Drivers of plant and lichen diversity in grasslands on mineral islands surrounded by peatlands (Biebrza Valley, NE Poland)

Abstract

We studied fine-scale patterns of plant and lichen species richness in the grasslands of small mineral 'islands' in the extensive Biebrza River peatlands (NE Poland). Despite their relatively small size (up to several tens of hectares), the islands with their mesic to dry grasslands are regional biodiversity hotspots. We asked: (i) What is the species richness of vascular plants, non-vascular taxa (bryophytes and lichens), and threatened vascular plant species in such grasslands and how does it vary at different spatial scales? (ii) To what extent are the observed biodiversity patterns of different groups determined by geographic, topographic, soil, productivity and anthropogenic factors? We sampled environmental data and all vascular plant, bryophyte and lichen species in 38 nested plot series of seven grain sizes (0.0001–100 m²) located within grasslands on these habitat islands. We used GLMMs, GLMs and multi-model inference to assess the importance of 16 variables related with geographical location and landscape, topography, soil, productivity, and human impact as predictors of species richness. For seven predictors, we confirmed the scale dependence of their importance. For most variables and grain sizes, richness of non-vascular taxa showed the inverse pattern to vascular plants both in case of scale-dependency of variable importance as well in the case of the direction of the relationship. For vascular plant richness mostly local factors were important predictors (e.g. unimodal relationship with soil moisture and organic matter), but surprisingly not soil pH, in contrast to many other studies. The amount of surrounding grassland was an important positive predictor only for the richness of threatened species. Conversely, richness of threatened species was negatively related to the size of the island. This result suggests that small habitat islands should receive special attention in conservation planning and management.