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Placostylus snails are vegetarian, feeding almost exclusively on the fallen leaves of karaka and mahoe except for one instance at the Three Kings Islands, where a colony has adapted itself to ngaio leaves.
These snails are restricted to the coastal headlands of the east and north coasts of North Auckland from Whangarei northward, and on certain of the outlying islands of this area.
An interesting study is presented by the remnant colonies which have persisted under very adverse conditions at the Three Kings Islands. On Great Island, which is one and a-quarter miles in length and roughly hour-glass shaped, there are three small colonies of the large land snail, Placostylus bollonsi, and each has developed individualistic characteristics.
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No. 1 colony from the north-east end is short and broad.
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No. 2 colony from the cliff faces of the narrow divide is small and slender, and
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No. 3 colony from the south-west cliff face is very large and slender.
It is at once apparent that the vegetation on Great Island has been greatly modified. Great Island, in spite of its being well watered, is the one semi-arid member of the group. By contrast with the other islands, the division between the rocky cliffs and the vegetation shows barren expanses of yellow and reddish earth and greyish shingle screes. There can be little doubt that Great Island once resembled the other members where the giant puka, Meryta sinclairi, is a dominant feature, but now Great Island is largely a monotonous expanse of kanuka and stunted pohutukawa.
The factors causing this change in character of the vegetation of Great Island are presumed to be:
| (1) |
The long residence in former times of a moderately large Maori population. These Maoris probably occupied the island for over three centuries. At the time of Tasman's discovery of the islands in 1643 the Maoris were in occupation, and Tasman noted in his journal that most of the more accessible parts were under cultivation. ‘This was probably the factor that reduced the snail populations to the perimeter of more or less inaccessible cliff faces and caused the initial segregation of these snails into at least three separate colonies. |
| (2) |
The present marked deterioration of the vegetation when compared with Cheeseman's description of the island in 1889 is directly attributable to the depredation of goats introduced at about the time of Cheeseman's visit. Prior to the extermination of the goats in 1946 by Internal Affairs, the succulent vegetation had been reduced to odd large-leaf trees in a general expanse of kanuka. |
Colony No. 1. This colony was found at the north-east end of Great Island and consisted of only eleven snails. The colony was restricted to the leaf-spread area afforded by a group of seven trees of wharangi (Melicope ternata) and one of mahoe (Melicytus ramiflorus). This colony exists solely within the leaf-spread area of this grove, which is from 5 to 10 ft. wide and 30 ft. long down a slope of 45°.
Colony No. 2. Probably about 25 snails occupying a similar grove on the south-west side of the island.
Colony No. 3. About 40 living snails and over 200 dead ones were taken bv Mr. E. G. Turbott in April last year. These were from a ngaio grove high up on the north-west landing slope. About 30 of the dead snails were comparatively fresh, and probably died during the drought of a few months prior to Mr. Turbott's visit.
The island was systematically combed by Mr. Turbott and the members of the expedition sent by Internal Affairs to exterminate the goats, with the result that no further colonies were found.
![Thumbnail: [Volume 77, 1948-49 page 208]](/journals/rsnz/images/rsnz_77/thumbnails/rsnz_77_05_0985_0208_ac_02.gif)
Colony No. 4. In December, 1946, Major G. A. Buddle succeeded in landing upon the formerly considered inaccessible North-East Island, which lies immediately to the North-East of Great Island. Here, amidst luxuriant vegetation dominated by the large puka, Major Buddle found Placostylus bollonsi to be quite abundant.
A point of special interest is that these snails are identical with those of the north-east colony on Great Island, in spite of the intervening waterway. North-East Island, however, showed abundant evidence of Maori occupation, and so it seems very probable that the Maoris either intentionally or unintentionally were responsible for acclimatising the snails to this island. The South-West King was visited by Major Buddle also, but no Placostylus was seen.