Ecotones of riverine ecosystems: Role and typology, spatio-temporal dynamics, and river regulation
Treść / Zawartość
Natural riverine ecosystems are characterized by a high level of heterogeneity manifest across a range of spatio-temporal scales. Ecotonal habitats are both the result of and contributors to the spatio-temporal dynamics of riverine ecosystems. Natural disturbances play an important rôle in maintaining a diversity of ecotonal habitats. A typology of riverine ecotones is developed that provides an expansive perspective scaled along four dimensions (longitudinal, lateral, vertical, and temporal) and that encompasses environmental gradients and boundaries as well as distinct transition zones between adjacent patches. From this broad perspective it is apparent that riverine ecotones play important rôles relating to speciation, evolutionary invasion of fresh waters, biodiversity, bioproduction, and ecological connectivity. River regulation disrupts the natural disturbance regime downstream, thereby reducing the diversity of ecotonal habitats and their connectivity with the main river channel. The altered rôle of ecotonal habitats induced by regulation is especially pronounced in alluvial floodplaing rivers, which are characterized by a mosaic of habitat patches that collectively occupy a wide range of successional stages. Downstream hydrologic changes, such as truncated sediment transport and reductions in the frequency and intensity of flooding, typically lead to altered successional trajectories, desiccated floodplain waterbodies, severed migration pathways, and reduced exchange rates of nutrients and organic detritus across ecotone boundaries. Effective management of regulated rivers should focus on maintaining or restoring the important rôles of ecotones by re-establishing interactive pathways and by reconstituting a disturbance regime that leads to a diversity of habitat patches and successional stages.