Department of Planning, Industry and Environment, Water, PO Box 53, Wollongong, NSW, Australia.
School of Geography, The University of Melbourne, Victoria, Australia.
Oecologia. 2020 Dec;194(4):709-722. doi: 10.1007/s00442-020-04799-2. Epub 2020 Nov 11.
Barriers preventing species from dispersing to a location can have a major influence on how communities assemble. Dispersal success may also depend on whether dispersers have to colonise an established community or a largely depauperate location. In freshwater systems, dams and weirs have fragmented rivers, potentially limiting dispersal of biota along rivers. Decommissioning aqueducts on two weirs, each within a tributary of different regulated rivers, delivered flow to previously dry riverbeds and additional flows to the main stem, regulated rivers further downstream. This provided an opportunity to test how removal of dispersal constraints affected community assembly in new habitats and whether changed dispersal can alter existing communities. The results were very similar for the two systems. Even with dispersal constrained via reduced drift rates, the new communities in the newly formed habitat in tributaries rapidly resembled unimpacted reference communities that were the source of colonists. For established communities (regulated rivers), greater flow increased the densities of filter feeders but this was due to greater areas of fast-flowing habitat (a change in environmental constraints) rather than higher dispersal rates. Our study illustrates that communities can quickly re-assemble when natural channels that have been dry for decades are re-wetted by flows that deliver dispersers from intact locations upstream. Nevertheless, boosting flows and concomitant densities of dispersers had no strong effects on existing communities. Instead, increased discharges effected a reduction in environmental constraints, which altered trophic structure. Thus, increases in discharge and dispersal produced different outcomes in new versus established communities.
阻碍物种扩散到特定地点的因素可能会对群落的形成产生重大影响。扩散的成功与否也可能取决于扩散者是要在已建立的群落中还是在资源匮乏的地点进行殖民。在淡水系统中，水坝和堰将河流分割开来，可能会限制生物沿河流的扩散。拆除两座水坝上的渡槽，这两座水坝分别位于不同受管制河流的支流上，为原本干涸的河床输送水流，并为下游受管制的干流增加水流。这为测试去除扩散限制如何影响新栖息地中的群落组成以及扩散的变化是否会改变现有群落提供了机会。这两个系统的结果非常相似。即使通过降低漂流率来限制扩散，支流中新形成栖息地中的新群落也迅速类似于未受影响的参照群落，这些参照群落是殖民者的来源。对于已建立的群落（受管制的河流）来说，更多的水流增加了滤食者的密度，但这是由于快速流动的栖息地面积更大（环境限制的变化），而不是扩散率更高。我们的研究表明，当几十年未被水流浸湿的自然河道重新被来自完整上游地区的水流重新润湿时，群落可以迅速重新组合。然而，增加水流和随之而来的扩散者密度对现有群落没有强烈影响。相反，增加的排放会减少环境限制，从而改变营养结构。因此，增加排放和扩散对新群落和已建立群落产生了不同的结果。
