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Grazer Diversity, Competition and the Response of the Periphyton Community

Susan E. Gresens
Oikos
Vol. 73, No. 3 (Sep., 1995), pp. 336-346
Published by: Wiley on behalf of Nordic Society Oikos
DOI: 10.2307/3545957
Stable URL: http://www.jstor.org/stable/3545957
Page Count: 11
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Grazer Diversity, Competition and the Response of the Periphyton Community
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Abstract

Enclosure of known densities of three species of benthic grazers demonstrated 1) that grazer species had qualitatively different effects on periphyton, and 2) that competitive interactions decreased growth and development of grazers at natural densities. Grazers (chironomid midge larvae, snails, chydorid Cladocera) did not significantly decrease biomass of periphyton attached to artificial substrates. Midge larvae and snails strongly increased the amount of deposited organic matter (feces) in enclosures. Interaction between midges and snails significantly decreased the amount of deposit present at high grazer densities. Snails increased biomass-specific chlorophyll content of attached algae. Grazers did not significantly alter the taxonomic composition of intact algae on substrates. However, midge density increased up to 70% the proportion of total algal biovolume on substrates which occured in fecal pellets. Total organic matter in enclosures was significantly increased at high midge density. Although periphyton food resources were not depleted, negative density dependent effects existed among grazers. High snail density decreased development of midge larvae by 50%, and reproduction of chydorids by 84%. Snails and midges also suffered from intraspecific competition. In the laboratory, activity of midge larvae decreased 29% in the presence of snails. Interference appears to be the predominant form of competition among these littoral grazers. Competitive interactions among benthic grazers may provide a negative feedback to perturbations at the top or bottom of littoral foodwebs.

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