Lord Howe Island Rodent Eradication Project NSW Species Impact Statement February 2017
Total Page：16
File Type：pdf, Size：1020Kb
Lord Howe Island Rodent Eradication Project NSW Species Impact Statement February 2017 Appendix K - Land Snail Survey 2016 K.1 Australian Museum Assessment of Potential Impacts on Land Snails Report Lord Howe Island Rodent Eradication Project: Assessment of potential impacts on land snails Frank Köhler1#, Isabel Hyman1, Adnan Moussalli2 1 – Australian Museum, Sydney, 2 – Museum Victoria, Melbourne, # - [email protected] 19 September 2016 Contents Summary ................................................................................................................................................. 2 Introduction: General characterisation of the land snail fauna ............................................................. 3 Diversity, endemism and distribution ................................................................................................. 3 Biology of the endemic species........................................................................................................... 6 General ecology of different land snail families ................................................................................. 7 Current status of the endangered species .......................................................................................... 9 Susceptibility to the baiting program ................................................................................................... 14 Toxicity of brodifacoum .................................................................................................................... 14 Likelihood of toxin intake .................................................................................................................. 16 Risk assessment for land snails and mitigation................................................................................. 16 Assessment summary for endangered and critically endangered species ....................................... 19 Sources .................................................................................................................................................. 21 Appendix ............................................................................................................................................... 23 Table 1. Native species: Endemism, conservation status, ecology and risk assessment.................. 23 Table 2. Introduced land snails on LHI .............................................................................................. 26 Table 3. Possibly undescribed species .............................................................................................. 27 Plate. Photographs of some relevant species. .................................................................................. 28 1 Summary Predation by introduced rats and mice is the most significant threat to land snails on Lord Howe Island (LHI). This impact is of national environmental significance as it has been causing a long-term decrease in the size of populations of critically endangered species. Given the restricted distribution and population size of these island endemics, especially the endangered species, addressing this impact will significantly contribute to their long term conservation. A possible side effect of the planned broadcast of poisoned baits (brodifacoum), however, is the lethal poisoning of the snails themselves. The susceptibility of one Lord Howe Island species, Placostylus bivaricosus, has been tested in an experimental setup indicating that this species may not be susceptible to this toxin. In other studies, however, it has been shown that some species may be susceptible to brodifacoum poisoning. To some extent, these conflicting results could be explained by differing ecologies and behaviours among the species so far examined. Nevertheless, given these mixed results, it is not possible to fully predict the effects of the baiting on the indigenous land snails on LHI, especially given these species belong to divergent systematic groups. To assess the potential impact on the local malacofauna, we have reviewed possible bait exposure on a per species basis taking into consideration the respective ecology and behaviour. Accordingly, we consider that all arboreal species are at a low risk of becoming exposed to baits (i.e., only a small number of individuals may get in contact with baits), that minute to small leaf litter-dwellers with small activity ranges are at moderate risk of exposure (i.e., some but not all individuals may get in contact with baits), and that large ground-dwelling species with large activity ranges are at high risk of exposure (i.e., most or all individuals may get in contact with baits). Three of the critically endangered land snails (Mystivagor mastersi, Peudocharopa ledgbirdi, P. whiteleggei) are placed in the moderate risk category, but the critically endangered subspecies Gudeconcha sophiae magnifica is in the high risk category. This taxon belongs to the same family as and is ecologically similar to Pachystyla bicolor from Mauritius, a species shown to be susceptible to brodifacoum. We recommend that for species of the high risk category experimental tests be conducted to examine their susceptibility to brodifacoum in analogy to the tests for P. bivaricosus. Individuals of the common subspecies G. sophiae sophiae can be used as surrogates for the critically endangered subspecies magnifica. More common species of the Charopidae can be used as surrogates for the critically endangered species belonging to the same family. We also recommend that, where possible, insurance populations of endangered or brodifacoum- susceptible species are kept in captivity over the duration of the baiting program. However, this is probably not a realistic option for the very rare and hard to find species M. mastersi, P. ledgbirdi, and P. whiteleggei and may also prove challenging for the rare taxon G. sophiae magnifica. Lastly, possible effects of the baiting on land snails should be monitored at selected survey sites for the duration of the program. Overall, we consider the eradication of rodents, which represent the greatest threat to the indigenous snails, to outweigh any potential short-term negative effect. We therefore support the implementation of the program. The alternative not to conduct the program is very likely to result in the extinction of more species, in particular the critically endangered species living at high altitudes, where they are currently largely unprotected from rodent predation due to the inaccessibility of the area. 2 Introduction: General characterisation of the land snail fauna Diversity, endemism and distribution The highly diverse land snail fauna of Lord Howe Island (LHI from here on) comprises 62 extant native species (or subspecies), of which 59 are endemic to the island. One species, Epiglypta howinsulae, is thought to be extinct as result of rodent predation. In addition, there are twelve introduced species, some of which are pests (Stanisic et al. 2010; AM 2016) (Tables 1-2, Appendix). The endemic species represent ten different families of both major non-marine gastropod groups, Pulmonata and Caenogastropoda: Charopidae (13 species), Punctidae (12 species), Microcystidae (11 species), Diplommatinidae (10 species), Helicarionidae (6 species or subspecies), Achatinellidae (4 species), Assimineidae (3 species), Bulimulidae (1 species), and Hydrocenidae (1 species). In addition, up to 37 potentially undescribed species have been identified among material held by the Australian Museum through curatorial work (Table 3, Appendix). The status of these candidate species requires verification by detailed comparative taxonomic study. Five species are currently listed as Endangered or Critically Endangered nationally (EPBC Act 1999), and one species is considered to be extinct (Table 1, Appendix). Predation by introduced mice and rats is the single most significant threat causing on-going decline in these species. However, based on the limited distribution of all island species and the predation by rodents, it is probable that additional species are eligible for listing as endangered or critically endangered in accordance with the criteria of the EPBC Act. Current knowledge of the distribution and abundance of most species is based on several comprehensive surveys undertaken since the early 19070s, namely 1971 (534 lots), 1978 (325 lots), 1999 (866 lots), 2000 (898 lots), 2001 (762 lots), and 2002 (550 lots) (Fig. 1). Previous surveys have covered nearly the entire surface of the island, but some inaccessible parts have remained unsurveyed or poorly represented in the collection (Fig. 2). These past surveys can provide a baseline for estimating trends in the distributions and abundance over the past few decades. Such an assessment, however, would require a comprehensive survey to evaluate their current status, one beyond the scope of the assessment provided herein. 3 Total number of samples per year (AM) 1000 900 800 700 600 500 400 300 200 100 0 1865 1870 1875 1880 1885 1890 1895 1900 1905 1910 1915 1920 1925 1930 1935 1940 1945 1950 1955 1960 1965 1970 1975 1980 1985 1990 1995 2000 2005 2010 2015 2020 Fig. 1. Time line showing the number of terrestrial land snail samples collected per year based on 4,747 records from the Australian Museum collection between 1865 and 2015, and samples collected during the current survey in September 2016. Previous surveys provide positive occurrence records that are based on specimens (live specimens, shells) picked up either