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The relative importance of direct and indirect effects in ecological communities remains unresolved. Indirect effects may diminish as they propagate through 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 providing the top predator (Didinium ) with either one (Paramecium ) or two (Paramecium and Colpidium ) species of protists as prey, we created linear or reticulate 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 indirect effect on Didinium . We expected the proportional response of Didinium to Chlamydomonas addition would be smaller in the reticulate web containing alternative, unperturbed prey (Colpidium ). We measured predator response as predator yield, the maximum number of predators produced prior to overexploitation of prey and subsequent predator decline. The ratio of yield in perturbed bottles to yield in unperturbed bottles measures the proportional response of Didinium to Chlamydomonas addition. We expected this ratio to be smaller with Colpidium present. Contrary to expectations, alternative prey enhanced rather than diminished predator response to the perturbation. This resulted from competition between the prey species, a factor ignored in some simple verbal arguments. Food web complexity may have unanticipated consequences for the strength of indirect effects. [ABSTRACT FROM AUTHOR]

This paper evaluates the multiple factors that determine the production of plant biomass and its distribution among producers and varioustrophic groups of consumers. In rough order of their importance, water and nutrient availability, factors that deter herbivores (plant defenses, environmental heterogeneity and disturbance, nutrient stoichiometry), and consumption by herbivores appear to the most universal determinants of the production and distribution of plant biomass. In some times and places, indirect effects from enemies of herbivores (predators, parasites, parasitoids and pathogens) propagate through the food web to influence plant biomass, in a manner somewhat consistent with green world and exploitation ecosystem mechanisms. I discuss whysuch food web dynamics appear to be much more important in water than on land. The only demonstrated cases of community-level trophic cascades occur in water. Although species-level cascades are moderately frequent on land, community-level cascades rarely or never occur. [ABSTRACT FROM AUTHOR]

Carbon stable isotopic composition was determined for zooplankton, POM, DOM and, in two cases, phytoplankton from 12 small forest lakes in southern Finland during summer 1997. The lakes were selected to provide a wide range of water colour (12-365 mg Pt l-1) but minimum variability in other limnological characteristics. POM and DOMshowed similar stable isotope ratios (delta13C around -28, comparable with values commonly reported for terrestrial C3 vegetation) and these showed no relationship to water colour. Zooplankton were consistently depleted in 13C relative to the bulk POM on which they might feed. This relative 13C depletion increased significantly with water colour. Therefore the zooplankton must have been feeding selectively on isotopically light food sources and doing so to a greater extent in the more humic lakes. Although phytoplankton were 13C-depleted relative to POM, their abundance and their variability between lakes did not appear to explain adequately the observed trend in zooplankton stable isotope ratios. We propose that some zooplankton were grazing on methanotrophic bacteria which utilize isotopically light methane derived from allochthonous organic matter decomposed under anaerobic conditions. This process would be more pronounced in colored lakes with a higher loadingof allochthonous organic matter and greater development of hypolimnetic anoxia. [ABSTRACT FROM AUTHOR]