All ecosystems allow energy to flow through them from photosynthesising organisms to predators. This is achieved through food chains and food webs. Lichens are part of an ecosystem’s primary producers and they are a source of food for many organisms from tiny invertebrates to large vertebrates. There is little doubt that in ancient Ireland the Elk would have fed on lichen species such as Reindeer moss (Cladonia species). Ireland does not have a high enough lichen biomass to support mammals today.
Many lichens, particularly the foliose types have a rich biodiversity of invertebrates associated with them. Some feed on the lichen, others use them for shelter. Birds also feed on lichens. As lichens are good bio accumulators (see Lichens as Biomonitors) and part of an ecosystem’s food chain, they can pass on toxic metals and chemicals to organisms that feed on them. This was a problem after the Chernobyl nuclear disaster where the local lichen flora was contaminated. The lichens were eaten by reindeer, and they in turn were eaten by humans. This led to illness (cancers mainly) and birth defects in humans.