Effects of Bythotrephes longimanus (Crustacea, Cladocera) on the abundance, morphology, and prey community of Leptodora kindtii (Crustacea, Cladocera)
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We hypothesized that native Leptodora kindtii would be shorter and have smaller feeding baskets in central Ontario lakes with greater abundances of small-bodied zooplankton prey, and that differences in zooplankton size among lakes could be attributed to the invasive cladoceran Bythotrephes longimanus. We evaluated these conjectures by comparing size metrics of Leptodora and the size of their preferred cladoceran prey in lakes invaded or not by Bythotrephes. Leptodora was less abundant in invaded lakes, but were smaller bodied with smaller feeding baskets only in lakes with long invasion histories. Small cladoceran abundance was greater in non-invaded lakes and was directly related to Leptodora abundance although not to Leptodora size. Mean Leptodora body size declined with increasing abundance of Bythotrephes. We evaluated three possible explanations for these patterns in Leptodora—(a) competition with Bythotrephes for zooplankton prey, (b) direct predation by Bythotrephes, and (c) size-selective predation by fish. While we were unable to unequivocally distinguish among these hypotheses, our observations are most consistent with predation by Bythotrephes changing zooplankton community composition and size structure in a manner that is detrimental to Leptodora. Our results indicate that Bythotrephes invasion may trigger more complex and subtle changes in food webs than previously thought.
KeywordsLeptodora size Feeding basket size Bythotrephes invasion Cladoceran size Boreal lakes
We thank K. Krupica and W. B. Wissemeyer for their help in the field and C. Tudorancea and J. Kilgour for counting and measuring zooplankton. N. Collins, B. Shuter, P. Abrams, D, Branstrator, and anonymous reviewers provided helpful comments and editorial corrections. Funding for this project was provided by a Natural Sciences and Engineering Research Council (NSERC) Discovery Grant to WGS, a Mary Beatty Scholarship from the University of Toronto Graduate School and other scholarships from the Department of Zoology to SEF, and a NSERC post-graduate scholarship to ALS. S. Arnott provided funding, logistical support, and advice to ALS. We are also grateful to the Ontario Ministry of Environment’s Dorset Environmental Science Center and the Ontario Ministry of Natural Resources Provincial Co-ordination Center for additional field support.
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