1. Despite growing interest in ecological consequences of parasitism in food webs, relatively little is known about effects of parasites on long-term population dynamics of non-host species or about whether such effects are density- or trait- mediated. 2. We studied a tri-trophic food chain comprised of: (i) a bacterial basal resource (Serratia fonticola), (ii) an intermediate consumer (Paramecium caudatum), (iii) a top predator (Didinium nasutum), and (iv) a parasite of the intermediate consumer (Holospora undulata). A fully-factorial experimental manipulation of predator and parasite presence/absence was combined with analyses of population dynamics, modelling, and analyses of host (Paramecium) morphology and behavior. 3. Predation and parasitism each reduced the abundance of the intermediate consumer (Paramecium), and parasitism indirectly reduced the abundance of the basal resource (Serratia). However, in combination, predation and parasitism had non-additive effects on the abundance of the intermediate consumer, as well as on that of the basal resource. In both cases, the negative effect of parasitism seemed to be effaced by predation. 4. Infection of the intermediate consumer reduced predator abundance. Modelling and additional experimentation revealed that this was most likely due to parasite reduction of intermediate host abundance (a density-mediated effect), as opposed to changes in predator functional or numerical response. 5. Parasitism altered morphological and behavioural traits, by reducing host cell length and increasing the swimming speed of cells with moderate parasite loads. Additional tests showed no significant difference in Didinium feeding rate on infected and uninfected hosts, suggesting that the combination of these modifications does not affect host vulnerability to predation. However, estimated rates of encounter with Serratia based on these modifications were higher for infected Paramecium than for uninfected Paramecium. 6. A mixture of density-mediated and trait-mediated indirect effects of parasitism on non-host species creates rich and complex possibilities for effects of parasites in food webs that should be included in assessments of possible impacts of parasite eradication or introduction.

Paramecium & Didinium AbundanceRaw data from the main experiment, pertaining to ciliate abundances over time. Observations were input by row. Column titles specify the following: "day" -- day of the experiment on which the observation was made; "repID" -- replicate label of the microcosm; "trtmt" -- treatment label of the microcosm; "prey" -- presence (yes) or absence (no) of Paramecium in the microcosm; "predator" -- presence (yes) or absence (no) of Didinium in the microcosm; "parasite" -- presence (yes) or absence (no) of Holospora in the microcosm; "species" -- the species to which the observation pertains ("PARA" -- Paramecium; "DIDI" -- Didinium); "vol" -- the volume sampled for the observation; "count" -- the number of individual present within the volume sampled.p-d-h_raw_data3.csvSerratia AbundanceRaw data from the main experiment, pertaining to Serratia abundance over time. Observations were input by row. Column titles specify the following: "day" -- day of the experiment on which the observation was made; "repID" -- replicate label of the microcosm; "trtmt" -- treatment label of the microcosm; "prey" -- presence (yes) or absence (no) of Paramecium in the microcosm; "predator -- presence (yes) or absence (no) of Didinium in the microcosm; "parasite" -- presence (yes) or absence (no) of Holospora in the microcosm; "species" -- the species to which the observation pertains ("SERRA" -- Serratia); "Platevol" -- volume of sample (in microliters) plated on agar gel for incubation; "Dilution" -- the factor by which the original sample from the microcosm had been diluted before plating; "Colonies" -- the number of colony-forming units counted on the plate.p-d-h.serra3.csvParamecium Morphology & TrajectoryRaw data obtained from video capture and image analysis of swimming Paramecium ("un" -- uninfected; "inf" -- infected by Holospora).MorphTrajExptData.csv