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

Abstract

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 northern and southern areas are an alternative explanation for the observed deviations from random expectation. Nevertheless, the relative differences between the north and the south remain as an interpretable pattern, whether one or several source pools are assumed. One explanation for this relative difference might be the higher number of polyploids in northern areas.