Diversity patterns of Pampean stream vegetation at different spatial scales

•Changes in the slopes of species–area relationships (SAR) for macrophytes were examined.•Stream vegetation was sampled at four scales: reach, stream, watershed and ecoregion.•SAR slopes increased significantly with scale, from reach to ecoregion.•A linear relationship between reach scale and stream scale richness was found.•Macrophyte assemblages in streams can be considered metacommunities. We examined diversity patterns at different spatial scales by observing the changes in the slopes of Species–Area relationships (SAR) and Local–Regional (L–R) relationships. Stream vegetation was sampled at four scales (reach (N=50), stream (N=25), watershed (N=9), and ecoregion (N=2)) in the Pampa Region (Buenos Aires, Argentina). The slopes of the SARs fitted to the logarithmic model increased significantly with scale from reach to ecoregion. For the L–R relationships, the slopes were significantly different from zero when analyzing stream richness in relation to reach richness, and undistinguishable from zero when analyzing watershed richness in relation to stream richness. The differences found in the slopes of the SAR mean that the scales proposed a priori are indeed different diversity scales. On the other hand, the linear relationship between reach and stream richness suggests that macrophyte assemblages in streams function as metacommunities, whose dynamics could be explained by the dispersal process. Thus, we propose that the metacommunities in streams and the assemblages in watersheds and ecoregions can be considered as the three diversity scales most relevant when attempting to understand macrophyte dynamics in Pampean streams. The increase in the species accumulation rate across all scales indicates that any actions for the conservation of macrophyte diversity in Pampean streams should be taken at the largest scale, that is, the ecoregion scale; on the other hand, the loss of species in the headwaters not only implies a decrease in local diversity, but also a loss of species downstream, that is why particular attention should be paid to the headwaters.