Shifts in the ecological behaviour of plant species between two distant regions: evidence from the base richness gradient in mires

Aim Water pH and conductivity are known to be major environmental factors controlling the species composition of nutrient‐poor wetlands. Based on the analysis of two large data sets of species co‐occurrence, sampled along the entire pH/calcium gradient, we explored whether species exhibit similar or different ecological behaviour in the two regions. Location West Carpathians (central Europe) and Bulgaria (south‐eastern Europe), situated 800 km apart. Bulgaria represents a range margin for many mire species. Methods The probability of occurrence of the 41 most common species along the pH and conductivity gradients was assessed using logistic regression fitted by means of generalized additive models. The species optimum and amplitude were determined. To check the possible effect of competitive release, we estimated where the potential maximum number of species (maximum overlap in realized niches) occurs along the base richness gradient. Results Most of the 41 frequently occurring species showed a significant response to water pH and ln‐transformed conductivity (approximating total mineral richness) in both regions. Eight species showed a shift in pH optimum greater than one unit, while 12 species showed the same or a larger shift along the conductivity gradient. Nearly all these striking shifts were connected to an extension of species tolerance towards mineral‐poor acid habitats in Bulgaria, which causes links between species and measured factors to be conspicuously weaker in Bulgaria than in the West Carpathians. Regarding ecological amplitude, 24 species exhibited a wider tolerance to water conductivity in the West Carpathians, whereas 17 species exhibited a wider tolerance in Bulgaria. Main conclusions A distinctive variation in the realized niche was observed in a large portion of the species examined. Niche shifts between local populations of the same species were similar to those of closely related vicariant species. Ecotypic adaptation within species is a possible explanation for this pattern. Other possible explanations (competitive release, specific habitat conditions, compensation for climate) seem to be less justified. The local populations of rich‐fen species may have adapted to mineral‐poor acid conditions in the high crystalline mountains of Bulgaria during dry periods of pleniglacials. Nomenclature Marhold & Hindák (1998) ; for Balkan elements not included in this source, Andreev et al. (1992) .