Text by Ave Suija and Jaan Liira
Man made disturbances and environmental pollution are the reality of contemporary world, however, such long-term created environmental gradients are handy to describe ecological processes (community dynamics) otherwise difficult to observe in natural conditions or test in experiments. We assumed that at regional scale, many community responses should not be always interpreted as a degradation, but as a part of the long-term dynamic equilibrium of various community states, if they are supported by different environment-specific species pools, harboured in different habitats. At the regional scale, such spatially clustered pool system forms a meta-pool of species. A community degradation should be declared only when the meta-pool cannot support the community’s switching between alternate states. The difficulty in applied ecology relies in adequate delineation and evaluation of these alternate states – usually, expert knowledge or species niche data have been used, but their accuracy has been sometimes questioned. A different approach is to compare new community states to other communities in the region, as they harbour alternative species pools within meta-pool.
We studied the re-assembly of lichen community on Scots pine (Pinus sylvestris) in response to the long-term alkaline dust pollution around a cement factory. Along pollution gradient we observed (1) the change of substrate (pine bark) pH from very acid to pH analogous to limestone, and (2) a respective alteration of natural community to unusual to pine lichen communities. The loss of diversity reflected the difference between source species pools. We found also that (3) expert knowledge (ecological indicator values) can reflect the overall trend, however, it does not describe humped variability of niche range of species, the divergence in alternate community states two different reference communities, add the revealed hidden interaction between limiting environmental drivers (specifically substrate pH and understorey density).
The study indicates that community responses to man-made disturbances should be interpreted with care, particularly as community reformations take long time assuming the availability of respective species sources. Still, as disturbance intensity may change over time by improving technologies or policies, such alterations can be reversed, if original pools are preserved somewhere in the neighbourhood.
Citation: Suija, A., & Liira, J. (2016). Community response to alkaline pollution as an adjusting reassembly between alternative stable states. Journal of Vegetation Science, DOI: link to full text)(
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.
Around a cement factory in Kunda, Estonia.
Lichen communities on Scots pine (Pinus sylvestris) trunks in 40 stands around a cement factory and in nine distant limestone habitats were assessed.
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.
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.