1. Generalist herbivores such as slugs have the potential not only to reduce plant density and biomass, but also to alter species diversity within vegetation. Their impact on species diversity may be either negative, if they concentrate feeding on less abundant plant species, or positive, if they feed on the most abundant species.2. This study investigated the influence of slugs on plant species diversity in experimental swards produced by sowing aLolium perenne/Trifolium repensseed mixture in field plots with a large seed bank of mainly arable species. Half of the plots were grazed byArion lusitanicusMabille. Plant cover, above-ground biomass and number of plant species were measured over a 3-year period.3. Vegetation cover increased in the control plots from 50% in the first year to 90% in the third year. Cover was significantly lower in the slug plots in the first year (>22%), while there were only small differences between treatments in the third year. Slugs reduced total above-ground biomass by>25% in both the first and third years.4. Slugs had a negative impact on plant species diversity in the first year, particularly by reducing forb species. In contrast, plant species diversity after 3 years was higher in the slug plots than in the controls, because of the higher number of forb species. Under slug grazing, the biomass and cover of annual and palatable species were reduced, but not the numbers of these species.5. Our results suggest that slugs can have a significant effect on plant species diversity in plant communities, but that the direction of the effect changes during the course of succession. In the earliest stages, when most species are present as seedlings or juveniles, slug grazing leads to reduced species diversity because favoured species are eliminated. In closed vegetation, in which competitive interactions are important, slugs may reduce the dominance of the more competitive species and thus provide gaps in which plants can establish from seed. As a consequence, slugs tend to cause an increase in plant species diversity, and may also reduce the rate of successional change by promoting the persistence of annual species.Functional Ecology(2005)19, 291–298doi: 10.1111/j.1365-2435.2005.00960.x [ABSTRACT FROM AUTHOR]