Omnivory and the stability of simple food webs |
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Authors: | Marcel Holyoak Sambhav Sachdev |
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Institution: | (1) Entomology Department, University of California, 1 Shields Avenue, Davis, CA 95616-8584, USA e-mail: maholyoak@ucdavis.edu, Fax: +1-916-7521537, US;(2) Gahr High School, 1111 Artesia Blvd., Cerritos, CA 90703, USA, US |
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Abstract: | Traditional ecological theory predicts that the stability of simple food webs will decline with an increasing number of trophic
levels and increasing amounts of omnivory. These ideas have been tested using protozoans in laboratory microcosms. However,
the results are equivocal, and contrary to expectation, omnivory is common in natural food webs. Two recent developments lead
us to re-evaluate these predictions using food webs assembled from protists and bacteria. First, recent modelling work suggests
that omnivory is actually stabilizing, providing that interactions are not too strong. Second, it is difficult to evaluate
the degree of omnivory of some protozoan species without explicit experimental tests. This study used seven species of ciliated
protozoa and a mixed bacterial flora to assemble four food webs with two trophic levels, and four webs with three trophic
levels. Protist species were assigned a rank for their degree of omnivory using information in the literature and the results
of experiments that tested whether the starvation rate of predators was influenced by the amount of bacteria on which they
may have fed and whether cannibalism (a form of omnivory) occurred. Consistent with recent modelling work, both bacterivorous
and predatory species with higher degrees of omnivory showed more stable dynamics, measured using time until extinction and
the temporal variability of population density. Systems with two protist species were less persistent than systems with one
protist species, supporting the prediction that longer food chains will be less stable dynamically.
Received: 28 December 1997 / Accepted: 22 June 1998 |
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Keywords: | Cannibalism Omnivory Population persistence Stability Starvation |
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