Scale-dependent dynamics: Zooplankton and the stability of freshwater food webs

The study of freshwater pelagic communities is entering an exciting and controversial phase. Recent efforts to clarify how food web interactions differ from food chain interactions have emphasized the various, often subtle, repercussions of top predators on communities. Predators can modify community structure not only through directly imposed death rates, but also through direct and indirect effects on prey interactions, behavior, life-styles and morphology (e.g. induction of defenses). In some cases, the effects influence ecosystem properties (material fluxes, turnover rates and primary production). Attempts to trace food web impacts in enclosure and lake studies have revealed important time-dependent system properties. Severe resource limitation of fast variables (phytoplankton and small zooplankton) stabilizes lower trophic levels, whereas the potentially destabilizing effects of fish population oscillations are long compared to the growing season and subject to year-to-year climatic vagaries. The time-scale dependent approach is important because it emphasizes how local (transient) solutions may be more ecologically relevant to stability calculations than overall (global) solutions.