Gastropod grazing shapes the vertical distribution of epiphytic lichens in forest canopies

Abstract

Summary 1. Species composition and biomass of epiphytic lichens varies along a complex environmental gradient from the ground to the top of the forest canopy. It is not known if this gradient, considered to be shaped by succession (age of bark surface) and climatic factors, is also influenced by invertebrate grazing. 2. To investigate the grazing hypothesis, the natural height ranges of four old‐forest Lobaria species on tree trunks were quantified. These large foliose epiphytes with different secondary chemistry were transplanted onto Fraxinus excelsior trunks 0.5, 3 and 6 m above‐ground in five broad‐leaved deciduous forests in southern Norway. After 4.5 months of exposure to natural climbing gastropods, grazing was quantified. 3. Grazing pressure strongly increased with increasing proximity to the ground. At all heights, gastropods clearly preferred Lobaria scrobiculata followed by L. amplissima. Lobaria pulmonaria, highest in carbon‐based secondary compounds (CBSCs), and L. virens, nearly deficient in CBSCs, were both much less grazed. Therefore, CBSCs cannot explain the preferences. According to existing literature the stictic acid complex, present in L. pulmonaria and L. scrobiculata, represents a herbivore defence when occurring in quantities as high as those in L. pulmonaria. The identity of the strong defence in the CBSC‐deficient L. virens is unknown. 4. Gastropods’ preference for these epiphytes mirrors the distribution of the lichens in nature. The highly palatable L. scrobiculata occurs mainly in localities with low gastropod abundance such as boreal forests and on bark with slightly lower pH. Lobaria amplissima occurs in gastropod‐rich localities, but above the lower parts of the trunk. Lobaria pulmonaria and L. virens frequently grow down to a level of < 1 m above the ground. 5. Synthesis. Our results suggest that climbing gastropods play a role in determining the lower distribution limit of epiphytic lichens along a vertical canopy gradient and influence the spatial pattern of susceptible lichen species. By grazing lichens in a species‐specific way, gastropods can shape epiphytic communities in broad‐leaved deciduous forests.