Just add water: rapid assembly of new communities in previously dry riverbeds, and limited long-distance effects on exis
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COMMUNITY ECOLOGY – ORIGINAL RESEARCH
Just add water: rapid assembly of new communities in previously dry riverbeds, and limited long‑distance effects on existing communities Andrew J. Brooks1,2 · Jill Lancaster2 · Barbara J. Downes2 · Benjamin Wolfenden3 Received: 2 February 2020 / Accepted: 3 November 2020 © crown 2020
Abstract 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. Keywords Community assembly · Colonisation · Dispersal · Freshwater invertebrates · Barriers
Introduction
Communicated by Leon A. Barmuta. Electronic supplementary material The online version of this article (https://doi.org/10.1007/s00442-020-04799-2) contains supplementary material, which is available to authorized users. * Andrew J. Brooks [email protected] 1
Department of Planning, Industry and Environment, Water, PO Box 53, Wollongong, NSW, Australia
2
School of Geography, The University of Melbourne, Victoria, Australia
3
Department of Planning, Industry and Environment, Biodiversity, Conservation and Science, Albury, NSW, Australia
Constraints on dispersal are a fundamental component of community assembly, determining the order, frequency and identity of species arrival at a location from the pool of potential colonists (Harrison and Hastings 1996; Belyea and Lancaster 1999). Upon arrival, an o
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