Community response to alkaline pollution as an adjusting re-assembly between alternative stable states

Abstract

AIMS: We hypothesize that the community response to disturbances can be interpreted as a large‐scale dynamic equilibrium between multiple alternate states stemming from different species pools within a regional meta‐pool and being limited by species’ multi‐dimensional niches. We explore this hypothesis by examining the re‐assembly of an acidophilous lichen community in response to long‐term alkaline dust pollution, assuming understorey as a potential side‐factor. LOCATION: Around a cement factory in Kunda, Estonia. METHODS: Lichen communities on Scots pine (Pinus sylvestris) trunks in 40 stands around a cement factory and in nine distant limestone habitats were assessed. RESULTS: The formed bark pH gradient from pH 2.4 to 8.1 was reflected in a continuum of lichen communities on pines from acidophilous to basidophilous communities. Besides suppressing species richness, understorey density more evidently caused the compositional divergence from neutral bark conditions. The effect of hidden interactions among drivers was explained through reactions of individual species – almost all species across the pollution gradient were pH‐limited, whereas species adapted to neutral or alkaline substrate were additionally sensitive to understorey conditions. The hump‐shaped distribution of pH niche ranges along the observed niche optima, rather than ecological indicator values, showed that the shape of species’ multi‐dimensional niche‐space still needs to be quantified. CONCLUSIONS: Each alternative community state along the disturbance gradient represents a realization of its specific species pool within the meta‐pool. Degradation can be defined if the community state is not supported by a meta‐pool. Species infiltration during community re‐assembly can be predicted using species source communities as cost‐efficient proxies.