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Presented at the NABS Annual meeting, Athens, Georgia, 2003
in Food Webs
Impacts of predatory fish in grassland and forested streams: does forest cover influence the relative importance of top-down processes?
A.R. McIntosh1, P. Nyström1,2, and M.J. Winterbourn1. 1Department of Zoology, University of Canterbury, Private Bag 4800, Christchurch, New Zealand, 2Department of Ecology, Lund University, Sweden
The influence of predatory fish on stream food webs may be altered by forest cover, and consequent differences in allochthonous inputs and light. Eight sites containing trout (Salmo trutta) and eight sites that did not were sampled. For each predator treatment, half the sites were located in beech (Nothofagus) forest and the rest in grassland. Total invertebrate biomass did not differ between streams with and without trout, but total biomass and taxon richness were significantly higher at grassland sites than forested sites. However, species composition differed between trout and troutless sites, irrespective of forest cover. Trout streams had a low biomass of predatory invertebrates and large shredders, but a high proportion of cased caddisflies. The biomass of coarse organic matter was higher at forested sites, but there was less fine organic matter and more algae at sites with trout, regardless of forest cover. At trout sites, the carbon signals of Deleatidium mayflies, the most abundant invertebrate primary consumer, were closely related to biofilm values, but there was no relationship at troutless sites. These results support a role for both bottom-up and top-down processes in controlling community structure, but indicate that predatory fish and forest cover had largely independent effects.
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