Inducible defences and the paradox of enrichment
Vos, M, Verschoor, A. M., Kooi, B. W., W\'{a}ckers, F. L., DeAngelis, D. L. and Mooij, W. M. 2004.
Inducible defences and the paradox of enrichment. Ecology 85: 2783-2794.
Abstract
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.