Pacific Research and Policy Centre

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Browsing Pacific Research and Policy Centre by Author "Brescia, Fabrice"

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    Ecology and population trends in New Caledonian Placostylus snails (Mollusca: Gastropoda: Bulimulidae) : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Zoology at Massey University, Palmerston North, New Zealand
    (Massey University, 2011) Brescia, Fabrice
    This study focuses on two endemic New Caledonian land snails: Placostylus fibratus and Placostylus porphyrostomus (known locally as bulimes) which are in decline and listed as vulnerable by the IUCN. On the Isle of Pines, both species are highly-valued commercially and traditionally harvested species suffering from exploitation for human consumption. In the dry forests of the New Caledonian mainland, P. porphyrostomus, especially, is threatened due to habitat degradation and loss, and rodent predation. Prior to this study, the life histories, impact of human harvest, and population trends remained largely unknown for the New Caledonian Placostylus species and restoration trials for their conservation had not been undertaken. Addressing these deficiencies forms the foundation for the thesis, and the findings are used to formulate recommendations for management and conservation. On Isle of Pines, the extent and densities of P. fibratus are greater than the scattered and isolated populations of P. porphyrostomus found on the island and in the dry forests of the Mainland. Placostylus snails are long-lived (estimated at 19 to 39 years for P. fibratus in this study) and relatively slow growing, taking up to four years to reach sexual maturity (aperture lip ≥3.5 mm). Non-exploitative mortality factors impinge disproportionately on juveniles. Consequently, the age-structure of populations is changing, with juveniles becoming increasingly rare. Annual survival rates for P. fibratus are estimated as 59.0% for juveniles and 70.0% for adults. The major cause of mortality of juveniles was predation by introduced rodents (the ship rat Rattus rattus, the Polynesian rat Rattus exulans and the mouse Mus musculus) that are present in very high densities compared with those reported elsewhere where Placostylus occurs, such as New Zealand. I found density estimates of 25.4 rats ha-1 in the rainforest (23.0 – 34.6 rats ha-1, 95% confidence intervals) and 19.1 rats ha-1 in the dry forest (18.9 – 23.6 rats ha-1, 95% confidence intervals). In the dry forest, 87.6% of all of the empty juvenile shells found for P. porphyrostomus appeared to have died from rodent predation and 73.2% for P. fibratus on Isle of Pines. The introduced little fire ant Wasmannia auropunctata had a negative impact on Placostylus growth but not on mortality under semi-natural conditions. I showed evidence for a decline in the total population of P. fibratus snails on the Isle of Pines between 1993 and 2008, with a particularly sharp decline in the population of juveniles from 2001 to 2008. About 60,000 adult snails are collected annually from the wild for sale, and an additional 69,000 snails are estimated to be eaten annually by Kuniés (the local people of the Isle of Pines). Thus, the actual annual harvest represents approximately 6% of the estimated wild adult stock (ca. 2.5 million snails on average over the period 2001-2008). A stage-structured population dynamics model developed here showed that if the harvest rate is maintained as it is (ca. 120,000 adult snails per year), then the population is likely to decline to very low numbers within the next 50 years. If the harvest rate increases, the rate of decline is accelerated. Sustainable exploitation is predicted to be attained at a 3% annual harvest rate (c.a. 70, 000 snails per annum), which would preclude collection for commercial purpose since the daily consumption by locals on Isle of Pines would account for most or all of this number. The model is very sensitive to rodent predation rates and predicts that the Placostylus population would likely recover if rodent predation was decreased even slightly. Restoration trials were undertaken for P. porphyrostomus populations on the mainland. I successfully controlled rodent populations for 22 months by continuous poisoning in 5 ha of dry forest. After 15 months, the poisoning was sufficient to reduce and maintain rodent activity at low levels but I was unable to conclusively demonstrate a significant benefit to snail populations in the poisoned areas during this period, probably due to the specific life history traits of these snails and the flow-on effects of poisoning also reducing the density of rodents in non-poisoned areas. A trial release of 21 captive-bred P. porphyrostomus snails was conducted in an isolated patch of dry forest including two release procedures (soft- vs. hard-release). Twenty five months after the release the trial was deemed a success. The mean survival rate over this period was 100% for hard-released snails, which was significantly higher than the 70% survivorship for soft-release snails. No differences in snail growth (weight and aperture lip thickness) existed between the two release-procedures. Soft-released snails travelled shorter distances from the release point than hard-released snails, and showed significantly higher site fidelity. Supplementation with captive-bred snails appeared viable as a conservation strategy for New Caledonian Placostylus in dry forests. The key findings of the thesis have direct implications for the conservation and management of New Caledonian Placostylus; recovery plans highlighting urgent actions that need to be undertaken for each species of New Caledonian Placostylus have been proposed to provide guidance for both managers and local people.