Consumption patterns, complexity and enrichment in aquatic food chains

In English The interactions between consumers and prey, and their impact on biomass distribution among trophic
levels, are central issues in both empirical and theoretical ecology. In a long-term experiment, where all
organisms, including the top predator, were allowed to respond to environmental conditions by
reproduction, we tested predictions from `prey-dependent' and `ratio-dependent' models. Prey-dependent
models made correct predictions only in the presence of strong interactors in simple food chains, but
failed to predict patterns in more complex situations. Processes such as omnivory, consumer excretion,
and unsuitable prey-size windows (invulnerable prey) increased the complexity and created patterns
resembling ratio-dependent consumption. However, whereas the prey-dependent patterns were created
by the mechanisms predicted by the model, ratio-dependent patterns were not, suggesting that they may
be `right for the wrong reason'. We show here that despite the enormous complexity of ecosystems, it is
possible to identify and disentangle mechanisms responsible for observed patterns in community structure,
as well as in biomass development of organisms ranging in size from bacteria to ¢sh.