Inducible defences and trophic structure.

M. Vos, A.M. Verschoor, B.W. Kooi, F.L. Wackers, D.L. DeAngelis, W.M. Mooij

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Resource edibility is a crucial factor in ecological theory on the relative
importance of bottom-up and top-down control. Current theory explains trophic structure
in terms of the relative abundance and succession of edible and inedible species across
gradients of primary productivity. We argue that this explanation is incomplete owing to
its focus on inedibility and the assumption that plants and herbivores have fixed defense
levels. Consumer-induced defenses are an important source of variation in the vulnerability
of prey and are prevalent in natural communities. Such induced defenses decrease per capita
consumption rates of consumers but hardly ever result in complete inedibility. When defenses
are inducible a prey population may consist of both undefended and defended individuals.
Here we use food chain models with realistic parameter values to show that
variation in consumption rates on different prey types causes a gradual instead of stepwise
increase in the biomass of all trophic levels in response to enrichment. Such all-level
responses have been observed in both aquatic and terrestrial ecosystems and in microbial
food chains in the laboratory. We stress that, in addition to the known food web effects of
interspecific variation in edibility, intraspecific variation in edibility is another form of
within-trophic-level heterogeneity that also has such effects. We conclude that inducible
defenses increase the relative importance of bottom-up control.
Original languageEnglish
Pages (from-to)2783-2794
Issue number10
Publication statusPublished - 2004


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