Food web structure and the strength of transient indirect effects

The relative importance of direct and indirect effects in ecological commun ities remains unresolved. Indirect effects may diminish as they propagate t hrough highly reticulate food webs. We tested this hypothesis by assembling replicate food webs of different complexity in laboratory microcosms, and comparing the transmission of indirect effects through these webs. By provi ding the top predator (Didinium) with either one (Paramecium) or two (Param ecium and Colpidium) species of protists as prey, we created linear or reti culate food webs where we could examine the transient response of predators to an indirect effect. Addition of Chlamydomonas, a small alga consumed by Paramecium, but not by Colpidium?, perturbed the system and generated an i ndirect effect on Didinium. We expected the proportional response of Didini um to Chlamydomonas addition would be smaller in the reticulate web contain ing alternative, unperturbed prey (Colpidium). We measured predator respons e as predator yield, the maximum number of predators produced prior to over exploitation of prey and subsequent predator decline. The ratio of yield in perturbed bottles to yield in unperturbed bottles measures the proportiona l response of Didinium to Chlamydomonas addition. We expected this ratio to be smaller with Colpidium present. Contrary to expectations, alternative p rey enhanced rather than diminished predator response to the perturbation. This resulted from competition between the prey species, a factor ignored i n some simple verbal arguments. Food web complexity may have unanticipated consequences for the strength of indirect effects.