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Indication of interference competition between the EU-protected Kerry slug Geomalacus maculosus and the native tree slug Lehmannia marginata in Ireland


 Interspecific competition plays a major role in organizing biological communities. Competition can involve direct (e.g. aggression) or indirect (e.g. avoidance) interactions, and can influence the behaviour, distribution and fitness of interacting species. Competition appears to be common among marine and terrestrial snail species, but the potential role of interspecific competition in the structure of terrestrial slug communities remains poorly known. This study examined whether competition between two terrestrial slug species, Geomalacus maculosus and Lehmannia marginata, contributes to the abundance of each species in a modified habitat (commercial conifer forest) in Ireland. Geomalacus maculosus is an EU-protected species but was probably an ancient introduction to Ireland, whereas L. marginata is unprotected but is native and widespread. A negative association was observed between the abundances of each species in field plots, sampled fortnightly over a 19-month period. The presence of other slug species and some environmental variables also influenced the abundances of L. marginata and G. maculosus. Behavioural experiments found no evidence of direct aggressive interactions between G. maculosus and L. marginata. However, L. marginata exhibited an aversion to G. maculosus mucus trails, suggesting an indirect mechanism of competition. Conversely, G. maculosus behaviour was not modified by the presence of L. marginata mucus. Results from field and laboratory studies together suggest that both species may compete in modified habitats. To our knowledge, these results provide the first experimental evidence of a negative long-term association between sympatric slugs, and the first evidence that mucus trails can act as a mechanism of interspecific interference competition in slugs.