Streams experience a constant redistribution of species and in-streamplant stands due to the extensive disturbance caused by flow variation, highcontinuity of habitats and efficient dispersal in the stream network. Theseconditions increase the homogeneity of environmental conditions and compositionof vegetation among stream localities and are likely to promote a positiveinterspecific relationship between abundance and range size through mechanismsof metapopulation dynamics and use of common widely distributed resources.Usingdata from 206 localities in 29 stream systems distributed throughout thecultivated lowlands of Denmark, we examined the overall relationship betweenlocal abundance and geographical range size of the vascular flora. We found asignificant positive relationship for all species at all stream localities andan even stronger relationship for ecologically similar species such asobligatory submerged or amphibious species. The amphibious species, which caneasily disperse by seeds between stream systems and by vegetative growth frompermanent bank populations to the open streambed, had a significantly strongerabundance-range relationship than obligatory submerged species probably due tomore effective dispersal. The importance of metapopulation dynamics was alsosupported by the fact that species in seven of the ten stream systems withmultiple localities showed a stronger positive relationship than the overallrelationship for all streams, while the relationship for distinct streamhabitattypes of the same width and lengthwise locations in different stream systemsdidnot show a stronger positive relationship than the overall relationship. Thefewobligatory submerged species having a high local abundance and low range sizecould not be regarded as specialist species on narrowly distributed resources.Their occurrence is probably a temporal phenomenon due to the profound habitatdestruction during the last 100 years, restricting to a few localities thespecies that were previously widely distributed. Many species presently survivein a marginal position of low range size and low local abundance. They face adouble jeopardy of extinction and may have difficulty benefitting fromenvironmental improvements.