1 We tested whether slug herbivory is a factor restricting the rare perennial Arnica montana to high elevations in the Harz mountains, Lower Saxony.
2 In one experiment we artificially increased the mollusc population density in plots containing native Arnica montana populations. Leaf loss and damage to Arnica increased significantly, whilst damage to other plant species in the same plots was unaffected by mollusc density.
3 A second experiment examined damage by molluscs of transplanted Arnica montana plants at three different altitudes. Whilst damage to Arnica was negligible at 610 m a.s.l. (where natural populations occur), molluscs removed 8% of Arnica leaf area at 385 m a.s.l. and 75% at 180 m a.s.l. At the two lower sites, protective caging of Arnica plants significantly reduced the amount of leaf tissue consumed by molluscs. The impact of mollusc herbivory on Arnica montana therefore appears to increase with decreasing altitude.
4 Despite the weak relationship between leaf damage to Arnica montana and mollusc abundance, periods of peak mollusc activity may well coincide with those phases in Arnica life history that are most sensitive to tissue removal by herbivores.
5 Our results support the hypothesis that polyphagous herbivores play a major role in limiting the distribution of preferred plant species. We discuss how selective herbivory may influence the distribution of Arnica montana populations in these grassland communities.