Variation in plant species richness in and adjacent to 64 ponds in SE Norwegian agricultural landscapes

All vascular plants, classified by life and growth form into six groups, four groups of hydrophytes (lemnids, nymphaeids, elodeids and isoetids), helophytes, and terrestrial species of pond margins, were inventoried in 64 SE Norwegian agricultural landscape ponds and their adjacent margins. The study sites varied considerably with respect to species richness; 0–4 for each hydrophyte group, 0–9 for helophytes, and 13–77 for terrestrial species. A total of 56 explanatory variables were recorded for each pond and adjacent margin to explain the observed richness variability.Species richness in each plant group was modelled separately by generalised linear models (GLM), using the recorded explanatory variables as predictors. Pond area, water depth and water chemical variables were the most important predictors of hydrophyte and helophyte species richness, followed by fluctuation and human impact variables such as liming and presence of ducks and fish. Pond margin area and geographical variables were significant predictors of richness of terrestrial species of pond margins.The contribution of different structuring processes to explanation of species richness patterns is discussed. The generally weak explanatory power of the selected variables indicates high importance of apparent randomness in this ecosystem, and, notably, that the ponds and their adjacent margins represent islands in the agricultural landscape that accumulate species more or less individualistically.