Time, space and ecology: why some clades have more species than others

Abstract

Aim We seek biotic and abiotic explanations for differences in lineage sizes of Afromontane sedges (Cyperaceae, Carex) and buttercups (Ranunculaceae, Ranunculus). Location Mountains of sub-Saharan Africa and Madagascar. Methods We investigated differences in the species richness and diversification rates of 18 lineages of the pan-temperate plant groups Carex and Ranunculus, established by long-distance dispersal on African sky islands. We built generalized linear models to test the individual and the cumulative power of biotic and abiotic factors for predicting variation in the size of lineages. Tested variables were: ages of the lineages, their geographic distributions, number of mountain systems occupied, isolation/distance from ancestral areas, elevation range, number of vegetation zones and habitat types in which lineages are found, light requirement and water availability for each lineage, and the sum of the habitat factors, representing habitat heterogeneity. Habitat conservatism was measured by the overlap in habitats among the species within each lineage. Diversification rate changes were investigated using ‘laser’ in R. Results The number of Carex and Ranunculus lineages on the African mountains accumulated gradually through time. The size of these lineages could best be explained by a model combining age and distribution together with a measure of environmental heterogeneity (either elevation and water availability or habitat heterogeneity). Extensive overlap in environmental characteristics and distribution ranges among the species indicates a relatively high degree of conservatism of these characters. Main conclusions Lineages that are species-rich are those that have the ability and time to occupy many mountain regions and a wide range of habitats. If allopatric or ecological speciation plays a role, then secondary dispersal and/or niche expansion soon obscures the patterns that may have existed at the point of speciation.