Drifting or walking? Colonisation routes used by different instars and species of lotic, macroinvertebrate filter feeders
Barbara J. Downes A C , Alecia Bellgrove B and Jodie L. Street AA School of Anthropology, Geography & Environmental Studies, 221 Bouverie St, The University of Melbourne, Vic. 3010, Australia.
B Present address: School of Ecology and Environment, Deakin University, PO Box 423, Warrnambool, Vic. 3280, Australia.
C Corresponding author. Email: barbarad@unimelb.edu.au
Marine and Freshwater Research 56(6) 815-824 https://doi.org/10.1071/MF04281
Submitted: 25 November 2004 Accepted: 26 May 2005 Published: 27 September 2005
Abstract
Many views of stream invertebrate populations centre on drift as the major route of larval dispersal, but few studies have presented unambiguous information about the role of drift. We present the results from an experiment designed to determine whether the major route of colonisation of substrata by hydropsychid larvae (commonly found in the drift) is by drifting directly onto substrata or by walking along the stream bottom. The experimental design contained four treatments: substrata open to drifters and walkers; fenced substrata open to drifters only; and two treatments open to drifters and walkers that provided forms of fence controls. Fifteen replicates of each treatment were set out at random locations within a riffle at each of three sites, with each site on a different river (the Little River, the Steavenson River and the Acheron River) in the Acheron River catchment. The experiment was run twice, once during autumn (April 1999) and once during early summer (January 2000). Both experiments were colonised by three species of hydropsychids, Asmicridea sp. AV1, and Smicrophylax sp. AV1 and AV2. We found that 2nd/3rd instars of Asmicridea sp. AV1 walked as well as drifted, whereas all others primarily drifted. No relation between numbers of recruits and water speed was found when substrata were open only to drifters, whereas substrata open also to walkers gained more recruits in faster flows. Additionally, larvae more frequently abandoned nets in slow than fast flows, indicating that drifting into unfavourable flow environments may result in mortality or redispersal of larvae. These findings demonstrate that, although drift is important, it is not necessarily the only method used by hydropsychids to colonise substrata. Larvae may have more capacity to choose substrata in fast flows when they colonise substrata by walking. Spot measures of hydropsychid distribution cannot distinguish between these explanations. The finding that walkers can sometimes comprise significant numbers of recruits raises the prospect that hydropsychids can be sourced locally and have not inevitably drifted in from upstream locations.
Extra keywords: dispersal, habitat selection, hydropsychids, recruitment.
Acknowledgments
This work was supported by a Discovery Grant to BJD from the Australian Research Council. We are grateful to several folk who helped with fieldwork: Owen Hammond, Paulo Lay, Lucy Nairn, Paul Reich and Andrew Sharpe.
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