Habitat extremity and conservation management stabilise endangered calcareous fens in a changing world

Calcareous fens represent an endangered type of peatlands, acting as refugia for stress-tolerant species in the currently changing landscapes. The resurveys across many regions have reported their recent disappearance or deterioration despite both the extreme habitat conditions (carbonate richness, presence of calcareous tufa, nutrient limitation, high water level) and conservation management. To test the stability of their biotic communities in different environmental and management configurations, we repeatedly sampled molluscs (terrestrial and aquatic), vascular plants, and bryophytes at 30 calcareous fens in the Inner Western Carpathians (Slovakia, Poland) after 13–17 years of warm summers and land-use changes. We found a small yet statistically significant effect of sampling period (old versus new survey) on the species composition of all three groups of organisms when the effect of various positions of sites along ecological gradients was controlled for. The compositional changes, interpreted with the help of Ellenberg Indicator Values, suggest an incipient succession towards grasslands and shrublands, driven by decreasing soil moisture and increasing nutrient availability. Although the number of habitat specialists did not change, the number of matrix-derived vascular plant and bryophyte species significantly increased, with six ubiquitous species of productive habitats being significantly more represented currently, while the richness of aquatic molluscs significantly decreased. Fens in which potentially strongly competitive plant species were less stressed because of less intense management and lower habitat extremity were more prone to such succession. There was no single factor that could predict the magnitude of composition changes; instead, tested factors were found to act synergistically. Conservation management was predominantly important for bryophytes, while extreme habitat conditions were predominantly important for terrestrial snails. We suggested a way how nature conservancy authorities can prioritise the management needs by applying an abiotic indicator system, with less environmentally extreme fens requiring more intense conservation management.