EU-protected slug Geomalacus maculosus and sympatric Lehmannia marginata in conifer plantations: What does mark-recapture method reveal about population densities?

Geomalacus maculosus Allman, 1843 is an EU-protected slug species which is only found in the west of Ireland and northern Iberia. There is little knowledge of its population sizes throughout its range and no long-term studies have been conducted to calculate estimates. Localized populations of G. maculosus and the sympatric slug Lehmannia marginata (Müller, 1774) were monitored over one year using the markrecapture approach at five different sites within a commercial conifer plantation in Ireland. Slugs were marked using visible-implant elastomers injected into the foot. Up to 23.9/m² (G. maculosus) and 5.6/m² (L. marginata) specimens were observed during the first survey in August; however, these densities varied greatly in different areas of the plantation and throughout the year. Population-size estimates were also calculated using the Jolly-Seber method. Temperature was significantly correlated with capture success, with fewer captures during the colder months. This is the first long-term study that uses visible-implant elastomers as markers for slugs and we evaluate their use, provide guidelines for trapping intensity, discuss limitations of the mark-recapture method and advise on the optimal survey time and conditions.