Abstract
Herbivores are known to decrease plant species diversity in ecosystems with low productivity. Limestone pavements are low-productive habitats harboring specialized communities of cyanobacteria, and endo- and epilithic lichens exposed to extreme temperature and humidity fluctuations. Pavements of the Great Alvar (O¨ land, Sweden) are covered by free-living cyanobacteria giving the rock surface a dark color. Based on cyanobacterial abundance along the edges, two types of cracks intersecting the pavements have been described: Type one with abundant cyanobacteria and type two without cyanobacteria resulting in light-colored edges. Erosion and different lengths of inundation by melt water have been suggested to cause the conspicuous differences in community composition and hence color between cracks. We hypothesized that this pattern results from the grazing activity of the cyanobacteria- and lichen-feeding snail Chondrina clienta, which reduces cyanobacterial cover along light-colored cracks and facilitates endolithic lichens. Three dark and three light-colored cracks were investigated at each of three localities. Crack characteristics (i.e., aspect, width, depth and erosion) and snail density were assessed at the crack level. Cyanobacterial cover and lichen diversity were recorded in 1-cm sections, sampled every 5 cm along eight 160-cm-long transects per crack. Model selection was applied to estimate effects of snail density and distance from crack edges on cyanobacterial abundance and lichen diversity. Crack characteristics explained no differences in cyanobacterial cover or lichen diversity. However, cyanobacterial cover decreased towards the edges of cracks with high snail densities. A transplant experiment supported the correlational evidence. The abundant cyanobacterial cover on pieces of stone placed close to cracks with high snail densities was completely grazed within 19 months. By contrast, cyanobacteria recolonized initially completely grazed pieces of stone when fixed near cracks without snails. Abundance and diversity of endolithic lichens increased along cracks with high compared to low snail densities but decreased in epilithic lichens and lichens with cyanobacterial symbionts. However, the presence of the gastropod herbivore decreased overall lichen diversity. Comparing presence-absence matrices with null models revealed that species co-occurred less frequently than expected. Taken together, we provide evidence that herbivory indirectly released endolithic lichens from competition for light by reducing cyanobacterial cover. (Less)
Highlights
Indirect interactions occur when changes in one species or a group of species affect others through a chain of intermediate species
We aimed to elucidate the effects of snail grazing on cyanobacterial abundance and on the diversity and abundance of the lichens occurring on limestone pavements
Direct effects of herbivore presence The present study showed that cyanobacterial cover was nearly completely removed along the edges of cracks inhabited by gastropods and that endolithic lichen abundance, as well as diversity increased in close association with high snail densities
Summary
Indirect interactions occur when changes in one species or a group of species affect others through a chain of intermediate species. Diversity of primary producers is influenced by large-scale processes such as dispersal, disturbances and isolation and by small-scale processes such as interactions between organisms of the same or different trophic levels and by indirect effects. Herbivory and competition may interact to increase or decrease diversity depending on the competitive ability, defense mechanisms and regeneration ability of primary producers, the size and selectivity of herbivores, nutrient availability and productivity of ecosystems (Bakker et al 2006). It has been suggested that if herbivory reduces competition among plant species, it may be more important than competition in structuring communities of primary producers (Sih et al 1985, Hulme 1996). In contrast to the well-studied direct effects (e.g., consumption) of vertebrates on plant species diversity, the impact of invertebrate herbivory on the diversity of lichen communities are largely unexplored with few exceptions. Indirect effects (e.g., competitive release) of invertebrates on the diversity of lichens have so far not been investigated
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