Geomalacus maculosus: An assessment of trapping methods, forestry management impacts, and feeding preferences
Johnston, Erin Ashleigh
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The EU protected Lusitanian slug species Geomalacus maculosus Allman (Gastropoda: Arionidae) is found only in northern Iberia and the west of Ireland. Once thought to inhabit undisturbed habitats such as blanket bogs and forests, in 2010 was found to be breeding in commercial forestry, a cause for concern in relation to the adequate conservation of the species. Chapter 1 provides an overview of the current knowledge of the species’ distribution, ecology and associated legislation and identifies areas in need of further research. Chapter 2: Given that automated behavioural analysis has never been undertaken for G. maculosus, a procedure to allow for Ethovision XT10 behavioural software use with G. maculosus was established. Observational trials indicated that when G. maculosus was in contact with food, it was actively feeding for 95% (median) of the time. Ethovision XT10 software was used to establish contact time with a range of lichens, mosses and liverworts, and analyse feeding behaviour for the first time under darkened conditions. While preferences for some bryophytes and lichens were observed, overall results indicate that G. maculosus is a generalist lichen and bryophyte herbivore with no differences in feeding preferences shown by the two colour morphs, associated with forested and open habitats respectively. Maximum mean distance moved in the laboratory over two hours by G. maculosus was 6.7m (+ 2.9 SE) with mean meander by colour morphs of open habitats being significantly greater (P = 0.008) than that of colour morphs of forested habitats. The results of this study are discussed in the context of maximising the food resource of G. maculosus during the forest cycle. Chapter 3 examines the efficacy of De Sangosse refuge traps across three habitats frequently found associated with commercial forest plantations in Ireland, comparing them with hand searching, a commonly used method for slug monitoring. Catch data during different seasons and under different weather conditions indicate that autumn is the optimal time for sampling G. maculosus. Refuge traps placed at 1.5m on trees in mature conifer plantations and directly on exposed rock in blanket peatlands result in significantly greater catches, however hand searching is the most successful approach for clear-fell areas. Hand searches in clear-fell preceded by rain are likely to result in greater numbers caught. Chapter 4: The discovery of EU protected G. maculosus in commercial plantations requires knowledge regarding implications of forestry practices on the species in the context of sustainable management in Ireland. The aims of the study were to compare G. maculosus captures across mature planted, existing clear-felled and unplanted habitats, assess models for estimating G. maculosus population sizes, and determine impacts of clear-felling on G. maculosus. Mean catches of G. maculosus were greatest in mature forest compartments. The Schnabel model for estimating population size was most suited for mature forest stands but could not be utilised for other habitats. BACIP analysis showed 95% reductions in G. maculosus mean catches post-felling where no individuals marked prior to felling were recaptured compared to 21% recapture rates at the control site. Greater tree circumference correlated with greater catches. Chapter 5 summarizes and discusses the main outcomes of the three preceding chapters through the establishment of key points, examines potential mitigation measures that could be established for G. maculosus and highlights areas in need of further research.
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