Interacting environmental gradients, trade-offs and reversals in the abundance–environment relationships of stream insects: when flow is unimportant
Jill Lancaster A C , Barbara J. Downes B and Alena Glaister BA Institute of Evolutionary Biology, University of Edinburgh, Ashworth Labs West Mains Road, Edinburgh, UK.
B Department of Resource Management and Geography, 221 Bouverie Street, The University of Melbourne, Melbourne, Vic. 3010, Australia.
C Corresponding author. Email: J.Lancaster@ed.ac.uk
Marine and Freshwater Research 60(3) 259-270 https://doi.org/10.1071/MF08226
Submitted: 6 August 2008 Accepted: 3 November 2008 Published: 27 March 2009
Abstract
Flow is often presumed to determine the distribution of stream invertebrates across stream beds. When temperatures are high, however, dissolved oxygen (DO) and its interactions with other environmental gradients may be more important. Field surveys were carried out in summer at two sites in a sand-bed stream in south-east Australia. Using quantile regression, we quantified the abundance–environment relationships of a caenid mayfly and an ecnomid caddisfly, and determined whether DO, fine detritus or velocity was the dominant limiting variable, and to gain insight into the causal mechanisms. Local densities of caenids were driven by food resources (detritus) at a site with a short DO gradient. The relationship was completely reversed where long DO and detritus gradients interacted, and here DO appeared to limit density. Densities of ecnomids were limited by prey-rich detritus patches at both sites. The velocity gradient did not explain the distribution patterns in either species. Ecnomid diet altered with changes in the spatial distribution of caenids between sites; caenids were the dominant prey at one site, but proportionately fewer were consumed where there was a negative spatial overlap of predators and prey. These results show that invertebrate responses to environmental gradients can be complex and that flow may be unimportant.
Additional keywords: Caenidae, dissolved oxygen, Ecnomidae, quantile regression.
Acknowledgements
Thanks to Adrian Glaister for field assistance and to David Cartwright and Ros StClair for taxonomic assistance. Thanks to Brian Cade and an anonymous referee for comments on an earlier draft of this paper. No thanks to the innumerable bedbugs to whom we unwillingly and unwittingly donated blood during this project. This project was supported by an E.C. Dyason Universitas 21 Fellowship awarded to J. Lancaster, held at the University of Melbourne, and by an Australian Research Council Discovery Grant awarded to B. J. Downes and J. Lancaster.
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