Body size and food web structure: testing the equiprobability assumption of the cascade model |
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Authors: | M G Neubert S C Blumenshine D E Duplisea T Jonsson B Rashleigh |
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Institution: | Biology Department, MS #34, Woods Hole Oceanographic Institution, Woods Hole, MA 02543-1049, USA e-mail: mneubert@whoi.edu, US Department of Biological Sciences, Arkansas State University, State University, AR 72467, USA, US CEFAS Lowestoft Laboratory, Pakefield Rd, Lowestoft, Suffolk, NR33 0HT, UK, GB Department of Biology, Link?ping University, S-58183 Link?ping, Sweden, SE EPA Ecosystems Assessment Branch, 960 College Station Road, Athens, GA 30605, USA, US
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Abstract: | The cascade model successfuly predicts many patterns in reported food webs. A key assumption of this model is the existence
of a predetermined trophic hierarchy; prey are always lower in the hierarchy than their predators. At least three studies
have suggested that, in animal food webs, this hierarchy can be explained to a large extent by body size relationships. A
second assumption of the standard cascade model is that trophic links not prohibited by the hierarchy occur with equal probability.
Using nonparametric contingency table analyses, we tested this ”equiprobability hypothesis” in 16 published animal food webs
for which the adult body masses of the species had been estimated. We found that when the hierarchy was based on body size,
the equiprobability hypothesis was rejected in favor of an alternative, ”predator-dominance” hypothesis wherein the probability
of a trophic link varies with the identity of the predator. Another alternative to equiprobabilty is that the probability
of a trophic link depends upon the ratio of the body sizes of the two species. Using nonparametric regression and liklihood
ratio tests, we show that a size-ratio based model represents a significant improvement over the cascade model. These results
suggest that models with heterogeneous predation probabilities will fit food web data better than the homogeneous cascade
model. They also suggest a new way to bridge the gap between static and dynamic food web models.
Received: 3 February 1999 / Accepted: 26 October 1999 |
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Keywords: | Food webs Body size Cascade model Contingency tables Kernel smoothing |
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