises about 120 species on numerous islands in the Western Pacific, from Palau and the Marianas until the Marquesas and the society islands (Fr. Polynesia).
The variety of different species on single islands is standing against the uniqueness of those species and the smallness of their distribution area. The latter is a factor, that makes the Partula snails specially vulnerable to influences in their ecosystem.
On the Polynesian islands belonging to France in the 1960s African achatine snails (Achatina fulica) were imported. They were supposed to grant the local farmers a possibility to aquire food and to trade. The expected economic success failed to appear and thus the snails were released into freedom.
On the Pacific islands the achataine snails did not have any natural enemies and therefore were able to reproduce freely and thus numerously. The effect were large damages to the agriculture or the area.
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Picture right: Euglandina rosea.
Source: R. Hue, Hawaii Dept. of Agriculture.
To fight the achatine snail plague on several islands the Central American predator snail (Euglandina rosea) was introduced. The predator, however, did not decimate the large achatine snails, which was what it had been introduced for, but the tree snails, that were much easier to kill. Besides scientists predicting otherwise, Euglandina rosea followed its prey up the highest trees, as well as a certain distance under water.
Luckily for the Partula snails, Frech genticists alarmed, after they discovered the dissapearing of snail species on several islands. International aid programmes ran on and several Partula snail species could at least in small populations be saved and secured in certain zoological gardens. Surveys resulted in a large endangerment of tree snails. An american study on the island of Moorea near Tahiti (Clarke et al., 1984) resulted in that the endemic tree snail species in the island would be extinct by three years' time. And indeed 1987 during a survey on the island not one free living tree snail could be found.
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Picture left: Moorea, Society Islands, Fr. Polynesia.
Source: Destination World - Moorea.
In the meantime samples of most tree snail species had been able to be collected, so that they could be bred in captivity, safe from predatory snails. So today there are tree snail colonies in the zoological gardens on the channel island of Jersey (founded by the naturalist and author Gerald Durrell), in the Shedd-Aquarium in Chicagi, Illinois, in the London Zoo and several other. Breeding Partula snails is everthing but easy. The snails must be kept in an environment conforming exactly with their ecosystem at home (humidity, temperature, light and food), otherwise they will cancel reproduction and the population will die out. Though, some colonies could be held and even the effort is done to put populations back to nature, of course in safe areas.
For some Partula snail species anyway all help was too late. They must be regarded extinct and forever gone.
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