Geographical trends in the species-to-family ratio of vascular plants in Fennoscandia

The ratio between the number of species and the number of higher-ranked taxa in biotic assemblages has long been used to evaluate the importance of competition and other ecological processes in determining assemblage composition. We present a study where the variation in the species-to-family (S/F) ratio of vascular plants in 229 (75 × 75 km) grid squares within Fennoscandia is related to latitude and longitude. Randomization tests were performed to allow for the statistical artefact that species-rich areas will tend to have more species per family. When this is accounted for, a clear geographical pattern remains. An S/F ratio equal to or slightly lower than expected is found in southern areas, whereas an S/F ratio higher than expected is found in northern areas and in alpine areas of southern Fennoscandia. Similar climatic demands or dispersal ability for species within a family compared to species from different families may help explain the high S/F ratios in the north. Different source pools for the n...