Gastropods slow down succession and maintain diversity in cryptogam communities.

Herbivore effects on diversity and succession were often studied in plants, but not in cryptogams. Besides direct herbivore effects on cryptogams, we expected indirect effects by changes in competitive interactions among cryptogams. Therefore, we conducted a long-term gastropod exclusion experiment testing for grazing effects on epiphytic cryptogam communities. We estimated the grazing damage, cover and diversity of cryptogams before gastropods were excluded and three and six years thereafter. Gastropod herbivory pronouncedly affected cryptogams, except for bryophytes, strongly depending on host tree species and duration of gastropod exclusion. On control trees, gastropod grazing regulated the growth of algae and non-lichenized fungi and thereby maintained a high lichen diversity and cover. On European beech, the release from gastropod grazing temporarily increased lichen vitality, cover, and species richness, but later caused rapid succession where algae and fungi overgrew lichens and thereby reduced their cover and diversity compared with the control. On Norway spruce, without gastropods lichen richness decreased and lichen cover increased compared with the control. Our findings highlight the importance of long-term exclusion experiments to disentangle short-term, direct effects from longer-term, indirect effects via changes in competitive relationships between taxa. We further demonstrated that gastropod feeding maintains the diversity of cryptogam communities.