Selective mortality of seedlings in response to predation by the slug Arion vulgaris (Mollusca: Arionidae)

Abstract We investigated the preference of the invasive slug Arion vulgaris for seedlings of dicotyledonous herbs. In the field experiment made in Prague (50.086 N 14.297E), seedlings of 25 herb species were exposed to predation by a natural population of slugs. Seeds, spread on the surface of moist potting compost pressed into plastic cups, were allowed to germinate and grow until the cotyledon stage. The cups, each containing the seedlings of one species, were exposed in a 5 × 5 block and replicated five times. After 7 days of exposure, the average mortality of the herb species (percentage of seedlings with devoured hypocotyl and cotyledon leaves) differed significantly and ranged from > 95% ( Atriplex sagittata , Portulaca oleracea , Spergula arvensis ) to < 20% ( Cerastium holosteoides , Vicia cracca , Euphorbia peplus ). The mortality of seedling species was positively related to their size and different between therophytes vs. hemicryptophytes. In the laboratory experiment, seedlings of 11 species were exposed to predation for 9 h, at constant 20 °C and in darkness. The mortality of seedling species also differed significantly and correlated marginally significantly with the mortality in the field experiment. Slugs affected seedling mortality selectively in the period when feeding activity of their population was low. In temperate climate of Central Europe, such period occurs in spring and autumn, at the time when seedlings of most herb species emerge. It is likely that slugs, by selectively feeding on herb species at the seedling stage, can influence the composition of herb populations.