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.

阻止物种扩散到某个位置的障碍可能会对社区的聚集方式产生重大影响。分散的成功也可能取决于分散者是否必须殖民一个已建立的社区或一个绝大部分地区。在淡水系统中，水坝和堰坝使河流支离破碎，有可能限制生物群沿河的扩散。两条堰上的退役渡槽分别位于不同管制河流的支流中，将水流输送至先前干燥的河床，并将其他水流输送至主干河下游的受管制河流。这提供了一个机会来测试散布限制的消除如何影响新栖息地中的社区聚集以及改变的散布是否可以改变现有社区。这两个系统的结果非常相似。即使通过降低漂移率来限制散布，在支流中新形成的栖息地中的新群落也迅速类似于未受到影响的参比群落，后者是殖民者的来源。对于已建立的社区（受管制的河流），更大的流量增加了滤嘴的密度，但这是由于快速流动的栖息地的面积更大（环境限制的变化）而不是更高的分散率所致。我们的研究表明，当几十年来干燥的自然渠道被从上游完好无损的位置输送分散剂的流重新浸湿时，社区可以迅速重新聚集。但是，增加分散剂的流量和随之而来的密度不会对现有社区产生重大影响。相反，增加的排放量减少了对环境的限制，改变了营养结构。因此，排放量和扩散量的增加在新社区和老社区中产生了不同的结果。